1

DOSIMETRY PROTOCOLS

2

I. Introduction

First generation: SCRAD(1971), ICRU14, 21 (1969, 1972) Exposure calibration factor, Nx for ion chambers in 60Co beam Look up dose conversion factors vs. Nominal energy(cλ,CE) No special considerations in either the type of chamber used or

the actual quality of the beam Omission of these considerations led to errors up to 5% M: corrected to the standard environment

Second generation: TG-21(1983) Third generation: TG-51(1999)

λxw CMND

3

I. Introduction(con‘ t) First generation:SCRAD(1971), ICRU14, 21(1969, 1972) Second generation: TG-21(1983) Many of these problems were reduced at the expense of added

complexity Accuracy was better, but it required complex calculations The chamber dependent factors and their variation with beam

quality increased potential for errors in the clinic

Third generation: TG-51(1999)

wallreplionwgasgasw PPP)

ρ

L(MND

humidaircapen

capair

airwallen

wallair

wallionxgas K])/()/L)(()/()/L([

AA)e/w(KNN

1

4

I. Introduction(con‘ t) First generation:SCRAD(1971), ICRU14, 21 (1969, 1972) Second generation: TG-21(1983) Third generation: TG-51(1999) Requires absorbed dose to water calibration factors, Conceptually easier to understand and simpler to implement

Requires a quality conversion factor, kQ ,change in modality, energy, gradient

N CowD,

60

CowD,Q

Qw

60

NMkD

rawpolelecTPion MPPPPM

5

II. TG-21 protocol (Med Phys, 1983)

Part I: How to obtain the calibration factors Nx (exposure) and Ngas(dose to cavity gas) in a Co-60 beam.

if only Nx is provided by the standard laboratory, then the user needs to convert Nx to Ngas.

Part II: How to use Ngas in a user’s beam, which can be any modalities (photon or electron beams) and energies, and the chamber can be placed in a plastic medium.

Dose to cavity gas dose to medium dose to water

TG-21 protocol has two major components:

6

II. TG-51 protocol (Med Phys 26, 1847-70, 1999 )

The TG-51 protocol is based on “absorbed dose to water” calibration (also in a Co-60 beam)

The chamber calibration factor is denoted The calibrated chamber can be used in any

beam modality (photon or electron beams) and any energy, in water.

The formalism is simpler than the TG-21, but it is applicable in water only.

Co

w,DN

60

7

Suitable norminal energy photon beams: 60Co~50MV electron beams: 4~50MeV Ion chambers calibrated in terms of absorbed dose to water in a 60Co beam. Purpose to ensure uniformity of reference dosimetry in

external beam radiation therapy with high-energy photons and electrons.

II. TG-51 protocol (con‘ t)

8

Obtaining An Absorbed-dose To Water Calibration Factor

Is the absorbed dose to water (in Gy) in the calibration laboratory’s 60Co beam under standard environment.

first purchased, repaired, redundant checks suggest a need, once every two years.

check sources, regular measurements in a 60C0 beam, multiple independent dosimetry systems.

(Gy/C or Gy/rdg) M

DN

CowCo

wD,

60

60

CowD

60

9

In a Co-60 beam:

Co

w,DQ

Q

wNkMD

60

In any other photon beam:(only cylindrical chamber allowed at present)

)correctedisM(NMD Co

w,D

Co

w

6060

In any electron beam:(both cylindrical and parallel-plate chambers allowed)

Co

w,Decal

'

R

Q

gr

Q

wNkkPMD

60

50

General Formalism rawpolelecTPion MPPPPM

10

kQ values for cylindrical chambers in photon beams

NRC-CNRCNRC-CNRC

11

Measurement Phantoms

A water phantom with dimensions of at least 303030 cm3

12

Charge Measurement

Polarity corrections, Ppol

Electrometer correction factor, Pelec

rawpolelecTPion MPPPPM

raw

rawrawpol 2M

)M(MP

(C or rdg)

13

Charge Measurement(con‘ t)

Standard environmental condition: 22oC,101.33kPa,humidity between 20%~80%(variation

0.15%)

PTP one corrects charge or meter readings to standard environmental condition.

