Absorbed Dose Determination in Photon and Electron Beams ... · Physical Scientists and Engineers...

Absorbed Dose Determination inPhoton and Electron Beams:

An adaptation of the IAEA InternationalCodes of Practice

Australasian College of Physical Scientists & Engineers in Medicine

2nd Edition

November, 1998

Preface

The first edition of this document was prepared by the National Radiation Laboratory (NewZealand) in 1989 on behalf of the Dosimetry Topic Group of the Australasian College ofPhysical Scientists and Engineers in Medicine and was based on IAEA TRS 277 (1987). Sincethat time, the IAEA has reviewed the data and methods used in TRS 277 (which is containedin a technical report IAEA TECDOC-897 (1996)) and published a code of practice for the useof plane parallel ionization chambers in high energy electron and photon beams, viz IAEATRS 381(1997).

This second edition of the adaptation of the IAEA Codes of Practice has been revised toinclude the recommendations contained in the technical report1 and the new code of practicefor plane parallel chambers. The document has also been updated to improve its usabilitybased on comments from a number of physicists. Appendix 2 contains details of the changesmade to this document.

It should be noted that it was agreed at the Dosimetry Topic Group meeting held in 1997 thatall physicists would move towards adopting the IPEMB dosimetry protocol published in 1996for low and medium voltage x-ray dosimetry by January 1, 1999.

1 The IAEA has published a 2nd edition of TRS 277, however this has not been used here because it was not

available at the time. Some changes may occur in this document in the near future when recommendationsof the 2nd edition are incorporated.

Disclaimer: Every reasonable effort has been made to ensure that the methods and data havebeen accurately transferred from the original documents. The author, DosimetryTopic Group or Australasian College of Physical Scientists and Engineers inMedicine accepts no liability for the accuracy of the material or for theconsequences of its use. Radiation beams used for the treatment of cancerpatients should only be calibrated by a qualified radiation oncology physicist.

Introduction

This is an adaptation of the IAEA dosimetry protocols for the absorbed dose determination inmegavoltage photon and electron beams to suit the three cylindrical ionization chambers andthree plane parallel ionization chambers which are most commonly used in the Australasianregion. It is intended as an easy reference manual for routine use and should be used inconjunction with the IAEA documents. Only high energy beams are covered here. Low andmedium energy x-ray dosimetry is also included in IAEA TRS 277.

Some tables have been condensed for simplicity, but the values have all been taken from IAEATRS 277 & 381.

An appendix is included which covers the use of field dose meters which are calibrated bymeans of a local reference instrument.

The Protocol

1. Ionization Chambers

1.1 For photon dosimetry, one of the three cylindrical chambers listed in Table 1 shall beused. For electrons, one of the three chambers in Table 1 may be used, or one of theplane parallel chambers designed especially for electron dosimetry in Table 2. The latter

is recommended for absolute dose measurements under reference conditions when E0 <

10 MeV 2 and required when E0 � 5 MeV, and is recommended for all relative dosemeasurements in electron beams.

1.2 Plane parallel ionization chambers are not recommended for determination of absolutedose in photon beams.

1.3 The chamber used shall have an absorbed dose to air chamber factor ND,air traceable to aNational Primary Standards Laboratory. For a cylindrical chamber, this is obtainedfrom the air kerma calibration factor NK or exposure calibration factor NX, and therelationship given in Table 1. For a plane parallel chamber, ND,air can be obtaineddirectly from a standards laboratory, or by intercomparison with a calibrated cylindricalchamber (refer to Appendix A).

1.4 Every reading taken with the user's dosemeter shall be corrected for deviation fromstandard conditions of temperature, pressure, and humidity, and for ionicrecombination, polarity effects and leakage3. The resultant corrected reading isdesignated MQ. In practice, it will usually be the average of several such readings.

2 Note that the cylindrical chambers included in this document have a radius greater than 2 mm.

Consequently, strict adherence to TRS 277 would require a plane parallel chamber to be used for E0 < 10MeV.

3 It is preferable not to use chambers with leakage greater than 0.1% of the current generated by the radiationexposure, otherwise measurements should be corrected for leakage.

2. Phantoms

2.1 A water phantom shall be used for absorbed dose measurements in electron beams with

E0 � 10 MeV and in photon beams. For absolute dose measurements in electron

beams with E0 < 10 MeV and for relative dose measurements, a plastic phantom4 maybe used but depths and ranges must be converted to the water equivalent as in Section3.1.2. There should be a margin of at least 5 cm on all sides of the largest field size usedat measurement depth, and beyond the maximum depth of measurement.

2.2 The chamber is suspended in the water phantom without a buildup cap. A thin well-fitted PMMA tube or cover (wall thickness about 0.5 mm) may be used to waterproofthe chamber.

2.3 The chamber is always used with its effective point of measurement (peff) at thereference depth. For a cylindrical chamber, this involves a displacement downstream ofthe chamber centre by an amount shown in Table 3. The effective point of measurementfor a plane parallel chamber is the inside surface of the front electrode.

2.4 Reference conditions for each radiation type and energy are given in Table 3.

3. Electrons

3.1 Energy Specification

3.1.1 The practical range Rp and half-value depth R50 for the electron beam are determinedfrom the depth dose or depth ionization curve using an SSD of 100 cm and a field size

of at least l2 cm x l2 cm for E0 < 15 MeV or 20 cm x 20 cm for E0 � 15 MeV. If anionization chamber is used, each reading must be corrected for ionic recombination andpolarity, and if the chamber is cylindrical, the effective point of measurement must be

used. A plane parallel chamber should be used for E0 � 10 MeV, and is recommendedfor all relative dose measurements.

