Dosimetry, Detectors and Traceability.
Transcript of Dosimetry, Detectors and Traceability.
Dosimetry, Detectors and Traceability.
Russell ThomasScience Area Leader
Medical Radiation Science Group
National Physical Laboratory
BIR, May 2021
Radiotherapy- cause and effect,
how much what, does what?
(PS Sorry to Margaret Bidmead for blatantly stealing your picture, but it is such a good one!)
Exposure Measurement
▪ Film Exposure
▪ Threshold Erythema Dose
https://www.iaea.org/resources/rpop/health-
professionals/radiology/erythema
Exposure Measurement
▪ Ionisationgoldleaf electroscope
condenser chambers
PRIMARY STANDARDS FOR
AIR KERMA
Free air chambersFree-air ionisation chambers are the primary standard for air kerma in air
for superficial and orthovoltage X rays (up to 300 kV).
reference volume
high voltage
measuringelectrode
collimatedbeam
secondaryelectrons
Principle:
The reference volume
(blue) is defined by the collimation of the beam
and by the size of
the measuring electrode. Secondary electron equilibrium
in air is fulfilled.
Monday, 17 May 2021
From exposure to air kerma
= measured charge
= mass of air
= mean energy required to produce an ion pair in dry air
= correction factors applied to the chamber
= fraction of energy lost to bremsstrahlung
e
Wair
Q
am
g
Fge
W
m
Q
dm
EdK air
aa
tra
−==
)1(
1
...4321 kkkkF =
Radium measurements - 1936
Free air chambers
Free air chambers cannot function as a primary standard for higher energies, such as for 192Ir brachytherapy and 60Co beams, since the air column surrounding the sensitive volume (for establishing the electronic equilibrium condition in air) would become very long.
• Photon energies between 0.18 and 2.198 MeV
• Electrodes 3.5 m high, spacing up to 3 m
• HT up to 20 kV
PRIMARY STANDARDS FOR
AIR KERMA
cavity ionisation chambers
50 cm
▪ Graphite cavity ionization chambers with and accurately known volume
are used as primary standards.
▪ The use of the graphite cavity chamber is based on the Bragg- Gray
cavity theory
▪ Know the chamber perturbation
Today this is determined using Monte Carlo calculations
▪ Estimate proportion of electrons produced in wall or build up cap of
chamber (to keep CPE)
▪ Only suitable for standards labs
▪ Methods that we could employ to measure
the dosimetric quantities of the radiation
beam include calorimetry, ionometry,
chemical dosimetry, solid state detectors
etc
▪ Measurement of ionisation in air was the
most technically achievable method in the
early years of radiotherapy
▪ First major development for primary
standards of external beam radiotherapy
was then the free air chamber
▪ Do we provide something that we can
reliably measure or something relevant or
perhaps even useful?
▪ Best result for the patient is what we want,
but we can also just want to be safe
▪ Sometimes good enough is the enemy of
progress
▪ We would like a precise knowledge of the
absorbed dose in the patient
Exposure based protocols – gave
absorbed dose but high uncertainty
Why consistency and accurate dosimetry are important?
▪ Tumor control and normal tissue complication probability
▪ Prediction of clinical results
Reproducible results for different patients
Transfer clinical results – clinical trials
• In 1976 – ICRU Report 24
• Dose delivery to the planning volume should be within 5% of the prescribed value (k=2)
• Uncertainty on 𝑫𝐰,𝐐: 1% (k=2)
• (uncertainty on 𝐷w,Q for photons: 1.5% (k=2))
International Organization for Standardization (ISO):
"Guide to the expression of uncertainty in measurement"
When reporting the result of a measurement of a physical quantity, it is
obligatory that some quantitative indication of the quality of the result be given
so that those who use it can assess its reliability.
▪ This guide provides a procedure for characterising the quality of a
measurement, i.e. for evaluating and expressing its uncertainty.
