Sudan Academy of Sciences

Dose Measurements for a Radiotherapy

Co-60 Machine

By

ALI MOHAMMED ELBASHIR ALI

A ^ j D (JAijJl Atlt (WU

Sudan Academy of Sciences Atomic Energy Council

Dose Measurements for a Radiotherapy

Co-60 Machine

By

ALI MOHAMMED ELBASHIR ALI

B.Sc (Honours)

A thesis submitted as a partial fulfillment for the

requirements of M.Sc. Degree in Medical Physics

Supervisor:

Dr. Ibrahim Idris Soliman

2008

Sudan Academy of Sciences

Atomic Energy Council

Examination Committee

Name Dr. Farouk Idris Habbani

Mr. Mohaned Mohammed

Elhassen

Dr. Ibrahim Idris

Soliman

Title External examiner

] W^cademic Affairs Representative

Supervisor

2008

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ABSTRACT

Measurements were conducted for the determination of the absorbed dose to water for

high energy photon beam of the Co-60 machine at the Institute of Nuclear Medicine,

Molecular Biology and Oncology (INMO), Jazeera State with the new dosimetry

equipments. The aims of this study were: 1- To make quality control for the Co-60

machine (CIRUS-Cis Biointernational-made at 1998) and to understand the relevant

dosimetry principles. 2- To verify the acquired data with published data such as PTW-

FREIBURG (German) and TRS398 -protocols. 3- To determine the absorbed dose to

the water using above two protocols.

A reference water phantom was used for the dosimetry measurements. The total

deviation in the determination of the dose to water during calculation of the beam was

found to be < 2.5 % and this magnitude was within the accepted limits of

recommendation of the IAEA protocol ^(2.5%).

The discrepancies in the determination of the absorbed dose to water between the two

protocols were 1.4%. Comparative measurements showed a deviation of less than 2%

between our measurements and protocol DIN-6800-2, and TG,51 protocol.

The results obtained showed that our measurements were within the accepted limit.

II

DEDICATION

c/fr'kMil ofmujht/ie* JMohammed <Q/6a6h'i*. and to- mn mo/Aep KJVOO-P and

for- mu wife Jffadeet' and fa* ma on/a one 6tite* a/tdfor a// my 6*etAexS and

fnend6.

Ill

ACKNOWLEDGMENTS

This investigation was -performed-within the framework of the TtRs -398

and (FItW-(F<^EI(BV(S^ protocols .In this research. I have 6een helped by

Mr.JAwed (Daraj and my great thanks go for my supervisor (Dr. I6rahim

Idris Soluman for supervising the research.

My great thanks go for the HeadofMedical(Physics (Department ofUMMO

who helped me to oBtain the data and to acquire the necessary and

sufficient skills in setting the dosimetry equipment for required

measurements.

IV

List of symbols

A Area; field size; atomic mass number; activity

0 Reference activity BQ becquerel (SI unit of activity)

C CEMA, converted energy per unit mass c speed of light C coulomb (SI unit of charge) °C degree Celsius (unit of Celsius temperature)

C* Curie (unit of activity)

d distance;

D Absorbed dose

D Dose rate

Dw Dose-to-water

Average energy transferred into kinetic energy of charged particles per interaction.

/ Gy

W-Q Absorbed dose to water at the reference depth, ref, in a water phantom irradiated by a beam of quality Q. The subscript Q is omitted when the reference beam quality is 60Co

Source-surface distance (SSD)

Gray (SI unit of dose)

HVL Half-value layer

J Joule (SI unit of energy) IS

Q.Qo Factor to correct for the difference between the response of an ionization chamber in the reference beam quality Q0 used for calibrating the chamber and in the actual user beam quality, Q. The subscript Q0 is omitted when the reference quality is ^ C o gamma radiation.

IT

P0' Polarization correction factor

s Ion recombination correction factor

T-p Temperature and pressure correction factor

K Kerma ? rt

K KERMA Rate K coi Collision kerma

rad radiative kerma LET Linear energy transfer m Mass M U monitor unit (unit of quantity MU)

™ Number of particles (photons); ionization chamber calibration coefficient

Number of electrons per gram

Avogadro's number

V

D.w.Qo Calibration factor in terms of absorbed dose to water for a dosimeter at a reference beam quality Q0.

P Pressure Pa Pascal (SI unit of pressure) p

eff Effective point of measurement P

0 Standard air pressure (101.325 kPa) PDD Percentage depth-dose. Q charge, point-of-interest in phantom; beam quality r radius; R roentgen (unit of exposure), Radiant energy Rad old unit of exposure and equal to 10"2 J.Kg"1

S linear stopping power; scatter function; cell surviving fraction S,

s s

Collimator scatter factor p Phantom scatter factor c-p Total scatter factor

"" Mass stopping power SAD Source-axis distance SCD Source-chamber distance SSD Source-surface distance

'1/2 Half-life T Temperature T° Standard air temperature (273.2K or 0°C) TMR Tissue-maximum ratio TPR Tissue-Phantom ratio. X Exposure

2 Depth in phantom; atomic number

Maximum depth

wa/i Water equivalent thickness of the phantom's entrance window Beta particle

^ Gamma ray P Density a Cross section (p fluence ^ Energy fluence Q Fluence rate or flux density

P Linear attenuation coefficient a^ Atomic attenuation coefficient

<"̂ Electronic attenuation coefficient

P

VI

CONTENTS Chapter 1

Introduction:

1.1 Preface 1 1.2 The aim of the study 2 1.3 The important of the study 2 1.4 The Hypothesis 2 1.5 Material and Method 3 1.5.1 Relative dose measurements 3 1.5.2 Absolute dose measurements 4 1.6 Quantities and Units 5 1.6.1 Quanitities 5 1.6.2 Interaction Coefficients 7 1.6.3 Dosimetric Quantities 11 1.7KERMA , 15 1.8 Energy fluence and kerma (photons) 15 1.9 Collision kerma and exposure 16 1.10 Absorbed dose and exposure 16 1.11 Charged particle equilibrium 18 1.12 Interaction of ionization radiation with matter 21 1.13 Beam quality 24

Chapter 2 Relative and Absolute dosimetry

2.1 Introduction 26 2.2 Fundamental Absorbed Dos-methods measurement 26 2.2.1 Calorimetry 27 2.2.2 Principles of absorbed does calibration 27 2.2.3 Chemcal Dosimetry 28 2.2.4 Ionization Dosimetry 29 2.2.5 Ionization measurements in water phantion 29 2.2.6 Ionization measurements in graphite phantion 30 2.2.7 Simplified Theory of TLD 30 2.3 PDD 31 2.4 Air temperature, pressure and humidity effects: KT,P 33 : ' . - > 2.5 Chamber polarity effects: polarity correction factor Kpoi 33 ?f-2.6 Chamber voltage effects: recombination correction factor Ks ; 3.4 2.7 Absorbed dose-to-water -. ........35

Chapter 3 Material and Methods:

3.1. Introduction 38

3.2. Equipment 38

3.2.1. Dosimetry of system 39

3.2.2. Phantoms 39

VII

3.2.2.1. Reference water phantom 39

3.3. Absolute measurement 39

3.3.1. Absorbed does to water measurment 39

3.3.1.1. Stability check and response characteristics 39

3.3.1.2. Absorbed does to water calculation 40

3.3.1.3. Determination of the polarity correction factor 41

3.3.1.4. Determination of the ion recombination factor 41

3.3.1.5. Determination of the temperature and pressure correction factor 42

3.4. Relative measurement 43

3.4.1. Beam quality Specification 43

Chapter 4

Result and Discussion:

4.1. Introduction 44

4.2. Absolute measurements 44

4.2.1 Determination of absorbed does to water 44

4.3. Results and discussion 45

4.4. Response characteristic of dosimetry system 47

Chapter 4

4.5. Conclusion 52

References 53

VIII

CHAPTER ONE

INTRODUCTION

1.1. Preface

After a therapy machine is accepted and before it can be placed in clinical service, the

physicist must acquire an extensive set of radiation measurements that characterize its

performance and to confirm the beam characteristics against the machine manufacturer's

specification. All acquired data is entered into the Radiotherapy Treatment Planning

System (RTPS) for the purpose of the dose calculation.