3.1.2 If a plastic phantom is used for measuring depth dose or ionization curves, the resultingvalues of R50 and Rp must be scaled to water equivalent ranges according to

R cm R cm Cwater phantomuser

tablepl[ ] [ ]= ρ

ρρ is the plastic density (in g.cm-3) and Cpl is the plastic to water range scaling factor.

Values of Cpl and Ψtable are given in Table 4. ρ user must be measured.

3.1.3 The mean electron energy at the water phantom surface E0 is determined by

E MeV R RD D0 50 50

20 656 2 059 0 022[ ] . . . ( )= + +

4 The plastic material should be conductive. However, insulating materials can be used provided users are

aware of problems resulting from charge storage. This effect can be minimized by using sheets not exceeding2 cm in thickness.

for RD50 [cm] determined from a depth dose curve or

E MeV R RJ J0 50 50

20 818 1935 0 040[ ] . . . ( )= + +

for RJ50 [cm] determined from a depth ionization curve. These equations are polynomial

fits to the data in Table 5 and yield stopping power ratios to within 0.4% of thosederived using Table 5 for depths up to 0.8Rp.

3.1.4 The mean electron energy EZ at depth z in water is determined from Table 6 as a

function of E0 and z/Rp. It is preferable that Rp is determined from the depth dosecurve, but there is not a large difference if Rp is taken from the depth ionization curve.This should however be checked once the depth dose curve has been measured.

3.2 Dose to Water

3.2.1 The dose to water Dw,Q for the user’s quality (Q) at the effective point of measurementpeff is given by

Dw,Q (peff) = MQ ND,air (sw,air)Q pQ

3.2.2 peff is located at the reference depth in water (Table 3). The above equation can be usedto measure dose at other depths, but pQ for cylindrical chambers has been determinedfor dose maximum, so the result will not be as accurate. It can be used for depths from0.2 to 0.5 Rp, but a plane parallel chamber is recommended away from the referencedepth.

3.2.3 MQ is the meter reading corrected as in Section 1.4.

3.2.4 ND,air is determined as in Section 1.3.

3.2.5 (sw,air)Q is the ratio of stopping powers in water and air at the user’s quality. Values are

given in Table 7 as a function of E0 and z.

3.2.6 pQ is the global perturbation correction factor. For cylindrical chambers, pQ = pcav pcel

which corrects for the influence of the air cavity on the electron fluence and for thepresence of the aluminium central electrode. It is obtained from Table 8 for each of the

three cylindrical chambers as a function of EZ . For plane parallel chambers, pQ = pcav

pwall (the latter factor corrects for non-medium equivalence of the chamber wallmaterial) and is given in Table 9 for a range of electron chambers.

3.2.7 The measurement may be done in a plastic phantom when E0 � 10 MeV. In this casethe depth to be used in plastic is given by z(plastic)

z cm z cmCplastic water

table

user pl

[ ] [ ]= ρρ

1

The meter reading MQ(plastic) must be converted to an equivalent reading in water by

MQ(water) = MQ(plastic) hm.

Values of Cpl , Ψtable and hm are given in Tables 4 and 10 respectively.

4. Photons

4.1 Energy Specification

The beam quality is specified byTPR1020. This is the ratio of ionization measurements

made at a constant source detector distance of 100 cm at depths of 20 cm and 10 cm inthe water phantom respectively and a field size of 10 cm x 10 cm at the effective point ofmeasurement. Alternatively D20/D10 can be used. This is the dose ratio (approximatelyequal to the ionization ratio J20/Jl0) at 20 cm and 10 cm but with a constant SSD of 100cm and 10 cm x 10 cm field at the phantom surface. Either of these parameters may beused as an index of beam quality in Table 11.

4.2 Dose to Water

4.2.1 The dose to water Dw,Q for user’s quality (Q) at the effective point of measurement peff isgiven by

Dw,Q (peff) = MQ ND,air (sw,air)Q pQ

4.2.2 peff should be located at the reference depth for a reference beam calibration (Table 3),but the above equation holds with acceptable accuracy at any depth for a given quality.

4.2.3 MQ is the meter reading corrected as in Section 1.4.

4.2.4 ND,air is determined as in Section 1.3.

4.2.5 Values of (sw,air)Q pQ are given as functions of TPR1020 and D20/D10 in Table 11. The

global perturbation factor pQ = pcav pcel.

References

International Atomic Energy Agency, Absorbed Dose Determination in Photon and ElectronBeams: An International Code of Practice, Technical Report Series No. 277, IAEA, Vienna(1987).

International Atomic Energy Agency, Review of Data and Methods Recommended in theInternational Code of Practice, IAEA Technical Report Series No. 277, on Absorbed DoseDetermination in High Energy Electron and Photon Beams, IAEA-TECDOC-897, IAEA,Vienna (1996).

International Atomic Energy Agency, The Use of Plane Parallel Ionization Chambers in HighEnergy Electron and Photon Beams: An International Code of Practice for Dosimetry,Technical Report Series No. 381, IAEA, Vienna (1997).

Institution of Physics & Engineering in Medicine & Biology, “The IPEMB code of practice forthe determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mmAl - 4 mm Cu HVL; 10-300 kV generating potential), Phys. Med. Biol. 41:2605-25 (1996).