▪ It defines uncertainty as a quantifiable attribute.
▪ It allows measurement results to be compared amongst themselves and
with reference values.
Assessing your uncertainty is vital
▪ Standard uncertainty:uncertainty of a result calculated from the standard deviation
▪ Type A standard uncertaintyis evaluated by a statistical analysis of a series of observations.
▪ Type B standard uncertaintyis evaluated using available knowledge.
Uncertainty components can sometimes be categorised as “random” and “systematic” and are associated with errors arising from random effects and known systematic effects,
respectively.
Combined uncertainties (I)
The determination of the final result is normally based on several components.
Example: Determination of the water absorbed dose Dw,Q in a radiation beam of quality Q by use of an ionisation chamber:
where MQ is the measured chargeND,w is the calibration factorkQ is the beam quality correction factor
, , , w Q Q D w Q Qo
D M N k=
Combined uncertainties (II)
The uncertainty of the charge MQ can be assessed by statistical
analysis of a series of observations the uncertainty of MQ is of
type A
The uncertainties of ND,w and kQ will be of type B
The combined uncertainty, uC, of the absorbed dose Dw,Q is the
quadratic addition of type A and type B uncertainties:
( ) ( ) ( ) ( )2 2 2
, , , C w Q A Q B D w Q B Qo
u D u M u N u k= + +
EXPANDED
uncertaintiesThe combined uncertainty is assumed to exhibit a normal distribution
Then the combined standard uncertainty uC corresponds to a confidence level of
68%.
A higher confidence level is obtained by multiplying uC with a coverage factor
denoted by k:
CU k u=
U is called the expanded uncertainty. For k = 2, the expanded uncertainty
corresponds to the 95% confidence level.
Confidence levels
Why consistency and accurate dosimetry are important?
▪ Tumor control and normal tissue complication probability
▪ Prediction of clinical results
Reproducible results for different patients
Transfer clinical results – clinical trials
• In 1976 – ICRU Report 24
• Dose delivery to the planning volume should be within 5% of the prescribed value (k=2)
• Uncertainty on 𝑫𝐰,𝐐: 1% (k=2)
• (uncertainty on 𝐷w,Q for photons: 1.5% (k=2))
Calorimetry – the move towards a more
accurate measurement of Absorbed Dose
▪ Radiation energy turns into heat
heat is tiny, but measurable – our primary standards for
absorbed dose are calorimeters
Absorbed Dose based protocols
Very similar, but very important differences, each with pros and cons
Ionisation chamber
dosimetry
Chambers and electrometers
central collecting electrode
gas filled cavity
outer wall
Basic design of a cylindrical Farmer-type ionisation chamber
• An ionisation chamber is basically a gas filled cavity surrounded by a conductive outer wall with a central collecting electrode.
• Measurements made with vented chambers must be corrected to standard temperature and pressure.
▪ The wall & collecting electrode are separated with a high quality
insulator to reduce the leakage current when a polarising voltage is
applied to the chamber.
▪ A guard electrode is usually provided in the chamber to further
reduce chamber leakage.
▪ The guard electrode intercepts the leakage current and allows it to
flow to ground directly, bypassing the collecting electrode.
▪ The guard electrode ensures improved field uniformity in the active
or sensitive volume of the chamber (for better charge collection).
Ionisation chamber
dosimetry
Chambers and electrometers
An electrometer is a high gain, negative feedback, operational
amplifier with a standard resistor or a standard capacitor in the
feedback path to measure the chamber current and charge,
respectively, collected over a fixed time interval.
Chambers and
electrometers
▪ Most popular design
▪ Independent of radial beam direction
▪ Typical volume between 0.05 -1.00 cm3
▪ Typical radius ~2-7 mm
▪ Length~ 4-25 mm
▪ Thin walls: ~0.1 g/cm2
▪ Used for:
electron, photon, proton, or ion beams.
Cylindrical (thimble type)
ionisation chamber
▪ Well-guarded chamber samples electron fluence incident
only through the front window.