The Institute of Nuclear Medicine, Molecular Biology and Oncology (INMO), Medani is

the one of radiotherapy centers in Sudan that continues to expand and recently instollted a

Co -60 machine.

Many dosimetry kits and tools were purchased by the INMO to improve the quality

assurance. A water phantom was purchased to facilitate beam data acquiring. The water

phantom permits the ion chamber to scan through the radiation field in two dimensions.

There is also therapy dosimeter device (dose 1) for measuring dose and dose rate in

radiation therapy.

With these tools the central axis (CA) data can be acquired and determination of the^

absorbed dose to water under reference conditions can be measured. The collected data%

verified by qualified medical physicists'and entered to the RTPS.

In terms of dosimetry the CA data is referred to as relative dosimetry, while determination

of the absorbed dose to water is referred to as absolute dosimetry.

Many national and international protocols are published to guide the medical physicists to

conduct the dosimetry. Examples of such protocols are American Association of Medical

1

Physicists (AAPM), International Atomic Energy Agency (IAEA). All these organizations

now recommend using water as a reference media for the measurements. Moreover all of

these protocols shift from exposure and kerma factors to absorbed dose to water. All these

issues will be discussed in depth in the body of this study.

1.2. The aim of the study:

1- To acquire necessary and sufficient skills in setting the new dosimetry equipment.

2- To commission the Co-60 machine and to understand the underlining dosimetry

principles .

3- To determine the absolute dose to the water using IAEA TRS- 398 and PTW-

FRIBURG protocols.

1.3. The importance of the study:

Commissioning of an external beam therapy device should consist of acquiring all radiation

beam data required for treatment. This data is used to develop the Treatment Planning

Systems (TPS) for use in external beam radiation therapy.

The uncertainty in data measurements acquired will be evaluated. These are necessary to be

estimated since the success of treatment depends on small values of uncertainty.

1.4. Hypothesis:

It is expected that the results of the central axis determined by this study will not differ

significantly from the one obtained by the hospital physicists and the published one by

IAEA TRS-398 and PTW-FRIBURG protocols . The difference in the determination of the

dose to water during calibration of the beam is estimated to be 2.5 % from the reference

value obtained from calibration laboratory [2].

2

1.5. Materials and Method:

The dose measurements in the field of dosimetry are divided into relative and absolute

dosimetry.

1.5.1. Relative dose measurements:

It is generally performed in a large water phantom and for this purpose a suitable ionization

chamber is used in the water phantom. In this way dose values are determined at many

points under varying treatment conditions, such as field size, source skin distance (SSD),

and the presence of beam modifiers[3].

When multiple fields are used for treatment of a particular tumor inside the patient,

isocentric, source axis distance (SAD) setups are often used. In contrast to source skin

distance SSD setups, which rely on percentage depth dose (PDD) distributions, SAD setups

rely on other functions, such as tissue-air ratios and tissue-phantom ratios, for dosimetric

calculations. So acquiring a group of measurements is divided to two groups the first group

is the central axis depth doses in water (SSD SET-UP) and the second group is the central

axis depth doses in water (SAD SET-UP).

Concerning the central axis depth doses in water (SSD SET-Up) the measuring sessions of

the PDD is to place the center of the ion chamber in the water phantom over the range of

depth of 0 to 25 cm in increments not larger than 1 cm and over the range of field sizes rf .

from 4cm x 4 cm to 40cm x 40cm in increments not larger than 5 cm for the side. The

central axis dose distributions inside the phantom are usually normalized to depth of

maximum D max = 100%.

The measurements of the central axis depth doses in water for (SAD SET-UP) consist of

the tissue-air ratio (TAR), tissue-phantom ratio (TPR), tissue maximum ratio (TMR),

3

output factor, blocking tray factor, and beam profiles.

Concerning the measuring sessions of the (TAR) (suggested by Johns in 1953), the ion

chamber should be placed in water phantom over the range of same depths and field size as

PDD measurements, in both water and air. The ratio between the absorbed dose in water at

the same point in free air is referred to as the (TAR). [7]

The measurements of the TPR at a point in a water phantom irradiated by a photon beam

can be done by divide the total absorbed dose at that point by the total absorbed dose at a

point or the beam axis at affixed reference depth, usually 5 cm or 10 cm. i

TMR is special case of TPR and may be determined by dividing the dose at a given point in

water phantom to the dose at the same point at the reference depth of maximum dose[7].

1.5.2 Absolute dose measurements:

Absolute dose measurements are also performed in a water phantom. For this purpose a

calibrated ionization chamber is used. The generally applied procedure for specification of

beam quality using IAEA TRS-398 protocol is to specify the tissue-phantom ratio,

TPR20,10. This is the ratio of the absorbed doses at depths of 20 cm and 10 cm in a water

phantom, measured with a constant source-chamber distance of 100 cm and a field size of

10x10 cm2 at the plane of the chamber [2]. -• J&

Determination of absorbed dose to water under the reference conditions is given' by pf'-

multiplying the corrected reading of the dosimeter with the calibration factor arid KQ go

which is the chamber-specific factor which corrects for the difference between the

reference beam quality go and the actual quality being used Q [2].

4

1.6. Quantities and Units:

The international System (SI) of units consists of, set of units for use in all branches of

science. The International System of Units, known as (SI) derives all the units in the

various technologies from the following seven base quantities and units: length (m), mass

(kg), time (second), electric current (Ampere), temperature (Kelvin), luminous intensity

(Candela), and amount of substance (mole) [9].

It is important to distinguish a quantity from a unit. The physical quantity is a phenomenon

capable of expression as the product of a number and a unit. While unit is a selected

reference sample of a quantity as follows:

Physical quantity = numerical value x some unit (1.1)

The international Commission on Radiation Units and Measures (ICRU) has published its

recommendation on the quantities and units to be used in the measurement of ionizing

radiations and activity. Only these quantities and unities will be discussed in this section.

These quantities and units can be classified into the following:

1.6.1. Radiometric quantities:

These quantities describe the radiation beam and they are divided into the

following:

•.-.•J*

1.6.1.1. The particles (photons) number, N. ^

It is the number of particles emitted, transformed, or received.

Unit of particles (photons) number is 1.

1.6.1.2 Radiant energy (excluding rest energy), R.

It's the energy of particles emitted, transformed, or received.

Unit of radiant energy is Joule.

5

1.6.1.3. Fluence or photon fluence, ^ .

It is the number of photons, dN ? (which represent monoenergetic beam) that pass a

cross-sectional area, "a. i.e.

o = dN_

da

number photon

area (1.2)

Unit of fluence is particles/m

1.6.1.4. Fluence rate or flux density, ̂ .

It is the number of photons that pass through unit area per unit time. i.e.

number of photon • JO r dt time x area (1.3)

where d<& is the increment of fluence in time interval dt _

Unit of fluence rate is particles/m2.s.

1.6.1.5. Energy Fluence, ^ .

The good way to describe the beam is energy fluence which is the sum of energies

of all particles, crossing unit area , " a . i.e.

¥ dN.hv

da

energy

area (1.4)

Unit of energy fluence is J.m"

1.6.1.6. Energy fluence rate or, energy flux density or Intensity

It is the energy ^ carried across unit area per unit time dt. j . e .

energy

Energy fluence rate

dif/

~!t time x area (1.5)

-2 _-l Unit of energy fluence rate is J.m" .s

6

.6.2. Interaction Coefficients

Interaction coefficients can be classified as the following:

1.6.2.1. Linear attenuation coefficient:

If we imagine monoenergetic photon beam is incident on an absorber of variable

thickness and detector has been placed at a fixed distance from this source. The

number of photons dN [s proportional to the number of incident photons ^ and to

the thickness of the absorber "X. Mathematically,

dNoc-Ndx (1.6)

.'. dN = -piNdx /i js

where ^ is called attenuation coefficient [4].

The above equation can be rewritten in the following form:

dl = -^ldx ( 1 8 )

where I is the intensity of the incident photons and it can be written instead of N.

Therefore,

While P is called linear attenuation coefficient if the thickness of the absorber x is

expressed as a length. p

Unit of linear attenuation coefficient is cm"1

7

1.6.2.2. Mass attenuation coefficient:

where P is the density. Mathematically,

H -dl 1

It is denoted by — KPJ

p I pdx

Unit of mass attenuation coefficient is cm /gm.