Initials: Page 6

Worksheet 1. Absorbed Dose to Water Calibration under ReferenceConditions for an Electron Beam.

Physicist: Date: / /

1. Radiation treatment unit:

Name, manufacturer & model:

Nominal energy: MeV

Nominal dose rate of accelerator: MU/min.

Field size : cm x cm at cm SSD

Depth of dose maximum in water, R100 = cm

Range in plastic must be converted to range in water (Table 4)R100 (water) = R100 (plastic) Cpl ρuser /ρtable

Reference depth in water of effective point of measurement, zpeff = cm (Table 3)

Cylindrical chamber

Depth in water phantom of centre of chamber, zp = zpeff + 0.5 r = cm

r = 0.315 cm for NE 2505/3A and NE 2571, or r = 0.370 cm for NE 2561.

Plane parallel chamber

Depth in water phantom of front of chamber cavity, zp = zpeff = cm

2. Dosemeter:

Ionization chamber manufacturer:

model :

serial no :

Electrometer manufacturer :

model :

serial no:

ND,air = Gy/div

Date of last calibration: / /

Response change compared to calibration date derived from checking against a radioactive source: %

Initials: Page 7

3. Electrometer reading corrections:

Range setting :

Range scale factor, kr =

Pressure, P = kPa

Temperature, T = oC

pP

T)TP = +1013 2732

2932

. ( .

.=

Relative humidity = % (should be between 20 & 70%)

Recombination correction

Polarizing voltage, V1 reading Ml =

V2 M2 =

V1 / V2 Ml / M2 =

ps = (Table l2)Polarity correction

Normal polarity reading Ml =

Reverse polarity reading M2 =

kp = (Ml + M2) / (2Ml) =

Electron fluence correction, plastic to water hm = (Table 10)

Combined correction factor pcombined = kr pTP ps kp hm =

4. Absorbed dose to water:

Phantom material used for measurements:

Density of phantom, Ψuser = g/cm3

Ranges obtained by measurement at SSD = 100 cm with absorbed dose / ionization curves

Ranges in phantom:

R phantom100, = cm R phantom50, = cm Rp phantom, = cm

Ranges in water:

R water100, = cm R water50, = cm Rp water, = cm

Mean energy at the surface , E o = MeV (Table 5)

zpeff/Rp = E Z / E o = (Table 6)

Mean energy at depth zpeff = cm in water, E Z = MeV

Initials: Page 8

Stopping power ratio water/air, (sw,air )Q = (Table 7)

Perturbation factor, pQ = (Table 8 for cylindrical chamber)(Table 9 for plane parallel chamber)

ABSORBED DOSE IN WATER AT DEPTH ZPeff

Monitor setting MU =

Average uncorrected electrometer reading M Q0 =

Dw(peff) = M p N s pQ combined D air w air Q Q0

, ,( ) = Gy

DOSIMETRY ADJUSTMENT FOR A LINEAR ACCELERATOR

Required dose per monitor unit Dw(peff)/MU = Gy/MU


Required uncorrected electrometer reading MD p

p N s pQw eff

combined D air w air Q Q

0 =( )

( ), ,

=

Initial average electrometer reading M Q0 = Error = %

Final average electrometer reading M Q0 = Error = %

Final Dw(peff) /MU = Gy/MU

Initials: Page 9

Worksheet 2. Absorbed Dose to Water Calibration under ReferenceConditions for a High Energy Photon Beam.


1. Radiation treatment unit:

Name, manufacturer & model:

Nominal accelerating potential: MV

Nominal dose rate of unit: MU/min

Field size : 10 cm x 10 cm at cm SSD / SAD

Depth in water of effective point of measurement, zpeff = cm (Table 3)

Depth in water of chamber centre, zp = cm

zp = zpeff + 0.5 r for 60Co or zp = zpeff + 0.6 r for x-rays

r = 0.315 cm for NE 2505/3A and NE 2571 , or r = 0.370 cm for NE 2561.

2. Dosemeter:


model :

serial no :

Electrometer manufacturer :

model :

serial no:

ND,air = Gy/div

Date of last calibration: / /

Response change compared to calibration date derived from checking against a radioactive source: %

Initials: Page 10

3. Electrometer reading corrections:

Range setting :


Pressure, P = kPa

Temperature, T = oC

pP

T)TP = +1013 2732

2932

. ( .

.=

Relative humidity = % (should be between 20 and 70%)

Recombination correction

Polarizing voltage, V1 reading Ml =

V2 M2 =

V1 / V2 Ml / M2 =

ps = (Table l2)

Combined correction factor pcombined = kr pTP ps =

4. Absorbed dose to water:

Quality of the beam for 10 x 10 cm2 field

TPR1020 or D20 / D10 = at 100 cm SCD / SSD

Correction factor (sw,air )QpQ = (Table 11)

Depth of effective measuring point zpeff = cm

ABSORBED DOSE AT DEPTH ZPeff


Average uncorrected electrometer reading M Q0 =

Dw(peff) = M p N s pQ combined D air w air Q Q0

, ,( ) = Gy

Initials: Page 11

DOSIMETRY ADJUSTMENT FOR A LINEAR ACCELERATOR

Required dose per monitor unit Dw(peff)/MU = Gy/MU


Required uncorrected electrometer reading MD p

p N s pQw eff

combined D air w air Q Q

0 =( )

( ), ,

=

Initial average electrometer reading = Error = %

Final average electrometer reading = Error = %

Final Dw(peff)/MU = Gy

Table 1. Cylindrical ionization chambers used in this protocol.