P

101.33

295.2

T273.2PTP

(C or rdg)rawpolelecTPion MPPPPM

14

Charge Measurement(con‘ t)

Corrections for ion-chamber collection inefficiency, Pion

if Pion 1.05, another ion chamber should be used Voltages should not be increased above normal

operating voltages. (~300V or less) 60Co pulse/swept beam

L

HLraw

Hraw

L

H

VV

M

M

VV

1

rawpolelecTPion MPPPPM (C or rdg)

2

L

HLraw

Hraw2

L

H )VV

(M

M)

VV

(1

15

Point of Measurement &Effective Point of Measurement

point of measurement

Effective point of

measurement

cylindrical parallel plate

rcav

r

Photon: r = 0.6 rcav

electron: r = 0.5 rcav

16

Beam Quality Specification (Photons)

CowD,Q

Qw

60

NMkD

For this protocol, the photon beam quality is specified by %dd(10)x, the percent depth-dose at 10 cm depth in water due to the photon component only, that is, excluding contaminated electrons.

For low energy photons (<10 MV with %dd(10) < 75%) %dd(10)x = %dd(10) (contaminated electron is negligible)

For high energy photons (>10 MV with 75%<%dd(10)<89%) %dd(10)x 1.267%dd(10) – 20.0

A more accurate method requires the use of a 1-mm thick lead foil placed about 50 cm from the surface. (505cm or 301cm)

%dd(10)x = [0.8905+0.00150%dd(10)pb] %dd(10)pb

[foil at 50 cm, %dd(10)pb>73%]

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Beam Quality Specification (electrons)

Percent depth ionization to be measured at SSD = 100 cm for field size 1010 cm2 (or 2020 cm2 for E>20 MeV).

Parallel-plate chamber: measured curve II.

Cylindrical chamber: measured curve I, needs to be shifted by 0.5 rcav to get curve II.

Curve II is the percent ionization curve.R50 = 1.029I50 – 0.06 (cm) for 2I50

10 cm

R50 = 1.059I50 – 0.37 (cm) for I50 > 10 cm

CowD,ecal

'R

Qgr

Qw

60

50NkkMPD

18

Beam Quality Specification

% depth-dose at 10 cm depth for curve shifted upstream by 0.6 rcav (photon), 0.5 rcav

(e-)

19

Beam Quality Specification(con‘ t)

20

kQ values for cylindrical chambers in photon beams

NRC-CNRCNRC-CNRC

21

kQ values for cylindrical chambers in photon beams

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Reference conditions

e- dref = 0.6 R50 - 0.1 cm photon dref = 10 cm

photon source

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Photon Beam Dosimetry

CowDQ

Qw NkMD

60

,

where

M is the fully corrected chamber reading,

kQ is the quality conversion factor,

is the absorbed dose to water chamber calibration factor

CowDN

60

,

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Electron Beam Dosimetry

CowDecalR

Qgr

Qw NkkPMD

60

50 ,'

where

M is the fully corrected chamber reading,

is the correction factor that accounts for the ionization gradient at the point of measurement (for cylindrical chamber only)

is the electron quality conversion factor.

kecal is the photon to electron conversion factor, fixed for a given chamber model

is the absorbed dose to water chamber calibration factor

CowDN

60

,

QgrP

'

50Rk

25

Electron Beam Dosimetry ( )

depends on user’s beam must be measured in clinic.

Cylindrical chamber:

parallel plate chamber:

0.1QgrP

QgrP

)(

)5.0(

refraw

cavrefrawQgr dM

rdMP

Same as shifting the point of

measurement upstream by 0.5rcav.

QgrP

CowD,ecal

'R

Qgr

Qw

60

50NkkMPD

26

Electron Beam Dosimetry ( Kecal )

Kecal is the photon-to-electron conversion factor for an arbitrary electron beam quality, taken as R50 = 7.5 cm.

The values of Kecal for a number of cylindrical and parallel-plate chambers are available in the TG-51 protocol.

CowD,ecal

'R

Qgr

Qw

60

50NkkMPD

27

'

50Rk

'

50Rk is the electron quality conversion factor converting from Qecal to Q.