1.6.2.3. Atomic attenuation coefficient, "^

It is given by the following equation:

P No

(1.10)

(1.11)

where Z is the atomic number and ° is the number of electrons per gram and

0 is given by:

xr NA-Z M = 0 A (1.12)

N A where A is Avogadro's number and w is the atomic weight.

Unit of atomic attenuation coefficient is cm /atom.

1.6.2.4. Electronic attenuation coefficient, e^ .

It is given by the following equation:

M 1 P ^o (1.13)

2/ Unit of electronic attenuation coefficient is cm /electron.

1.6.2.5. Energy transfer coefficient, ^,r,

It is given by the following equation:

hv (1.14)

where tr is the average energy transferred into kinetic energy of charged particles

per interaction.

Unit of energy transfer coefficient is m .kg" .

1.6.2.6. Energy absorption coefficient, ^ab.

It is given by the following equation:

hv (1.15)

where ab is the average energy deposited by charged particles in the attenuator.

Unit of energy absorption coefficient is m2.kg"'.

e 1.6.2.7. Mass stopping power, "" .

The term stopping power refers to the energy loss by electrons per unit path length

of material. The total mass stopping power, ^ ^'"", of a material for charged •.;-.Jk

particles is defined by the ICRU as the quotient of dE by ^ , where ^ H s the

total energy lost by the particle in traversing a path length dl \n the material of

density ^\,i.e.,

isip)lol = x-d-§ P dl (1.16)

v Phot consists of two components of the mass collision stopping power,

9

v Plcoi ^ resulting from electron- electron interactions (atomic excitations and

ionisations) and the mass radiative stopping power, ̂ Phad ^ resulting from

electron-nucleus interactions (bremsstrahlung production), i.e.,

{Slp\ol={Slp)col + (5/pL ( L 1 7 )

v Pfcoi plays an important role in radiation dosimetry, since the dose D in the

medium may be expressed as:

D=y(s/Ph ( U 8 )

where 9 is the fluence electrons.

Unit of mass stopping power is MeV.cm2/g

1.6.2.8. Linear energy transfer, LET.

Linear energy transfer denoted by LET and it coefficient describes the quality of

radiation and is defined as the quotient of A by ^ a n d is denoted by Adhere

dF A is the energy lost by a charged particle due to soft and hard collisions in

traversing a distance "* minus the total kinetic energy of the charged particles

released with kinetic energies in excess of A.- > J* -*£

L^=dEA/dI (1.19) :

Unit of linear energy transfer coefficient is eV/cm.

w

1.6.2.9. Mean energy expended in gas per ion, e ,

The mean energy required to produce an ion pair in air is almost constant for all

10

W - 33 85 electron energies and has a value of mr ' eV /ion pair. If e is the electronic

W air

charge (=1.602x10"19 C), then e is the average energy absorbed per unit charge

W air

of ionisation produced. Since 1 eV = 1.6xl0'19 J, then e = 33.85 J.C"1.

Unit of mean energy is J.C"1.

1.6.2.10. Cross Section,0",

For a particular interaction produced by incident charged or uncharged particles, a

is the quotient of P by $> where P is the probability of the interaction for a single

target entity when subjected to the particle fluence 3>, thus

P a - —

<*> (1.20)

Otherwise the mass, electronic, and atomic coefficients which are measured in units

of area per gm, or area per electron or area per atom are often called cross sections.

i.e.

» (1.21)

where n is the number of particles of matter per unit volume and ^ is the attenuation

coefficient. V

A special unit of cross section is barn (where lbarn = 10"24 cm2/atom)

1.6.3.Dosimetric Quantities

Dosimetric quantities are intended to provide a physical measure at a point or in a

region of interest, which is correlated, with actual or potential effects of ionizing

radiation. These quantities of dosimetry are classified as the following [4]:

11

1.6.3.1. KERMA, K.

KERMA stands for kinetic energy released in the medium. It is the quotient of

dE!r by dm ? where dE,r is the sum of the initial kinetic energies of all charged

ionizing particles (electrons and positrons) librated by uncharged particles

(photons) in a material of mass dm \ e>

K = dEtr

dm (1.22)

Unit of kerma is J.kg*1 while the special unit is the Gy.

1.6.3.2. KERMA Rate, K.

It is the quotient of the kerma dK by the time ^ . i.e.

K = — (1.23)

dt

Unit of kerma rate is Gy/sec

1.6.3.3. Exposure, X.

It is the quotient of ~ by dm ? where ^ is the total charge of the ions of one

sign produced in air when all the electrons and positrons liberated or created by

photons in mass dm 0f a j r are completely stopped in air. i.e.

XJ-V (1.24) >

dm •

Old unit of the exposure is Roentgen (R).

1R =2.58xlO-4Ckg1 (1.25)

The SI unit of the exposure is C kg"1

12

1.6.3.4. Exposure Rate , X.

X = ~ (1.26)

where t is the time.

1.6.3.5. Energy deposit

It is the energy deposited in a single interaction and leads to disrupting the

energy bonds, which hold atoms together.

Unit of Energy deposit is Joule.

1.6.3.6. CEMA, C;

A new quantity "CEMA" which stands for converted energy per unit mass, has

been introduced for charged particles, paralleling "kerma" which means (kinetic

energy released per unit mass) for uncharged particles.

dF J dF The CEMA C is the quotient of c by " w , where c is the energy lost by

charged particles, except secondary electrons, in electronic collisions in a mass

d™ of a material, i.e.

dm (1.27)

Unit of CEMA is J.kg' where the special unit is gray (Gy).

1.6.3.7. Energy imparted, e.

The total amount of all energy deposited in matter, called the imparted energy. It

is given by the following equation:

e = K-Z«*+YiQ (1.28)

where '" is the radiation energy incident on the volume, i.e. the sum of the

13

energy (excluding rest energy) of all those charged and uncharged ionization

particles which enter the volume.

D

<"" is the radiation energy emerging from the volume, i.e. the sum of the energy

(excluding rest energy) of all those charged and uncharged ionization particles

which leave the volume.

2-^ is the sum of all charges and elementary particles in any interactions

which occur in the volume.

Unit of imparted energy is joule [10].

1.6.3.8. Specific energy (imparted, z)

Is the quotient of e by m , where s is the energy imparted to matter of mass m ,

thus

s z =—

m (1.29)

Unit of specific energy is J.ks_1.

The special name for the unit of specific energy is Gray (Gy)

1.6.3.9. Absorbed dose ,D .

It is the quotient of dE by dm ? where dE }s the mean energy imparted bv

ionizing radiation to material of mass dm [ 1 ]. .£•'".

' dE_

D=dm (1.30)

Unit of absorbed dose is Gray (Gy).

14

1.6.3.10. Absorbed dose rate , D.

It is the quotient of dD by dt ? where dD [s the absorbed dose and dt stands

for the time. i.e.

dD_

D= dt (1.31)

Unit of absorbed dose rate is Gy.s"1

1.7. KERMA

K K

KERMA (see section 1.6.3.1) can be divided into two parts co/ and rad as the following

relation:

= co! + rad (1.32)

V

where co' collision kerma is that part of kerma that leads to the production of electrons

that dissipate their energy as ionisation in or near the electron tracks in the medium, and is

the result of Coulomb-force interactions with atomic electrons.

Thus, the collision kerma is the expectation value of the net energy transferred to charged

particles per unit mass at the point of interest, excluding both the radiative energy loss and

energy passed from one charged particle to another [1].

Radiative kerma Kra [s the second part of kerma that leads to the production of

bremsstrahlung as the secondary charged particles are decelerated in the medium. It is the

result of Coulomb-force interactions between the charged particle and the atomic nuclei [1].

1.8. Energy fluence and kerma (photons)

Energy can be transferred to electron by photons through collision interactions (soft

collisions, hard collisions) and through radiative interactions (bremsstrahlung, electron-

positron annihilation).

15

IS

Therefore, the total kerma is usually divided into two components: the collision kerma coi

is and the radiative kerma rad as mentioned before at 1.7.