NE 2505/3A NE 2561 NE 2571

Volume (cm3) 0.6 0.325 0.6

Length (mm) 24 9.2 24

Internal radius (mm) 3.15 3.70 3.15

Wall material graphite graphite graphite

Wall thickness (mm) 0.36 0.50 0.36

(g cm-2) 0.065 0.090 0.065

Buildup cap PMMA Delrin Delrin

Buildup thickness (mm) 4.7 4.3 3.9

(g cm-2) 0.551 0.600 0.551

Absorbed dose to airchamber factor,ND,air (Gy/div.)

0.978 NK

or8.60 x 10-3 NX

0.976 NK

or8.58 x 10-3 NX

0.982 NK

or8.63 x 10-3 NX

Note that the unit for NX is R/div.

Table 2. Plane parallel ionization chambers used in this protocol for electron dosimetry

Chamber Material Windowthickness

Electrodespacing(mm)

Collectingelectrode

diameter (mm)

Guardring width

(mm)

Recommendedphantom material

NACPa Mylar foil and graphitewindow, graphitedrexolite electrode,graphite body (backwall), rexolite housing

104mg/cm2

0.6 mm

2 10 3 Water, PMMA

Markus Graphited polyethylenefoil window, graphitedpolystyrene collector,PMMA body, PMMAcap

102mg/cm2

0.9 mm(includes

cap)

2 5.3 0.2 Water, PMMA

Roos PMMA, graphitedelectrodes

118mg/cm2

1 mm

2 16 4 Water, PMMA

a Model NACP02

Table 3. Reference conditions of the irradiation geometry for absorbed dose measurementsusing an ionization chamber in a phantom.

Reference depth inwater phantom

(cm)

Effective point ofmeasurement for

cylindricalchambersb

SSD Field size

(cm x cm)

Electrons

E o < 5 MeV Rl00 0.5rc Usual 10x10

5 α E o < 10 MeV Rl00 or la 0.5rc treatment 10x10

10 α E o < 20 MeV Rl00 or 2a 0.5r SSD 10x10

20 α E o < 50 MeV Rl00 or 3a 0.5r 15 x15

Photons

TPR1020 α 0.70 5 0.6r Usual 10x10

TPR1020 > 0.70 10 0.6r treatment 10x10

60Co 5 0.5r SSD or SAD 10x10

Notes:

a Whichever depth is greater.

b The effective point of measurement for a plane parallel chamber is the inside surface of the frontelectrode. For a cylindrical chamber the effective point of measurement is displaced on a linetowards the source from the chamber axis by the indicated fraction of the chamber internalradius.

c The cylindrical chambers included in this document have a radius greater than 2 mm.Consequently, strict adherence to TRS 277 would require a plane parallel chamber to be used for

E0 < 10 MeV.

Table 4. Scaling factor Cpl for ranges and depths in different plastics for electron beamdosimetry.

The mass density used in the calculations and the mean atomic numbera are included.

PMMAc Polystyrenec Polyethylenec A-150c Solid waterc

(WT1)

Plasticwaterd

Cpl 1.123 0.981b 0.908 1.067 0.967 0.991

Ψ (g.cm- 1.190 1.060b 0.940 1.127 1.020 1.013

Z 5.85 5.29 4.75 5.49 5.95 6.61

a For comparison, Zwater = 6.60.b These values can also be used for white (high impact) polystyrene.c Refer to ICRU 37 & 44 for composition.d Refer to Agostini et al (1992) Med. Phys. 19: 774 for composition.

Table 5. Relationship between the mean energy E o at the water phantom surface of anelectron beam and the half-value depth measured from absorbed dose andionization curves at 100 cm SSD for broad beams (RD

50and RJ50 respectively).

E o (MeV) RD50 (cm) RJ

50 (cm)

1 0.3 0.3

2 0.7 0.7

3 1.2 1.2

4 1.6 1.6

5 2.1 2.1

6 2.5 2.5

7 3.0 3.0

8 3.4 3.4

9 3.8 3.8

10 4.3 4.3

12 5.1 5.1

14 6.0 5.9

16 6.8 6.7

18 7.8 7.6

20 8.6 8.4

22 9.4 9.2

25 10.7 10.4

30 12.8 12.3

35 14.6 14.0

40 16.3 15.4

45 18.1 16.9

50 19.7 18.2

Table 6. Ratio of the mean energyE Z at depth z and the mean energy at the phantom

surface E o for electron beams in water.

Depths are given as a fraction of Rp.

Mean energy at surface E o (MeV)

z/Rp 2 MeV a 5 MeV 10 MeV 20 MeV 30 MeV 40 MeV 50 MeV

0.00 1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.05 0.945 0.943 0.941 0.936 0.929 0.922 0.915