▪ Recommended for dosimetry of electron beams
▪ It is useful for depth dose measurements.
▪ Effective point of measurement is at the front face.
▪ Used for surface dose and depth dose measurements in
the build-up region of megavoltage photon beams.
Parallel-plate (plane-parallel)
ionisation chamber
(1) polarising electrode
(2) measuring electrode
(3) guard ring
(a) height (electrode separation) of the air cavity
(d) diameter of the polarising electrode
(m) diameter of the collecting electrode
(g) width of the guard ring.
Parallel-plate (plane-parallel)
ionisation chamber
3 32
1
g
a
dm
Electrometer and Air Ionisation
chamber as a Dosimetry System
▪ Satisfies most of the requirements for a dosimeterReproducible construction/materials
Sensitive. For low doses use larger volume chambers
Electrometer and Air Ionisation
Chamber as a Dosimetry System
▪ Satisfies most of the requirements for a dosimeterReproducible construction/materials
Sensitive. For low doses use larger volume chambers
– Good precision i.e. signal/noise and repeatability
– Good accuracy. Conversion factors established at national labs
– Wide dose range
– Linear with accumulated dose
– Independent of dose rate (with small correction)
– Reasonable energy independence
– Good long term stability
Measurement corrections for
Ionisation chambers
▪ Mass of air in vented chamber
▪ Ion recombination
▪ Polarity
▪ Effective point of measurement
Measurement corrections for
Ionisation chambers
▪ Mass of air in vented chamber
▪ Ion recombination
▪ Polarity
▪ Effective point of measurement
Measurement corrections for
Ionisation chambers
▪ Mass of air in vented chamber
Gas laws
▪ Ion recombination
% loss of ion collection
decreases with higher polarising volts
increases with dose per pulse
▪ Polarity
Difference in reading whether collecting electrode -ve or +ve.
Mainly effects primary electron beams striking collecting
electrode in parallel plate chambers
▪ Effective point of measurement
Temperature-Pressure correction
▪ As temp. increases air density decreases
▪ As atmospheric pressure increases air density
increases
▪ R (T+273.15)/T0 (P0/P)
T ambient temp in degrees Centigrade
T0 = 293.15 °K (20°C)
Standard pressure P0 = 1013.25mb, 760mm Hg, 29.92in.Hg,
101.32kPa
P, ambient pressure in same units
Recombination correction, Pion
▪ Negative and positive ions recombine if insufficient
collecting voltage to sweep up ions quickly
▪ Increasing collecting voltage increases the chamber
collection efficiency until near saturation is reached
(E > 500V/cm)
▪ Beyond saturation, higher voltages (E > 1000V/cm)
may cause ion multiplication by collision
Recombination correction, Pion
▪ Typically correction tiny for continuous radiation,
and ~1% for pulsed beams,
The dose per pulse is the key element
For scanned or flattening filter free beams the correction will be
greater (use 400 Volt)
▪ Secondary standard chamber recombination (200V),
NPL equation;
Pion= 1.0014 + 0.23, is cGy/pulse
for 300cGy/min, 300pps, = 0.017, Pion= 1.005
for 400cGy/min, 200pps, = 0.033, Pion= 1.009
Recombination correction, Pion
▪ Plotting 1/Reading vs
1/Voltage is a straight line
▪ This can easily be
extrapolated to volts, or
100% collection efficiency
▪ Linear dependence permits
use of the 2 voltage method
Theory
( volt)
0
1/R
eadin
g
1/Volts
0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050.99
1
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
4 MeV NACP10 MeV NACP15 MeV NACP
oo 200 100 253350 VOLTS
0
1/VOLTS
1/R
ead
ing
Recombination Correction fion
Actual: NACP Parallel Plate chamber
Recombination Correction fion
Actual: Markus Parallel Plate chamber
1/VOLTS
1/R
ead
ing
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050.99
1
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
4 MeV, Markus4 MeV NACP10 MeV NACP15 MeV NACP
1/VOLTS
1/R
ead
ing
oo 200 100 253350 VOLTS
Recombination - general equation
Dose per pulse (Gy)
0.0000 0.0002 0.0004 0.0006 0.0008 0.0010
kio
n (
at
100
V)
1.000
1.005
1.010
1.015
1.020
▪ Recombination is only a
function of dose per pulse
(different energy beams may
not have same dose per
pulse)
▪ A linear fit derived on one
Linac should be “universal”
▪ Measurement at a number of
clinics gave agreement
between fit and measured
correction at the 0.05% level
▪ Recombination is linearly
related to Dp
Fion = c + m x Dp
Polarity correction
▪ Compton electron current striking the collecting electrode can
add or subtract from signal current, depending on polarity
▪ Taking readings at +ve and -ve polarity will first add then
subtract these extra currents.