For mono-energetic photons the collision kerma at a point in a medium is related to the

energy fluence ¥ in the medium by the following equation:

Kcol = H^) P (1.33)

where P is the mass-energy transfer coefficient of the medium for the given i

monoenergetic photon beam. For poly-energetic beams, similarly as above, spectrum-

averaged mass-energy transfer coefficients can be used in conjunction with total energy

fluence to obtain the total kerma [1].

1.9. Collision kerma and exposure

Multiplying the collision kerma by (e I Wair), the number of coulombs of charge created

per joule of energy deposited, gives the exposure:

X=(KMl)J-f-) 0-34)

1.10. Absorbed dose and exposure

The absorbed dose has been defined to describe the quantity of radiation for all^pes

of ionizing radiation, including charged and uncharged particles; on all materials; for all

energies. However, the exposure, which can be defined as the ability of an X or gamma

rays to produce ionization in air, have the different meaning from absorbed dose, where the

exposure applies only to x-rays and gamma rays and it is a measure of ionization in air

only, and cannot be used for photon energies above about 3 MeV.

16

The reason that we cannot use the concept exposure to the photon energy above 3 MeV

arises from understanding of the definition of exposure. Exposure can be defined as the

ratio of the absolute value of total charge of ions of one sign ( ^ ) produced in air when all

electrons liberated by photons of x-ray or gamma rays in a volume element of air having a

mass ^m are completely stopped in air. However, the increase of energy more than 3 MeV

makes the detection of all total charge of ions of one sign ( ^ ) produced in air impossible.

Also all measurements using the concept of exposure are done in air while some

radiological practice is required to do these measurements in other medium like water and

to use other charged or uncharged particles as electrons, protons, and neutrons.

Moreover, the only truly satisfactory way to measure exposure can be done by one

instrument (Standard Air Chamber) but to measure energies above 3 MeV another type of

ion chambers is required.

On other hand, the quantity absorbed dose, D, can be defined as quotient of dE by £m^

where "& is the mean energy imparted by ionizing radiation to material of mass dm [1].

dm (1-35)

ft

The old unit of absorbed dose is Rad but the SI unit for absorbed dose is the Gray (J.kg7 ) . The relation between absorbed dose and exposure, X, is readily accomplished under

condition of electron equilibrium (more details about electron equilibrium can be found in

K section 1.11). Therefore the dose is equal to the coi in the point of interaction. Dose to air

Dair under these conditions is given by the following equation:

17

« (1.36)

where X is the exposure with the SI unit C.kg" of air.

lR = 2.58xlO-4C.Kg-l.

W_ e is the mean energy and it is equal to 33.85 J.C'1. Therefore, the above equation can be

expressed by their units as the following:

Dair(J.kg-x) = XiRyi.SZxl^iCkg-'yiZ.ZSiJ.C'1) (1.37)

:.Da„{rad) = Q.mA.X(R) & (1.38)

where vaa> is the old unit of dose and equal to 10"2 J.kg"1, while 0.876 is called exposure

to dose conversion factor or Roentgen to Rad conversion factor.

Equation number (1.37) is considered one of the important principle, for the quality of

radiation at points within the scattering medium because the conversion factor of exposure

to absorbed dose depends on the photon energy [7].

1.11. Charged particle equilibrium

The relation between dose and kerma can be described by the ways of the interaction. Fig

1.1 illustrates the initial interaction, which is described by kerma, and the other interaction

described by the absorbed dose. Moreover, the interaction starts as shown in Fig 1.1 at

point (a) by transfer of some of the photon energy to an electron given as kinetic energy

(K.E) and this K.E. released in the medium is kerma. ^v is scattered from (a) and the

electron in turn gives up it as energy mostly in small collisions along its track (b). The

transfer of energy at (b) is known as absorbed dose. The ratio of dose and collision kerma is

18

often denoted as:

fl = — * - (1.39)

where, P is the status of escaping of radiative photons.

Figure 1.2 also illustrates the relationship between collision kerma and absorbed dose but

under build-up conditions, under conditions of charged particle equilibrium (CPE) in part

(a) and under conditions of transient charged particle equilibrium (TCPE) in part (b).

Charge particle equilibrium is established in a volume when the energy carried into the

volume by the charged particle is balanced by the energy carried out of the volume by the

charged particle. When charged particle equilibrium is established, kerma is equal to dose.

Charged particle equilibrium is not established for thin materials irradiated by gamma rays.

The range of secondary electrons produced is larger than the material dimensions, so that

much of the electron energy is lost simply by the electrons leaking out of the material [11].

On other hand the dose region between the surface (depth z = 0) and depth z = zmax in

megavoltage photon beams is referred to as the dose build-up region and in the region

immediately beneath the patient surface, the condition of charged particle equilibrium

(CPE) does not exist and the absorbed dose is much smaller than the collision kerma.

However, as the depth d increases, CPE is eventually reached at z — zmax where ;ZJ*S

approximately equal to the range of secondary charged particles and the dose becomes

comparable to the collision kerma.

Beyond zmax both the dose and collision kerma decrease because of the photon

attenuation in the patient, resulting in a transient rather than true CPE.

19

</5 vo

£ .•ES

a .

e> <u c: o> <u

.S o>

a :

<n E

"5 =3 CU O -a> <u

.a <q a>

DC

/c

ID

f build-up / region

<a)

P - 1

> charged particle equilibrium (CPE)

Depth in medium

transient charged particle equi l ibr ium (TCPE)

*-"•• D e p t h in m e d i t i m

Fig (1.1) Collision kerma and absorbed dose as a function of depth in a medium, irradiated

by a high-energy photon beaml. ?fr-

Fig. 1.1 (a) graph illustrates the increasing of the absorbed dose with the depth and showing

how the electron equilibrium is achieved between dose and kerma. The portion of the

medium from the surface to depth Zmax is called build-up region and the portion beyond it

is loosely called electronic equilibrium where the two curves of collision kerma and dose

m e e t ( ^ < 1 ) .

20

In Fig 1.1 (b) is shown a condition of attenuation of the photon beam that leads to not

attaining the electronic equilibrium. The kerma will decrease continually but the absorbed

dose will first increase and then decrease [1].

1.12. Interaction of ionizing radiation with matter

1.12.1. Particle interactions

The three primary types of radiation from nuclei, alpha, beta, and gamma rays, all

interact with matter by giving up all or part of their energy to electrons in the material they

are passing through. After the energy is lost in the interaction, the radiation has reduced

energy and may leave the material or interact further with it.

1.12.2. X- and Gamma Ray Interactions

All indirect photon interactions fall into one of the following four categories:

Bremsstrahlung (continuous x rays), characteristic x-rays, gamma rays, and annihilation

radiation [1]. Photons may undergo various possible interactions with atoms, the type of

photon interactions can be classified as the following:

Photoelectric (PE): all of the incident photon energy is transferred to an electron, which is

ejected from the atom. The kinetic energy of the ejected photo-electron (Ee) is equal to the

incident photon energy (Eo) minus the binding energy of the orbital electron (Eb). i.e.

Ee=E0-Eb (1.40) '"£

In order for photoelectric absorption to occur, the incident photon energy must be greater

than or equal to the binding energy of the electron that is ejected. The ejected electron is

most likely one whose binding energy is close to, but less than, the incident photon energy.

For example, for photons whose energies exceed the K-shell binding energy, photoelectric

interactions with K-shell electrons are most probable. Following a photoelectric interaction

21

the atom is ionized, with an inner shell electron vacancy. An electron from a shell with a

lower binding energy will fill this vacancy. This creates another vacancy, which, in turn, is

filled by an electron from an even lower binding energy shell. Thus an electron cascade

from outer to inner shells occurs. The difference in binding energy is released as either

characteristic x-rays or auger electrons. The probability of characteristic x-ray emission

decreases as the atomic number of the absorber decreases.

The probability of photoelectric absorption per unit mass is approximately proportional

to Z3 / £ 3 , where Z is the atomic number and E is incident photon energy.

Compton Scattering (C): also known as incoherent scattering, occurs when the incident x-

ray photon ejects an electron from an atom and an x-ray photon of lower energy is scattered

from the atom. Relativistic energy and momentum are conserved in this process and the

scattered x-ray photon has less energy and therefore greater wavelength than the incident

photon. Compton scattering is important for low atomic number specimens. At energies of

100 keV -10 MeV the absorption of radiation is mainly due to the Compton effect.