'

50Rk for a number of cylindrical and parallel-plate chambers are available in Figs. 5-8 in the TG-51 protocol. It can also be calculated from the following expressions:

Cylindrical:

67.3' 50

500710.09905.0)( R

R ecylk

Parallel-plate:

214.050

' )(145.02239.1)(50

RppkR

Electron Beam Dosimetry ( )

CowD,ecal

'R

Qgr

Qw

60

50NkkMPD

28

k’R50 for Cylindrical Chambers

NRC-CNRCNRC-CNRC

29

k’R50 for Parallel Plate Chambers

NRC-CNRCNRC-CNRC

30

Sample Worksheet A: Photon Beam(< 10MV) Worksheet A: Photon Beam(> = < 10MV) Worksheet B: Electron Beams - Cylindrical Chambers Worksheet C: for plane-parallel chambers Worksheet D: Electron Beams using Plane-Parallel

Chambers

Worksheet

CowDN

60

.ecalk

31

Using Other Ion Chambers

Finding the closest matching chamber for which data are given.

Critical features are in order: Wall material Radius of the air cavity Presence of an aluminum electrode Wall thickness

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III. Discussion TG-51 emphasis in primary standards laboratories move from standards for exposure or air kerma to those for

absorbed dose to water clinic reference dosimetry is directly related to the quantity absorbed dose can be developed in linear accelerator absorbed dose to water have an uncertainty (1σ) of less than

1% in 60Co~25MV TG-51 must be performed in a water phantom measurements in plastics, including water-equivalent

plastics, are not allowed, does not preclude the use of plastic material for QA.

in-water calibrations must be performed at least annually.

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III. Discussion (con‘ t)

TG-21 combined both the theory and practical application

TG-51 serves only as a “how to” The results of clinical reference dosimetry in photon

beam will not change more than 1% compare to TG-21

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Relation betweenabsorb-dose and air-kerma standards

A theoretical relationship between absorbed-dose and air-kerma standards (or calibration factors) can be derived by comparing the absorbed-dose to water determined for a 60Co beam using the TG-51 and TG-21 protocols. Using kQ =1.0 and assuming Ppol~1.0, the following relationship for a 60Co beam can be obtained

Co

wD,Q

Q

w

60

NMkD

replwall

water

air

gaswaterPP

LMND

35

Relation betweenabsorb-dose and air-kerma standards (con‘t)

replwall

water

air

gas

Co

WD,Q

water

Q

w

PPρL

MN

NMk

DD

60

kQ =1.0

Ppol~1.0

1

PPρL

NN0.8791

N

N

DD

replwall

water

air

gas

x

k

WD,

water

w

replwall

water

airx

gas

k

wD, PPρL

N0.8791

N

N

N

water

air

replwall

x

gas1

k

wD,

ρL

PPN

N0.8791

N

N

36

Relation betweenabsorb-dose and air-kerma standards (con‘t)

water

air

replwall

x

gas1

k

wD,

ρL

PPN

N0.8791

N

N

Note that no measured value is necessary. All the required numerical values can be obtained from the TG-21 report. The ratio also be directly determined using the absorbed-dose and air-kerma calibration factors measured at the calibration laboratory.

37

Compare Dose & Kerma Standards

water

air

replwall

x

gas1

k

wD,

ρL

PPN

N0.8791

N

N

- where Prepl, Pwall and L/ρ values were taken from TG-21 data and Ngas/Nx comes from Gastorf et al.

· The equation used to determine ND,W/NK is as follows:

38

Co

wD,Q

Q

w

60

NMkD

rawpolelecTPionMPPPPM

Co

wD,ecal

'

R

Q

gr

Q

w

60

50NkkMPD

TG-51 TG-21

rawelecTPionMPPPM

replwall

water

air

gaswaterPP

LMND

repl

water

air

gaswaterP

LMND

39

Photon Equations

Co

wD,QrawpolelecTPion

Q

w

60

NkMPPPPD TG-51

TG-21replwall

water

air

gasrawelecTPionwaterPP

LNMPPPD

· The equations used to determine absorbed dose for photons are as follows:

40

max

ref

ddd%

ddd%repl

water

air

gasrawelecTPionwaterP

LNMPPPD

Electron Equations

Co

wD,ecal

'

R

Q

grrawpolelecTPion

Q

w

60

50NkkPMPPPPD TG-51

TG-21

· The equations used to determine dose for electrons are as follows:

Both values are absorbed dose at dref.