▪ Therefore the mean reading of + and – polarity represents the
true signal
Fpol = (|M+| + |M-|)/2M
M is reading with ‘normal’ polarity
M+ and M- readings with respective polarity
▪ In an electron beam polarity can change with depth (and
energy)
Polarity correction
▪ In an electron beam polarity effect can increase at lower
beam energies (or at greater depths).
▪ Electrons are then more likely to stop in the collecting
electrode as they are then more scattered and more oblique.
▪ This effect can be appreciable in parallel plate chambers but
is minimized by making the collecting electrode very thin.
▪ In a well designed chamber the effect should be <0.5%
▪ Bass et al “The calibration of parallel plate electron
ionisation chambers at NPL for use with the IPEM 2003
code of practice.” PMB, Vol 54, No 8, pN115-N124, 2009
Polarising voltage: conventional
NACP
EARTH NEGATIVE
-100V
TYPE B
FLYING LEAD
Polarising voltage: modern electrometerElectrometer floats at polarising volts
NACP
EARTH POSITIVE
+100V
TYPE A
Effective point of measurement
▪ The ion chamber introduces a bubble of air into a phantom
perturbing the fluence, and so it does not sample the true
fluence in the undisturbed medium
▪ The effective point of measurement is the position in the
medium where the fluence would be the same as that entering
the cavity
For parallel plate chambers that is just inside the front window
If a significant electron contribution is backscatter from the rear
chamber wall the Peff will shift towards the cavity centre
▪ For cylindrical chambers, the curved entry face means it is
somewhere inside the front face, but forward of the physical
centre of the chamber.
Effective Point of Measurement
EFFECTIVE DEPTHS
1.18x 1mm
NACP FARMER
1.7mm
0.125ccROOS
1.8mm1.7x 0.6mm 0.5mm
DIODE
Effective point of measurement
▪ Peff may be dealt with by;
Positioning the effective point of measurement of the chamber, or
Positioning at the chamber centre and introducing a displacement
correction
▪ In the UK the NPL photon calibration adopts the latter
approach
When performing absolute measurements always use the chamber
centre
Absorbed Dose based protocols
Very similar, but very important differences, each with pros and cons
TRS 398 based on Absorbed dose
measurement in Cobalt 60
Reference chamber calibration
NB. Ideally chambers should be calibrated in the
same or similar beam to that which is used clinically
▪ Summarised as calibration in 60Co and kQ factor
Tissue Phantom Ratio, TPR20,10
0.55 0.60 0.65 0.70 0.75 0.80 0.85
Cali
bra
tion
coef
fici
ent
CC
, G
y /
C x 1
07
9.9
10.0
10.1
10.2
10.3
10.4
15.7669
-27.7357
47.5655
-27.5083
Equation of calibration:
CC = a + b (TPR) + c (TPR)2 + d (TPR)3
b =
a =
d =
c =x 107
x 107
x 107
x 107
Nominal beam
energy (MV)
Quality Index,
TPR20/10
kQ
(60Co)
4
6
8
10
15
18
25
0.568
0.633
0.682
0.713
0.733
0.758
0.775
0.800
1.000
0.998
0.994
0.989
0.985
0.979
0.973
0.963
TRS 398 flexibility and choice of
chamber
IPEM 1990 much less comprehensive
and single recommended secondary
standard system
50 cm…….but now with choice
of multiple electrometers
that confirm to the later