Pair Production (PP): can occur when the x-ray photon energy is greater than 1.02 MeV,

when an electron and positron are created. Positrons are very short lived and disappear

(positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair

production is of particular importance when high-energy photons pass through materialf^f

a high atomic number.

Thomson scattering (R): also known as Rayleigh, coherent, or classical scattering, occurs

when the x-ray photon interacts with the whole atom so that the photon is scattered with no

change in internal energy to the scattering atom, nor to the x-ray photon. Thomson

scattering is never more than a minor contributor to the absorption coefficient. The

22

scattering occurs without the loss of energy.

Photodisintegration (PD): is the process by which the x-ray photon is captured by the

nucleus of the atom with the ejection of a particle from the nucleus when all the energy of

the x-ray is given to the nucleus. Because of the enormously high energies involved, this

process may be neglected for the energies of x-rays used in radiography.

The linear attenuation coefficient is given by // = —.— as being mentioned (see section

/ dx

1.6.2.1). The linear attenuation coefficient is considered the sum of the individual linear

attenuation coefficients for each type of interaction:

r1 M Rayleigh ' M Photoelectric effect r^Compton Scatter r1 Pair production / i A-I\

1.12.3. Interaction of electrons

When electron traverses matter, it loses kinetic energy mainly through collisions or

scattering. Energy losses are described by stopping power; scattering is described by

scattering power.

The incident electrons interact with orbital electrons or nucleus of the atom through elastic

or inelastic collisions. When the electron is deflected from its original path but without any

energy loss we call this elastic collisions. While in an inelastic collision, the electron isj»

deflected from its original path and some of its energy is transferred to an orbital electron Or

emitted in the form of bremsstrahlung radiation. Therefore, we can classify the interaction

of electrons with matter in two types: electron-electron interactions as explained in the

above and electron-nucleus interactions.

Electron-nucleus interactions occur when the incident electron interacts with the nuclei of

the absorber atom resulting in electron scattering and energy loss of the electron through

23

production of x-ray photons (bremstrahlung). These types of energy losses are known as

radiative stopping powers while collision-stopping powers describe the energy losses in the

electron-electron interactions. The total stopping power is the summation of collision and

radiative stopping power, which was been described in the section 1.6.2.7. To compare the

stopping powers and linear energy transfer (LET) we can observe that stopping power been

focuses on energy losses, while LET focuses on the linear rate of energy absorbed

absorbing medium as the electron traverses the medium. Linear energy transfer can be

defined as the average energy locally imparted to the absorbing medium by an electron of

specified energy in traversing a given distance in the medium.

1.13. Beam quality

The penetrating ability of the radiation is often described as the quality of the radiation and

denoted by Q. It has been recommended that the quality of the clinical beams in the

superficial and orthovoltage range (below megavoltage range) be specified by the HVL and

the kVp in preference to the HVL alone. On the other hand the determination of beam

quality in megavoltage range can be specified by the peak energy and rarely by the HVL.

The most practical method of determining the megavoltage beam energy is by measuring

percent depth dose distribution, tissue-air ratios, or tissue maximum ratios (more detail's. j»

about these parameters will be discussed in chapter 3) and will be compared with published'"'

data such as those from the Hospital Physicist's Association [4].

The recent protocols are based on quantities that are related to beam penetration into water,

such as the tissue-phantom ratio (TPR) or the percentage depth dose (PDD).

For high-energy photons produced by clinical Co-60 the beam quality Q is specified by the

tissue-phantom ratio, TPR20,10. The parameter TPR20,10 is defined as the ratio of doses

24

on the beam central axis at depths of 20 cm and 10 cm in water obtained with a constant

source-detector distance of 100 cm and a field size of 10 x 10 cm at the position of the

detector.

TPR20,10 is a measure of the effective attenuation coefficient describing the approximately

exponential decrease of a photon depth dose curve beyond the depth of maximum dose

Zmax, and, more importantly, it is independent of electron contamination of the incident

photon beam [1]. The TPR20,10 can be related to measured PDD20,10 using the following

relationship [2]:

TPR20,10 =1.2661 PDD20,10 - 0.0595 (1.41)

where PDD20,10 is the ratio of percentage depth doses at depths of 20 cm and 10 cm for a

field of 10 x 10 cm2 defined at the water phantom surface with an SSD of 100 cm.

25

CHAPTERTWO

RELATIVE AND ABSOLUTE DOSIMETRY

2.1. Introduction

As mentioned before dosimetry is the science of measurement of radiation and it is divided

into two types, relative and absolute dosimetry. Relative dose measurements include

determination of percentage depth dose, output factors, tissue maximum ratio, tissue

phantom ratio, wedge transmission factor, tray transmission factor and beam profiles.

While absolute measurements include beam quality specification, calibration of ionisation

chamber and determination of absorbed dose (most of these measurements will be

explained in the following sections).

Relative dose measurements are generally performed in a large water phantom and for this

purpose a special type of ionization chamber will be used (e.g. Farmer Type Chamber

FC65-P or Cylindrical Chamber CC13)

2.2. Fundamental Absorbed Dose -Methods of Measurement

There are three methods for the fundamental determination of absorbed dose to water,

calorimetry, chemical dosimetry, and ionization dosimetry. These systems are accurate

enough to be classified as primary standards of absorbed dose to water ,i.e. ,graphite

calorimetry, water absorbed-dose calorimetry, Fricke dosimetry ,water/Fricke calorimetry?

and ionization dosimetry. Ionisation'dosimetry based on calibrated ionization chambers

does not belong to this category.

26

2.2.1. Calorimetry

The absorbed dose at a point within an irradiated medium can be determined by the

calorimetric methods. Absorbed-dose calorimeters use graphite as the detector medium and

as phantom material.

2.2.2. Principles of absorbed-dose calorimetry

The absorbed dose, D, at a given point in a given medium, measured by calorimetry in an

ideal condition, i.e. at the given point in a thermally isolated mass element dm, is given by

dm dm

where dE is the mean energy imparted by ionisaing radiation to matter of mass dm, dmh is

the energy which appears as heat and dEs is the heat defect .If there is no change of state,

DJdm = CpAT (2.2)

where Cp is the specific heat capacity of the medium at constant pressure, and AT is the

increase in temperature at the point of measurement .The expression for the absorbed

dosed may then be

D = CpATKh (2.3)

where Kh is the heat defect correction factor. The response of calorimeter is given by the

ratio AT/D and may be calculated using equation (2.3).

Different types of absorbed- dose calorimeters are use at present in photon dosimetry.

1. Absorbed- dose calorimeters made of solid medium. They consist of a thermally

isolated central absorber in which the heat resulting from the absorption of radiation

energy is measured. The core is surrounded by thermally isolated shells to provide

thermal equilibrium and temperature control. Graphite calorimeters are most

common, taking advantage of low atomic number (i.e. close to the mean atomic

27

number of water).

2. Water absorbed-dose calorimeters. An obvious advantage is the use of water, which

is the reference medium. Water absorbed-dose calorimeters do not have problems

associated with isolating vacuum gaps. The main problem is still the need to know

the heat defect in water due to exothermic or endothermic radio-chemical reaction.

3. Absorbed-dose calorimeters with the solid detector system in a water phantom.

With this design, the heat defect in the detector probe material must be known, and in

addition, the absorbed dose to the probe material has to be converted into the absorbed

dose to water.

2.2.3. Chemical Dosimetry

Chemical dosimetry is based on a quantitative determination of the chemical change

produced after the deposition of energy in matter by ionizing radiation. Although several

systems based on reactions in solid, liquids and gases have been studied only those based

on aqueous solutions are of importance as absorbed-dose-to-water primary standards. An

aqueous chemical dosimeter consists of water as solvent in which most of the absorbed

energy is deposited, and a solute which reacts with the radiation -induced species formed in

the solvent to produce the observed chemical change .The reactions involved in this process

are usually dependent on both the physical characteristic's of the radiation beam (type and

energy of radiation...) and on the chemical characteristic's of the solution (pH value, solute-

concentration...). For the use as an absorbed-dose standard, it is necessary to know the

response of the system to the absorption of radiation energy in terms of the radiation

chemical yield G(x), defined in ICRU. The radiation chemical yield, G(x)is the quotient of

n(x) by E, where n(x) is the mean amount of substance of a specified entity, x, which is

28

produced, destroyed or changed by the mean energy imparted, E, to the matter. The unit of

G(x) is mol/J.