0.10 0.890 0.888 0.884 0.875 0.863 0.849 0.835

0.15 0.835 0.831 0.826 0.815 0.797 0.779 0.761

0.20 0.777 0.772 0.766 0.754 0.732 0.712 0.692

0.25 0.717 0.712 0.705 0.692 0.669 0.648 0.627

0.30 0.656 0.651 0.645 0.633 0.607 0.584 0.561

0.35 0.592 0.587 0.583 0.574 0.547 0.525 0.503

0.40 0.531 0.527 0.523 0.514 0.488 0.466 0.444

0.45 0.468 0.465 0.462 0.456 0.432 0.411 0.390

0.50 0.415 0.411 0.407 0.399 0.379 0.362 0.345

0.55 0.362 0.359 0.355 0.348 0.329 0.314 0.299

0.60 0.317 0.313 0.309 0.300 0.282 0.269 0.256

0.65 0.274 0.270 0.265 0.255 0.239 0.228 0.217

0.70 0.235 0.231 0.226 0.216 0.202 0.192 0.182

0.75 0.201 0.197 0.191 0.180 0.168 0.159 0.150

0.80 0.168 0.164 0.159 0.149 0.138 0.131 0.124

0.85 0.141 0.137 0.131 0.120 0.111 0.105 0.099

0.90 0.118 0.114 0.108 0.096 0.089 0.084 0.079

0.95 0.095 0.091 0.086 0.076 0.069 0.065 0.061

1.00 0.081 0.077 0.071 0.059 0.053 0.049 0.045

a Values extrapolated by using a third degree polynomial fit at each depth.

Table 7. Spencer-Attix stopping power ratios (θ = 10 kev), water to air (sw,air) for electron

beams as a function of E 0 and depth in water

Electron beam energyE 0 (MeV)Depth

in 1 2 3 4 5 6 7 8 9 10

water Rp (cm)

(cm) 0.36 0.88 1.40 1.91 2.43 2.94 3.45 3.96 4.47 4.98

0.0 1.117 1.088 1.066 1.049 1.034 1.026 1.014 1.006 0.998 0.993

0.1 1.125 1.096 1.072 1.055 1.040 1.030 1.018 1.010 1.002 0.996

0.2 1.131 1.104 1.079 1.060 1.045 1.033 1.022 1.014 1.005 0.999

0.3 1.134 1.111 1.085 1.065 1.049 1.037 1.026 1.018 1.009 1.002

0.4 1.136 1.117 1.091 1.070 1.053 1.041 1.029 1.021 1.011 1.005

0.5 1.123 1.097 1.075 1.057 1.044 1.032 1.023 1.014 1.007

0.6 1.127 1.102 1.079 1.061 1.048 1.035 1.026 1.016 1.009

0.8 1.132 1.112 1.089 1.069 1.055 1.041 1.031 1.021 1.013

1.0 1.135 1.120 1.098 1.077 1.062 1.047 1.036 1.025 1.018

1.2 1.127 1.107 1.086 1.070 1.054 1.042 1.030 1.022

1.4 1.132 1.116 1.095 1.079 1.061 1.048 1.035 1.027

1.6 1.135 1.123 1.104 1.087 1.069 1.054 1.041 1.031

1.8 1.137 1.129 1.112 1.095 1.076 1.061 1.047 1.037

2.0 1.133 1.118 1.103 1.084 1.068 1.053 1.042

2.5 1.128 1.120 1.102 1.086 1.069 1.056

3.0 1.133 1.131 1.118 1.103 1.086 1.072

3.5 1.132 1.129 1.118 1.102 1.087

4.0 1.128 1.116 1.103

4.5 1.130 1.127 1.115

5.0 1.125

5.5 1.127

6.0 1.124

7.0

8.0

9.0

10.0

12.0

Table 7. (continued)

Electron beam energyE 0 (MeV)Depth

in 12 14 16 18 20 22 25 30

water Rp (cm)a

(cm) 5.99 6.99 7.99 8.98 9.96 10.93 12.38 14.77

0.0 0.981 0.969 0.961 0.955 0.948 0.943 0.936 0.924

0.1 0.985 0.973 0.965 0.959 0.951 0.946 0.938 0.927

0.2 0.988 0.976 0.968 0.962 0.954 0.948 0.941 0.929

0.3 0.990 0.979 0.971 0.964 0.957 0.951 0.943 0.932

0.4 0.993 0.982 0.973 0.966 0.959 0.953 0.945 0.934

0.5 0.995 0.984 0.975 0.968 0.961 0.955 0.946 0.935

0.6 0.997 0.986 0.977 0.970 0.963 0.957 0.948 0.937

0.8 1.001 0.989 0.980 0.973 0.966 0.960 0.951 0.940

1.0 1.004 0.992 0.983 0.975 0.969 0.962 0.953 0.943

1.2 1.008 0.995 0.985 0.978 0.971 0.964 0.956 0.945

1.4 1.011 0.998 0.988 0.981 0.973 0.966 0.958 0.947

1.6 1.015 1.001 0.991 0.983 0.975 0.969 0.960 0.948

1.8 1.018 1.004 0.994 0.986 0.977 0.971 0.962 0.950

2.0 1.023 1.008 0.997 0.988 0.980 0.973 0.964 0.952

2.5 1.034 1.016 1.004 0.994 0.986 0.978 0.969 0.956

3.0 1.047 1.027 1.012 1.002 0.992 0.984 0.974 0.960

3.5 1.060 1.038 1.021 1.008 0.998 0.989 0.978 0.964

4.0 1.074 1.050 1.031 1.016 1.005 0.996 0.984 0.969

4.5 1.088 1.062 1.041 1.026 1.012 1.002 0.990 0.973

5.0 1.102 1.075 1.053 1.035 1.021 1.009 0.995 0.978

5.5 1.114 1.088 1.065 1.045 1.029 1.016 1.001 0.983

6.0 1.123 1.100 1.077 1.056 1.038 1.024 1.007 0.987

7.0 1.122 1.120 1.099 1.078 1.058 1.041 1.021 0.998

8.0 1.118 1.118 1.099 1.078 1.060 1.037 1.009

9.0 1.114 1.116 1.099 1.079 1.053 1.022

10.0 1.114 1.098 1.071 1.036

12.0 1.109 1.104 1.065

Table 8. Correction factors pQ = pcav pcel for cylindrical ionization chambers for electron

beams as a function of mean energyE Z at reference depth.