ipem recommendations
(Phys Med Biol. 2000
Sep; 45(9):2445-57)
IPEM 1990 based
on calibration in
TPR 0.568 to 0.800
Dosimetry chain to the clinic
▪ Absorbed dose in a high-energy (MV) photon beam
▪ 1990 code of practice (also for TRS 398)
NPL primary
standard
NPL reference
chambersHospital secondary
standard
2611 ion chamber now
manufactured at NPL
Calibrated secondary
standard returned to hospital
Implementation checked by audit
Hospital field
instrument
Hospital secondary
standard
Linac output /
beam quality (TPR)lin nonelecionTPQ,Dw .f.N.f.fM.ND
w=
Audit – a vital part of the
dissemination of standards
Traceability via secondary standard calibration
consistent with MV CoP 1990
Traceability via secondary standard calibration
consistent with MV CoP 1990
(now incorporated in New 2020 CoP)
The problem with flexibility is it may
catch you out….
▪ Polarity effects and apparent ion recombination in
microionization chambers. Miller et al Med. Phys. 43
(5), May 2016
Proton & ion beam dosimetry
▪ Bring reference dosimetry onto the same level of uncertainty as
photon therapy.
▪ Establish primary standard for proton & ion beams
▪ IPEM code of practice on reference dosimetry for therapy level
proton beams
▪ Improve clinical dosimetry measurement through study &
development of such things as dosimeter characteristics and
water equivalent materials
Ratio of Dose from calorimeter to that derived
using TRS 398 calibration conversion
0.96
0.97
0.98
0.99
1.00
1.01
1.02
Dc
al/D
ion
0.98
0.99
1.00
1.01
1.02
1.03
1.04
NE2561 (Co-60)
NACP02 (Co-60)
Markus (Co-60)
NACP02 (e-19)
Markus (e-19)
modulated beam
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
non-modulated beam
It’s the basics that still catch
people out
e.g. Lessons from audit
– Labelling of equipment
– Valid Calibration
(eg Barometer, Thermometer)
– Care of equipment
– ***Uncertainties***
Dosimeter QC
▪ Strontium-90 check
(monthly?)
▪ Intercomparison with
standard (annual?) and other
systems
▪ Leakage
▪ Linearity
▪ Dosimeter interchange
▪ History
Sr-90 Sources
Constant Current Source for
Electrometer QC
HVL (mm Al)
0.1 1 10
Rel
ativ
e ca
libra
tion f
acto
r (e
xp
osu
re)
0.950
1.000
1.050
1.100
1.150
Autumn 1976
Autumn 1979
Before repair, 1982
After repair, 1982
Effect of contamination:
Care of equipment
NE2561 ionisation chamber
Effect of Corrosion:
HVL (mm)
0.1 1 10
Rel
ativ
e ca
lib
rati
on
fac
tor
(ex
po
sure
)
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
October 1982
October 1985
Before repair, 1989
After repair, 1989
NE2611 ionisation chamber
Chamber & case, one careful
owner.
Chamber & case, one careful
owner, part 2
Can you spot the problem with
this chamber ?
A Farmer type chamber with 2
obvious problems:
2 NACP chambers from different
manufacturers:
Packaging
problems
More packaging problems….
Read the manual, preferably the
right way up!
“Such an elusive sight had to be captured for posterity. A photo was taken
of a scientist taking a photo of another scientist who was taking a photo of
Russell Thomas hard at work, thus providing a chain of traceability for this
unusual phenomenon.”
Thank you.