2.2.4. Ionization Dosimetry

To obtain the absorbed dose to water in a water phantom using ionization

dosimetry, the measurements can be carried out directly in a water phantom, using a water

-tight graphite -cavity ionization chamber with an accurately known volume.

Alternatively, the measurement can be made in a graphite phantom followed by the transfer

of absorbed dose from graphite to water. For Co-60 gamma radiation, the first approach

has been chosen by the BIPM as the absorbed -dose -to-water primary standard, whereas

the latter serves as a validation system.

2.2.5. Ionization measurements in a water phantom

The conversion is made of the ionization measured in the cavity of the thick -walled

graphite chamber to absorbed dose to water in an unperturbed water phantom .The mean

specific energy (E/m) imparted to the air in an ideal cavity is given by

(E/m) = (q/m\W/e) (2.4)

where q is,charge created in the mass m of the cavity, W is mean energy required to

replace the cavity by graphite. The absorbed dose to graphite can be calculated using the

Bragg-Gray relation as r»

Dgr={E/m)Sgr,air. (2.5)

where Sgr, air is the ratio of the stopping powers of the graphite and air averaged over the

secondary electron spectrum produced by the Co-60 gamma radiation . From the graphite

kerma in graphite to the water, the relation between the corresponding absorbed dose can

be deduced.

29

Dw = — \Sgr,air 1a ^

JwBWigr. (2.6)

where r*cn

V P

is the ratio of mass energy -absorption coefficients averaged over the photon V P J

energy spectrum ,YWgr is the corresponding ratio of the photon energy fiuence at the

point of interest and BWgr is the quotient of the ratios of absorbed dose to collision kerma.

The subscripts w and gr refer to water and graphite, respectively.

2.2.6. Ionization measurement in a graphite phantom

For measurement in a graphite phantom, the walls and the electron of the ionization

chamber should be of same material as the phantom .The chamber must be electrically

insulated from the remainder of the phantom, introducing a foreign material into the

system. With suitable design, the effect may be made negligible, or it can be corrected for.

Markus and Kasten used a graphite double - extrapolation ionization chamber as an

absorbed -dosed standard for high energy photon and electron beams .The perturbation

effect due to the size of the ionization chamber volume can be experimentally studied with

well- defined sensitive volumes that can be varied both in height and radius.

This method requires, as subsequent step, the transfer of the absorbed -dose determination

in a graphite phantom to that in water phantom. •••••*

2.2.7. Simplified theory of thermoluminescent dosimetry

The chemical and physical theory of TLD is not exactly known, but simple models have

been proposed to explain the phenomenon qualitatively.

In an individual atom, electrons occupy discrete energy levels. In a crystal lattice, on the

other hand, electronic energy levels are perturbed by mutual interactions between atoms

30

and gives rise to energy bands: the allowed energy bands and the forbidden energy bands.

In addition, the presence of impurities in the crystal create energy traps in the forbidden

energy region, providing metastable states for the electrons. When the material is irradiated,

some of the electrons in the valence band (ground state) receive sufficient energy to be

raised to the conduction band. The vacancy thus created in the valence band is called a

positive hole. The electron and the hole move independently through their respective bands

until they recombine (electron returning to the ground state) or until they fall into a trap

(metastable state) .If there is instantaneous emission of light owing to these transitions, the

phenomenon is called fluorescence. If an electron in the trap requires energy to get out of

the trap and fall to the valence band, the emission of light in this case is called

phosphorescence (delayed fluorescence) .If phosphorescence at room temperature is very

slow, but can be speeded up significantly with a moderate amount of heating (~ 300° C),

the phenomenon is called thermoluminescent.

A polt of thermoluminescence against temperature is called a glow curve. As the

temperature of the TL material exposed to radiation is increased, the probability of

releasing trapped electrons increases. The light emitted (TL) first increases, reaches a

maximum value, and falls again to zero. Because most phosphors contain a number of traps

at various energy levels in the forbidden band, the glow curve may consist of a number of - '

glow peaks .The different peaks correspond to different trapped energy levels.

2.3. Percentage Depth Dose

The Percentage depth dose, which is denoted by PDD 5 is usually normalized to max =

2 100% at the depth of dose maximum max. In this way, dose values are determined at many

points under varying depths and field sizes (F.S). Fig 2.1 shows two beams of radiation

31

impinging normally on a water phantom. Farmer Type Chamber or any suitable chamber

capable of measuring the absorbed dose is placed at the surface where it measures p and

then at a depth z where it measures Q. The ratio of Q to p is called the PDD.

Do PDD = —2-jclOO

DB (2.7)

(a)

Source

/ Y

SSD

Source

(b)

EiOsrrsSr5B i _• 1**5

SAD

Aref 7^ ./**

-t*ffi S'"VHPj" 4. iWJ, i> ' , .9ht l ' . ji'-'

*•':•?:/; .: •• Q f : * « N l * * *" '

;f-;t« - 4 r ^ t . 4 ;# -i3»£

AMC M TfUb , . i J f t t lJ

F/g (2.7^ Diagram illustrating the two beams of radiation impinging the water phantom

32

2.4. Air temperature, pressure and humidity effects:

In most clinical situations the measurement conditions do not match the reference

conditions used in the standard laboratory. This may affect the response of the dosimeter

and then it is necessary to differentiate between the reference conditions used in the

standards laboratory and the clinical measurement conditions. So a correction factor should

be applied to the temperature and pressure to convert the cavity air mass to the reference

conditions. This correction can be applied by equation:

K = (273.2 + T)P0 T-P (273.2 + T0)P ( 2 8 )

where P and T are the cavity air pressure and temperature at the time of the

P T P T

measurements, and ° and ° are the reference values (generally ° = 101.3 kPa and ° =

20° C).

No corrections for humidity are needed if the calibration factor was referred to a relative

humidity of 50% and is used in a relative humidity between 20% and 80%. If the

calibration factor is referred to dry air a correction factor should be applied; for Co-60

calibrations Kh = 0.997.

2.5. Chamber polarity effects: polarity correction factor ( Po') , ^

Ionization chambers should be designed so that the effect on the response of reversing the

polarity of the voltage applied between the electrodes is negligible.

The polarity effect depends on the measuring depth of the chamber in the phantom and the

length of cable in the field and may even change sign between small and large depths. The

polarity effect generally increases with decreasing energy.

The effect on the chamber reading of using polarizing potentials of opposite polarity for

33

each used beam quality ^ can be accounted for using the polarity correction factor PoS.

i.e

KPol ~ t MA + \M

2M (2.9)

where + and - are the electrometer readings obtained at positive and negative polarity,

respectively, and Ad is the electrometer reading obtained with the polarity used routinely

(positive or negative). The readings + and - should be made with care, ensuring that

the chamber reading is stable following any change in polarity (our chambers can take up to

10 minutes to stabilize for one and the other about 20 min).

is 2.6. Chamber voltage effects: recombination correction factor ( s )

Recombination is one of the phenomena that can happen during the motion of electrons and

ions in gases. This phenomenon can be defined as the recombining of the negative electron

and positive ion when they are found very close together.

As the voltage difference between the electrodes of an ion chamber increases, the ionization

current increases at first almost linearity and later more slowly.

Ion recombination is usually small for continuous radiation sources, but can be significantly ' • ' . : - , J *

higher for the pulsed radiation emitted from a linear accelerator and should be measured.

The incomplete collection of charge in an ionization chamber cavity, due to the

recombination of ions requires the use of a correction factor 5 .

Also the correction for ion recombination may be derived using the 'half voltage technique'

(Boag and Currant, 1980). Then if the chamber reading decreases by X % when the

polarizing voltage V is reduced to V/2, then the correction to be applied to the chamber

34

reading with the normal polarizing voltage is + X %. An alternative method was used for

the 'two voltage technique' (Weinhous and Meli 1984).

According to the recommendation of the IAEA TRS - 398 the two voltage method is

recommended to derive a correction factor for recombination rather than Boag theory

because it cannot account for chamber-to-chamber variations within a given chamber type.