NE 2505/3A NE 2561

E Z (MeV) NE 2571

4 0.965 0.961

6 0.974 0.967

8 0.981 0.974

10 0.987 0.982

12 0.992 0.989

15 0.996 0.997

20 0.999 1.000

Note: The values in this table for cylindrical chambers are only true at dose maximum and are notrecommended outside the range 0.2 Rp to 0.5 Rp. A plane parallel chamber with pQ = 1.0should be used outside this range.

Table 9. Correction factor pQ = pcavpwall for plane parallel chambers in electron beams as a

function of mean energyE Z at reference depth.

E Z

(MeV)

NACP PTWMarkus

M 23 343

RoosFK-6

2 1.000 0.978 1.000

3 1.000 0.983 1.000

4 1.000 0.987 1.000

5 1.000 0.990 1.000

6 1.000 0.993 1.000

7 1.000 0.995 1.000

8 1.000 0.996 1.000

10 1.000 0.998 1.000

12 1.000 0.999 1.000

15 1.000 0.999 1.000

20 1.000 1.000 1.000

The data apply at, or close to, R100.

Table 10. Factor hm to correct for the difference in the electron fluence in a plastic phantomcompared to that in water at an equivalent depth.

E Z

(MeV)

Epoxy resin watersubstitutes

Clear polystyrene

Whitepolystyrene PMMA

1 1.013 1.028 1.021 1.010

2 1.012 1.027 1.019 1.009

3 1.011 1.025 1.018 1.008

4 1.010 1.023 1.017 1.008

5 1.009 1.021 1.016 1.007

6 1.008 1.020 1.015 1.007

8 1.006 1.017 1.012 1.005

10 1.004 1.014 1.010 1.004

12 1.002 1.010 1.008 1.003

14 1.000 1.007 1.005 1.002

16 --- 1.004 1.003 1.001

18 --- 1.001 1.001 1.000

20 --- 1.000 1.000 1.000

The values are appropriate to depths close to R100 and beyond; for depths between the surface and R100,an acceptable solution is to interpolate linearly between a surface value of 1.0 and the values at Rl00.

Table 11. The factor sw,air pQ as a function of photon beam quality.

Beam quality NE 2505/3A NE 2561

TPR1020 D20/D10 NE 2571

0.50 0.44 1.129 1.131

0.53 0.47 1.127 1.128

0.56 0.49 1.124 1.125

0.59 0.52 1.123 1.122

0.62 0.54 1.121 1.119

0.65 0.56 1.119 1.116

0.68 0.58 1.115 1.113

0.70 0.60 1.113 1.110

0.72 0.61 1.108 1.106

0.74 0.63 1.103 1.100

0.76 0.65 1.097 1.095

0.78 0.66 1.089 1.087

0.80 0.68 1.083 1.083

0.82 0.69 1.073 1.073

0.84 0.71 1.063 1.065

60Co 1.124 1.123

Note: pQ = pcav pcel. The values used for pcel for all three chambers are:

pcel = 1.000, TPR1020 < 0.80

pcel = 1.004, TPR1020 ≥ 0.80

Table 12. Coefficients for the ionic recombination correction factor ps determined using thetwo voltage method

M1 & M2 are the collected charge using bias voltages V1 & V2 . The ratio V1/V2 should begreater than or equal to 3.

(i) Continuous radiation: ps = 1.0 + a (Ml/M2 - 1.0)

Vl / V2 a

1.75 0.490

2.00 0.340

2.25 0.253

2.50 0.193

3.00 0.127

3.75 0.077

5.00 0.043

(ii) Pulsed radiation: ps = ao + al (Ml/M2) + a2 (Ml/M2)2

Vl / V2 ao al a2

2.0 2.337 - 3.636 2.299

2.5 1.474 - 1.587 1.114

3.0 1.198 - 0.8753 0.6773

3.5 1.080 - 0.5421 0.4627

4.0 1.022 - 0.3632 0.3413

5.0 0.9745 - 0.1875 0.2135

6.0 0.9584 - 0.1075 0.1495

8.0 0.9502 - 0.03732 0.08750

10.0 0.9516 - 0.01041 0.05909

Table 12 (continued)

(iii) Pulsed scanned radiation: ps = ao + al (Ml/M2) + a2 (Ml/M2)2

Vl / V2 ao al a2

2.0 4.711 - 8.242 4.533

2.5 2.719 - 3.977 2.261

3.0 2.001 - 2.402 1.404

3.5 1.665 - 1.647 0.9841

4.0 1.468 - 1.200 0.7340

5.0 1.279 - 0.7500 0.4741

6.0 1.177 - 0.5081 0.3342

8.0 1.089 - 0.2890 0.2020

10.0 1.052 - 0.1896 0.1398

Initials: Page A1

Appendix A: Use of Field Dosemeters

It may be preferred that the dosemeter which is calibrated directly by a Primary or SecondaryStandards Laboratory be used as a local reference standard, and that a more robust ionizationchamber be used for routine measurements after calibration against the reference chamber.

The calibration of a cylindrical chamber is a suggested application of the IAEA TRS 277protocol and not part of the protocol itself. The calibration of plane parallel chambers for usein electron beams is based on the IAEA TRS 381 protocol.

