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

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


▪ 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


▪ 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

dosimetry, detectors and traceability

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