In addition, a slight movement of the central electrode in cylindrical chambers might

invalidate the application of Boag's theory.

The two-voltage method assumes a linear dependence of M o n V ^ d u s e s m e measured

values of the collected charges ' and 2 at the polarizing voltages ' and 2,

respectively, measured at the same irradiation conditions. ' is the normal operating

n voltage and 2 is a lower voltage; the ratio 2 should ideally be equal to or larger than 3.

K V The recombination correction factor s at the normal operating voltage ' is obtained

from the following equation [2]. Ks = — (2.10) [V2j

(VIs

[V2,

2

2

-1

[M2)

2.7. Absorbed dose

The general formalism used by IAEA TRS-398 in the determination the adsorbed dose is as

follows: DV^NWKQQ. (2.11)

where Mq is the reading of the dosimeter under the reference conditions used in the

standard laboratory and this reading is corrected by the following equation:

35

Mq = M^K.K^K^ (2.12)

\A K K K K where 'is uncorrected dosimeter reading and r , /" s' Po" e/ecare temperature -

pressure, ion recombination, polarity, electrometer calibration factors, respectively.

D,W,Q0 j s ^g calibration factor in terms of absorbed dose to water of the dosimeter

obtained from a standard laboratory through calibration certificate of the dosimeter.

Q'Q' is factor to correct the difference between the response of an ionization chamber in

the reference beam quality r used for calibrating the chamber and in the actual used beam

quality, ^.

The reference conditions of the determination of the absorbed dose to water in high-energy

photon beams is as follows:

1. Field size 10 x 10 cm2,

2. Cylindrical ion chamber,

3. Water as phantom material,

4. SSD= 100 cm,

5. The reference point of chamber is on the central axis at the center of the

cavity volume,

7 6. The measurement depth ref for TPR20,10 < 0.7, 10 g.cm"2 (or 5 g.cm"2)'

forTPR20,10 ^0 .7 , 10 g.cm

36

The general formalism used in PTW- FREIBUFG protocol in the determination of the

absorbed dose Dw is as follows:

Dw = Kq

where Kq: radiation quality correction ,

N ̂ : calibration factor,

K p: check reading during calibration,

Km : mean value of check reading at use

M : display reading (in Gy)

p

\KmJ N^M (2-13)

37

CHAPTERTHREE

MATERIALS AND METHODS

3.1. Introduction

Measurements were performed at Cobalt-60 teletherapy unit located in INMO radiotherapy

centre, Sudan (CIRUS -Bio international made in FRANCE atl998). In this chapter the,

materials and methods used for absorbed dose measurements are presented.

3.2. Equipment

3.2.1. Dosimetry System (Ionization chambers and the electrometer)

The ion chamber used for measurements was a thimble therapy level chamber model/type

W-30001 volume 0.6 cc. (PTW, Freiburg, Germany) .The maximum polarizing voltage use

was + 400 Volt. The chamber is a water-proof chamber, with serial number 2117. The

chamber was calibrated at the IAEA Dosimetry Laboratory from 4-17 Mar 1999 .The

applications of this chamber include: absolute dosimetry of photon and electron beams in

radiotherapy, measurements in air, solid or water phantoms and ionization chamber for all

routine applications. .The electrometer from PTW - FREIBURG (Type/Ser-No. 10002-

20326) is a very sophisticated and accurate measuring device for dose and dose rate

measurements in radiation therapy. The ionization chambers and the dose electrometer •:-.JS»

(Type/Ser.-No W-30001/2117) were calibrated together, then the combined calibration^ i-V' .

factor was typically given in units of Gy/nC and no separate electrometer calibration factor

K elec was required. This electrometer has the ability to store all correction factors required

in the measurements and then compensate the corrected reading. This dosimetry system has

been calibrated at IAEA dosimetry laboratory located at Siebersdorf near Vienna by

substitution method using the IAEA reference standard chamber NE-2561/NPL(#321). The

38

IAEA standard had been calibrated at BIPM in June 1999 for Co-60 gamma radiation and

X-ray qualities recommended by the CCEMRI. The ionization chamber has absorbed dose

to water (NDW) calibration factor of 52.2mGy/nC for Co-60 radiation.

3.2.2. Phantoms

3.2.2.1. Reference water phantom

Dose measurements were performed in Scanditronix WellhQfer WP34 water phantom

designed for absolute dose measurements in radiation therapy beams with vertical beam

incidence (Scanditronix WellhOfer, Germany). Furthermore it is suitable for the calibration

of ionization chambers used in radiation therapy [15]. The phantom material is PMMA and

the wall thickness of entrance window is 4 mm. The phantom has external dimensions 41 x

32.6 x 32 cm3 and internal dimensions 30 x 30 x 30 cm3. The net weight of this phantom is

10 kg. The ruler in the upper frame of this phantom has been used for the determination the

reference depth.

3.3. Absolute measurements

3.3.1. Absorbed dose to water measurements

3.3.1.1. Stability check and response characteristics

Prior to any dosimetric measurements, radioactive stability check source measurements

were performed. This is so as to verify the response characteristics of the dosimetry

systems. The checks of the stability of cylindrical ionization chambers were performed

using radioactive (90Sr) check source.

39

Dose measurements

The cobalt-60 telerthrapy unit was operated for at least two hours for warming, and

mechanical and radiation safety checks were performed thereafter. The dosimetry system

composed of ion chamber and electrometer where connected and positioned for dose

measurements. Thermometer and a barometer were also positioned in the treatment room

for temperature and pressure measurements. Ionization chamber was connected to the

measuring electrometer for about a period of 30 minutes for warm-up. The chamber was

inserted in Scanditronix WellhOfer WP34 water phantom at a reference depth of 5 cm as

recommended in dosimetry protocols. Absorbed dose to water was measured for source to

detector distance of 80 cm, field size 10 cm x 10 cm, reference depth of 5 cm in water.

3.3.1.2. Absorbed dose to water calculations

The absorbed dose to water (Dw) was calculated by the following equation [2]:

A. =MchNDw (3.1)

where N DW : is the calibration factor,

M ch is the electrometer reading corrected for the polarity effect, ion recombination and air

density (temperature and pressure ) by this equation:

Mar M xKrpxK P0LxKsxKelec. (3.2)

where M is uncorrected electrometer reading. :'

KTP •. is temperature and pressure correction factor

KPOL : is polarity correction factor

Ks: is ion recombination correction factor

K. : is the electrometer correction factor

40

3.3.1.3. Determination of the polarity correction factor

The water phantom WP34 was filled with water and FC-65-P chamber was placed in the

specific holder in the phantom. The field size was 1 0 x 1 0 cm2 at the position of the

reference point of the chamber at SSD of 80 cm, reference depth of 5 cm .

Readings were obtained by using polarisation voltage of +400 volt and it was then changed

to -400 volt. The average of the readings of the positive polarity of the dosimeter was

denoted by + and the averages of the reading of the negative polarity of the dosimeter

was denoted by - . The values of + and - were substituted in equation (3.2) i.e.

\M\ + \M I KmL=l *' ' "' (3-3)

where KPOL stand for the polarity correction factor,

M is the electrometer reading obtained with the polarity used routinely .

3.3.1.4. Determination of the ion recombination factor

The reference water phantom with FC-65-P chamber has been used for the determination of

the ion recombination factor. Same previous settings of the polarity correction factor were

used instead of using lower voltage during measurements. Readings were obtained using

polarisation voltage of+400 volt and one at lower voltage of+100 volt.

The average of the reading at the + 400 volt is denoted by ' and the average of the

readings at the + 100 volt is denoted 2.

The values of ' and 2 are inserted in the following equation:

K _ (FT) -1 (3.4) s /yiY (M]

V2) I M2

41

3.3.1.5. Determination of the temperature and pressure correction factor

Due to change of temperature and pressure during the measurements compared with the

reference conditions, the correction factor was obtained to convert the cavity air mass using

the following equation:

(273.2 + T)£ TP (273.2 + T0) P

where P and T are the cavity air pressure and temperature at the time of the measurements.