The field and reference dose meters are compared consecutively at the same effective point ofmeasurement (at the reference depth given in Table 3) in a phantom. A water phantom shouldbe used in photon & electron beams, however, TRS 381 allows the use of a solid plasticphantom in electron beams where the absolute dose determination is to be performed in thesolid phantom. A photon beam should be used for cylindrical chambers and a high energyelectron beam for plane parallel chambers. Two worksheets are provided for the determinationof ND,air for cylindrical and plane parallel chambers. An external monitor detector is stronglyrecommended, positioned within the phantom at approximately the same depth at a distance of3 to 4 cm from the other chamber. Alternatively, the “sandwich” technique should be usedwhere readings with the reference chamber are taken before and after the field chamberreadings.

Initials: Page A2

Worksheet Al. Calibration of a cylindrical chamber in a photon beam.


1. Dosemeters:

Reference Field


model:

serial no:

Electrometer manufacturer:

model:

serial no:

Absorbed dose to air chamber factor for reference dosemeter, ND airref

, = Gy/div.

2. Calibration conditions:

Treatment unit:

Nominal accelerating potential: MV



Reference Field

Monitor units or time (corrected for transit) set =

Chamber radius, r (cm) =

Depth to chamber axis, zp (cm) =

r = 0.315 cm for NE 2505/3A, 2571, or r = 0.370 cm for NE 2561.zp = zpeff + 0.5 r ( 60Co); or zp = zpeff + 0.6 r (x-rays).

Initials: Page A3

3. Electrometer readings and corrections:

Reference Field

Average raw reading M Q0 =

(div/s or div/MU.)

Range setting:


Recombination and polarity corrections:

V1 =

V2 =

Vl / V2 =

Reading, Ml =

Reading, M2 =

Ml/M2 =

ps (Table 12) =

Normal polarizing volts, Ml =

Reverse polarizing volts, M2 =

kp = (Ml + M2) / (2Ml) =

Corrected reading, MQ =

MQ = M Q0 kr ps kp (div/s or div/MU.)

4. N D airf

, for field instrument

TPR1020 or D20/Dl0 = at 100 cm SCD / SSD

Reference Field

Perturbation factor, pQ (Table 13)

NM N p

M pD air

f Q

ref

D air

ref

Q

ref

Q

f

Q

f,

,=

= Gy/div.

Initials: Page A4

Worksheet A2. Calibration of a plane parallel chamber in an electronbeam.


1. Dosemeters:

Reference Field


model:

serial no:

Electrometer manufacturer:

model:

serial no:

Absorbed dose to air chamber factor for reference dosemeter, ND airref

, = Gy/div.

2. Calibration conditions:

Treatment Unit:

Nominal energy: MeV



A high energy electron beam (Eo > 10 MeV) should be used.

Reference Field

Monitor units set

Chamber radius, r (cm)

Depth to effective measuring point, zpeff (cm)

Depth to chamber axis, zp (cm)

Cylindrical: zp = zpeff + 0.5 r (electrons) Plane parallel: zp = zpeff. r = 0.315 cm for NE 2505/3A, 2571, or r = 0.370 cm for NE 2561.

Initials: Page A5

3. Electrometer readings and corrections:

Reference Field

Average raw reading M Q0 (div/MU) =

Range setting:


Recombination and polarity corrections:

V1 =

V2 =

Vl / V2 =

Reading, Ml =

Reading, M2 =

Ml/M2 =

ps (Table 12) =

Normal polarizing volts, Ml =

Reverse polarizing volts, M2 =

kp = (Ml + M2) / (2Ml) =

Corrected reading, MQ =

MQ = M Q0 kr ps kp (div/MU.)

4. N D air

f

, for field instrument

Phantom material used for measurements:

Density of phantom, Ψuser = g/cm3

Ranges obtained by measurement at SSD = 100 cm with absorbed dose / ionization curves

Ranges in phantom:

R phantom100, = cm R phantom50, = cm Rp phantom, = cm

Ranges in water:

R water100, = cm R water50, = cm Rp water, = cm

Mean energy at the surface , E o = MeV (Table 5)

zpeff/Rp = E Z / E o = (Table 6)

Initials: Page A6

Mean energy at depth zpeff = cm in water, E Z = MeV

Perturbation factor, pQref = = (Table 8)

Perturbation factor, pQrpp = = (Table 9)

NM N p

M pD airpp Q

refD airref

Qref

Qpp

Qpp,

,=

= Gy/div.

Page A7

Table 13. The factor pQ = pcav pcel as a function of photon beam quality.

Beam quality NE 2505/3A NE 2561

TPR1020 D20/D10 NE 2571

0.50 0.44 0.995 0.997

0.53 0.47 0.994 0.995

0.56 0.49 0.993 0.994

0.59 0.52 0.994 0.993

0.62 0.54 0.995 0.993

0.65 0.56 0.996 0.994

0.68 0.58 0.996 0.994

0.70 0.60 0.997 0.995

0.72 0.61 0.997 0.995

0.74 0.63 0.998 0.996

0.76 0.65 0.998 0.997

0.78 0.66 0.998 0.997

0.80 0.68 1.003 1.002

0.82 0.69 1.004 1.004

0.84 0.71 1.004 1.005

60CO 0.992 0.991

Page B1

APPENDIX B: Revision History

This appendix describes the major changes made to the document.