The reference values for pressure and temperature are given below.

i P 0 =101.3 kPa (reference value)

T 0 = 20° C (reference value)

The reference water phantom was accurately positioned in the treatment room and left there

for at least half hour (the recommendation is to leave the dosimetry tools at the room for at

least 24 h, with the water stored in water reservoir inside the treatment room) in order to

reach temperature stability. The absorbed dose to water was calculated using FC-65-P

chamber at SSD 80 cm, field size 10x10 cm2, reference depth of 5 cm and mode (Current

dt) range low 230 pA. Thus were have:

DW,Q ~ MQNDJVQKQQQ Q£\ ,. j

where, the corrected reading of the dosimeter was multiplied by the calibration factor'

ND-W>Q and KQQ. values.

42

3.4. Relative measurement

3.4.1. Beam quality specification

The definition of beam quality has been discussed in the previous chapter. To measure this

quantity the reference water phantom and FC-65-P chamber were used (either plane-

parallel chamber can be used according to the recommendation of the IAEA) at SSD 80 cm,

depth 5 cm and field size 10x10cm .

43

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1. Introduction

This study was done at INMO Jazera State where a Co-60 machine which gives direct

photon beam to the patient was used. A water phantom was used to determine the absorbed

dose by using two different protocols. The results were then compared to the a similar

study done in Academic Teaching Hospital of the University of Cologne, Germany using

DIN 6800-2 (2000) protocol;

The results and discussion of measurements at the reference depth (equivalent thickness) of

the phantom window are shown in section 4.2, together with the results of calibration of the

ion chamber and the correction factors for polarity, ion recombination, temperature and

pressure. Also the results and discussion of determination of the absorbed dose to water are

presented along with the results of specification of the beam quality in the same section,

and presented in Table 1. The measurements which describe the absorbed dose at reference

depth (5 cm) and at maximum depth, using PTW-protocol of Germany are also presented.

4.2. Absolute measurements

4.2.1. Determination of absorbed dose to water:

Absorbed dose to water was determined according to IAEA dosimetry protocol TRS-398

and PTW-FREIBURG(German medical products) protocol using ionization chamber,

electrometer Unidose and cobalt-60 machine. Depth of 5 cm was used instead of the

recommended depth of 5 cm as reference depth in most of relative and absolute

measurements [2]. INMO was uses a distance of (5 cm ) in the ruler mounted in the

phantom for the photon measurements. There was no difference shown in both results.

44

4.3. Results and discussion:

Table 1 and 2 show the measurement results done using the two protocols to determine

the absorbed dose to water at reference depth and maximum depth under reference

conditions for photon beam dosimetry. These results were taken in different days as shown

in the tables. Our results show small difference between the calculated absorbed doses. As

the accuracy in the absorbed dose measurement is highest in radiotherapy, it is important

that discrepancies be investigated.

Only cylindrical chamber was used for photon beams measurements. The measurements of

photon beams were performed using the two protocols. It was observed that in Table 1 the

values of the absorbed dose to water at reference depth and the dose at maximum depth

measured using the two protocols were comparable to the results of a similar study by

DIN6800-2 (1997) performed using four dosimetry protocols.

Also it can be seen from Table 1, there is a minor deviation observed for the protocol DIN-

2(1997) compared to the PTW protocols.

In an earlier works there were a comparison of IAEA- TRS 398 and AAPM-TG 51 against

DIN 6800-2 (1997). The discrepancies in the determination of absorbed dose to water for

photon beams were within 1%. The better agreement of this work in comparison to the

earlier work can probably be explained by two facts:

Firstly, we have used dosimeter system PTW-UNIDOS in the present work in stead of the

PTW- dosimeter system.

Secondly, the air density correction was done from the direct measurement of the

temperature and pressure instead of using the radioactive check source method. Moreover

DIN 6800-2(2006) protocol is used as a reference protocol.

45

Table 2 represents the measurement used with IAEA-TRS-398 taken at different days. The

absorbed dose to water at reference and maximum depth as illustrated in the table, shows

no significant deference between the protocols.

The result of polarity was between 1 and 1.001 representing 0.1% for the polarity effect of

Fc -65 chamber. While the polarity effect determined by INMO was 0.995 which

represents 0.5%.

IAEA recommends to use the chamber if it exhibits polarity less than 3%. Therefore it can

be deduced that the results were within acceptable limits recommended.

Also in this table the result of ion recombination was found to be between 1.002-1.008,

which represents 0.5% for the recombination effect for that chamber.

The result of ion recombination effect determined by INMO was found to be 1.000, which

represents a negligible effect of ion recombination.

The comparison of INMO measurements and measured results with the recommendation of

the IAEA represent a tolerance level.

Table 3 illustrates a compression between the two protocols for the absorbed dose at

maximum depth. The differences between the measurements is between 0.8% and 1.6%,

which within the tolerance level.

Table 4 represents the comparison between KpIKm and K TP which shows the difference ^

ranged between 0.01 to 0.04. It was observed that KP/Kmwas greater than-K^. This

difference gives small shift between the two protocols. But that difference does not affected

the absorbed dose to the patient

46

4-4 . Response characteristic of dosimetry system:

The result of response characteristic of dosimetry system represent increase of the

electrometer readings with the dose, as shown in Table 4.2.

• - »

47

Tablel: The measurement done by the PTW protocol

Date

19/4/008

26/4/008

03/5/008

10/5/008

17/5/008

24/5/008

24/5/008

KPIKm

1.057

1.092

1.0595

1.061

1.079

1.086

1.089

Measured

Dw(cGy/min)

93.75

92.45

90.59

91.95

91.94

92.11

87.80

£V(Zre/)cGy/min

99.09

100.96

96.03

97.56

99.25

100.03

95.61

Dw(zmJcGy/min

125.75

128.12

121.86

123.81

125.95

126.94

121.13

Note: Kp is check reading during calibration

Km is the mean value of check reading at use.

48

Table2: The measurement done by IAEA-TRS 398 protocol.

Date M KTP KPOL Ks Nm Dw(Zref) Dw(Zmax )cGy/min

(c Gy/nC) (c Gy/min)

99.98 126.87

99.24 125.94

97.58 123.38

98.91 125.52

97.63 123.89

96.93 123.01

96.76 122.79

19/4/2008

26/4/2008

03/5/2008

10/5/2008

17/5/2008

24/5/2008

24/5/2008

18.17

18.07

18.04

17.82

17.54

17.40

17.22

1.05

1.05

1.03

1.054

1.06

1.06

1.07

1

1

1

1.0008

1

1.0008

1

1.004

1.002

1.006

1.008

1.006

1.006

1.006

52.2

52.2

52.2

52.2

52.2

52.2

52.2

A,(zmax) = ioo rDw(zref)^

v PDD

49

Table3: compression between the absorbed dose at the maximum depth for the two

protocols.

PTW

Dw(ZmJ

125.75

128.12

121.86

123.81

125.95

126.94

121.13

TRs-398

w v max /

126.87

125.94 ! 123.38

125.52

123.89

123.01

122.79

Relative difference

(%)

0.8%

1%

1%

1%

1%

1.6%

1.4%

Date of

measurements

19/4/2008

26/4/2008

3/5/2008

10/5/2008

17/5/2008

24/5/2008

24/5/2008

50

Table 4: Kp/Km and KTP difference

KPIKm KTP Difference

1.092 L05 O04

1.062 1.05 0.01

1.087 1.03 0.06

1.087 1.054 0.03

1.09 1.06 0.03

1.06 1.05 0.01

1.057 1.07 0.01

51

CHAPTER FIVE

CONCLUSION

5. CONCLUSION

Measurements of absorbed dose to water were performed using PTW and TRS-398

dosimetry protocols. The deviation in the determination of the dose to water during

calibration of the beam was found to be < 2.5 % and this magnitude is within the accepted

limits of recommendation of the IAEA protocol (^2.5%).

It can be concluded that the IAEA protocol is much easier to use than their equivalents

from other protocols. This is due to the simplicity in the language and in the explanation

provided. One of the irritating features of the previous IAEA protocol is that the reader is

often referred from one section to another section, which causes a break in concentration

and tends to undermine the clarity of the explanation. The most important characteristics of

the new code of practice protocols are, it is easy to implement, even with limited

experience, the following examples given in the appendix sections. Moreover, the IAEA

uses more recent publications as their references and uses the latest data obtained by Monte

Carlo simulations [20].

52

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54