March-June 1998

A soft copy of the first edition was not available so it was scanned and converted into MSWord 6.0 format. The document was then revised to include the recommendations of IAEAReview of Data & Methods used in TRS 277 and IAEA TRS 381. Draft document was sent toVere Smythe, Malcolm Millar, Peter Christiansen, Bob Fitchew and Nick Menzies forcomment. Replies were received from Vere Smythe, Peter Christiansen and Bob Fitchew.Cathryn Lindsay and John Doody provided some suggestions for the worksheets andregarding clarity of the text.

K.N. Nitschke

November 1998

Document finalised and sent to Malcolm Millar & Vere Smythe in PDF format. Changes from1st to 2nd edition are given below.

IAEA 381 uses revised nomenclature which has been adopted in this document.

ND --> ND,air

Subscript Q (rather than u) has been used to designate quantities for the user’s quality,e.g. Mu --> MQ.A global perturbation factor pQ is used rather than specifying the individual sub-factors,pu & pcel. pu --> pcav for cylindrical chambers and pQ = pcav pwall for plane parallelchambers.

The correction for the non air equivalence of the central electrode of a thimble chamber isimplemented differently in TRS 277 and TRS 381. The change in chamber response due to thenon-air equivalence of the central electrode may be different for the calibration quality (60Co)and the user’s quality. For example, in TRS 277 the correction factor for 60Co was 1.008 andfor high energy electrons was 1.000. TRS 277 applied a global pcel correction factor ratherthan applying the correction twice, i.e. ND is calculated from NK ignoring the change inresponse due to the electrode in 60Co and a global factor is applied for dose determination inthe user’s beam to account for the change in response in both the 60Co chamber calibration andthe measurement in the user’s beam. TRS 381 applies these corrections separately, i.e. ND iscalculated from NK applying a correction (kcel) for the change in response in the 60Co beamand the pcel applied in dose determinations relates only to the change in response in the user’sbeam. TRS 381 also recommends a global pcel = 1.004 for high energy electrons compared to1.008 in TRS 277.

The method of correcting for the non air equivalence of the central electrode of athimble chamber given in TRS 277 is retained in this document.

Page B2

Section 1

Comparison of chamber readings in phantom in a high energy electron beam is now therecommended method of transferring ND,air to a plane parallel ionisation chamber. Clause 1.2has been added, i.e. plane parallel chambers are not recommended for absolute dosedetermination in photon beams.

Section 2

Clause 2.3 now contains a statement about the effective measuring point of a plane parallelchamber.

Section 3

Clause 3.1.2 has been revised to describe the new method in TRS 381 of scaling rangesmeasured in plastic phantoms to those in water. TRS 277 recommends using the ratio of csdaranges in the two materials. The new scaling factor Cpl is the ratio of the average depth ofpenetration for the two materials and is taken to be independent of energy.

Two polynomials have been added for the energy range relationships for depth dose & depthionization in clause 3.1.3.

The global perturbation correction factor for electrons has been described in clause 3.2.6.

Clause 3.2.7 has been modified as per clause 3.1.2.

Section 4

Only changes to the nomenclature have been made to clauses in this section.

Worksheets

Most changes to the worksheets are of a cosmetic nature. The humidity correction factor hasbeen removed since there are no recommendations provided in TRS 277 on what values to usewhen the humidity is outside the acceptable range (i.e. 20 to 70%). The humidity should stillbe recorded. A section has been added for use when adjusting the output of a linearaccelerator.

Tables

Added a table which contains a list of three common used and acceptable plane parallelionization chambers (Table 2) and a table which contains the perturbation factors for the planeparallel chambers when used in electron beams (Table 9) from TRS 381. This resulted inrenumbering of all the tables.

The displacement of the effective measurement point for photon beams has been changed to0.6r in Table 3.

Page B3

Photon beam calibration can be performed at either the usual treatment SSD or SAD (seeTable 2). Also clarified the use of cylindrical chambers in electron beams with energies lessthan 10 MeV in Table 2.

Table 4 has been replaced with Table VIII from TRS 381.

Added 2 MeV column to Table 6 from Table XII from TRS 381.

TRS 381 does not recommend scaling the depth with the ratio of measured to calculatedextrapolated range when selecting the water to air stopping power for electron beams, as wasthe case in TRS 277, since the latter method does not lead to “a practical improvement” in theaccuracy. For the example presented (i.e. 20 MeV electron beam), using the scaled depth doesnot improve the accuracy of sw,air selection. The sw,air in the previous version of this documentwas tabulated as a function of scaled depth. The sw,air values for electrons provided in TRS 381are a result of Monte Carlo calculations using the Ashley density effect correction. The densitycorrection used in TRS 277 is not known. Table 7 contains the revised sw,air values as afunction of depth given in Table XVI in TRS 381. This introduces a small inconsistency in sw,air

for electrons & photons since the photon values have not been recalculated with the Ashleydensity effect correction.

Table 8 has been recalculated using pcav from Table XI in TRS 381 and a global pcel = 1.004for high energy electrons. pcav for the NE 2561 was obtained by linear extrapolation. Therevised pQ values are lower than the previous values by up to 0.3%.

Table 10 has been replaced with Table XVIII from TRS 381.

Appendix

The calibration of chambers in terms of dose to water has been removed. Separate worksheetsare provided for the transfer of ND,air for cylindrical and plane parallel chambers.

Corrected typographical errors for pQ for the NE 2561 chamber in Table 13.

K.N. Nitschke

August 2000

Corrected error in table 7. The value of sw,air for 8 MeV at the surface was 1.066 which isclearly not correct. Same error occurs in TRS 381. Appears it should be 1.006.

K.N. Nitschke

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