41

TG-51 Photon Measurements

Search for dmax (apply a 0.6 rcav shift to effective point of measurement).

Place chamber at 10cm + 0.6 rcav to determine PDD at 10 cm.

Determine kQ value using the data from TG-51 report.

Move chamber to calibration depth, 10 cm (without a 0.6 rcav shift).

Make measurements for Polarity Correction (+ 300 and 300 Volts) and Pion (- 150 Volts).

Polarity Correction was less than 0.2% for all chambers studied.

42

TG-51 Electron Measurements

Search for Imax and I50 (apply a 0.5 rcav shift to effective point of measurement).

- From these; determine R50 and dref.

- From R50; k’R50 is determined.

Move center of chamber to the calibration depth, dref (without a 0.5 rcav shift).

Measure Polarity Correction (+ 300 and – 300 Volts) and Pion (- 150 Volts).

Move center of chamber to dref + 0.5rcav and calculate the gradient correction.

Polarity correction was less than 0.2% for all chambers studied.

43

RESULTSComparison Between Absorbed Dose and Air Kerma Calibration

44

The 1.2% discrepancy between the measured and calculated values represents a basic discrepancy between the Air Kerma and Absorbed dose standards and the TG-21 formalism used to convert Air Kerma into Dose.

NIST and NRC Canada are aware of a 0.7% difference between their two Air Kerma standards. This difference is in the right direction to suggest that ND,w / NK for NRC Canada should be closer to unity.

Comparison Between Absorbed Dose and Air Kerma CalibrationRESULTS

45

Comparison Between TG-51 and TG-21 Calibrations (photons)RESULTS

46

Comparison Between TG-51 and TG-21 Calibrations (electrons)RESULTS

47

Conclusions

Standards:

There is an apparent 1% discrepancy between the Absorbed Dose Standard and the Air Kerma Standard (converted to dose using TG-21).

Photons:

Institutions, which calibrate at dmax for TG-21, may see a different TG51/TG21 ratio, due to the difference in the determination of %dd for TG-51 and TG-21.

The 1% discrepancy in the standards is reflected in a 1% discrepancy between TG-51 and TG-21 for x-rays.

Comparison Between TG-51 and TG-21 Calibrations (electrons)

48

Electrons:

In addition to the 1% discrepancy in standards there is an additional discrepancy for electrons, partly due to new stopping power data used for the TG-51 protocol.

The magnitude of changes in electron beam output may depend on the electron energy.

Comparison Between TG-51 and TG-21 Calibrations (electrons)Conclusions

49

Ratios of absorbed dose to water for TG-51 and TG-21 protocols.

Dwater = M raw × PTP × CF

Stated energy CF ( cGy/nC) variation ( % )

TG-51 TG-216 MV photon beam 4.530 4.519 0.25

10 MV 4.487 4.495 -0.18

6 MeV electron beam 49.243 49.918 -1.35

8 MeV 49.098 49.597 -1.01

10 MeV 48.896 49.175 -0.57

12 MeV 48.755 48.975 -0.45

15 MeV 48.493 48.505 -0.02

18 MeV 48.005 48.009 -0.01

Experience

50

IV. Conclusions

Omissions of the type of chamber used or the actual quality of the beam in SCRAD led to errors up to 5%.

There are some trends which indicate that the AAPM protocol improves the consistency of the calculation of dose for the various dosimetry system use.

TG-21is more consistently than SCRAD in the various chamber and phantom combinations.

51

IV. Conclusions (con‘ t)

The results of clinical reference dosimetry in TG-51 photon beam will not change more than 1% compare to TG-21.

This protocol represents a major simplification compares to the AAPM’s TG21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors.

52

The comparison shows approximately a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed-dose to water determined by TG-51 and that determined by TG-21 for photon beam calibrations.

IV. Conclusions (con‘ t)

53

The typical change in a 6 MV photon beam calibration following the implementation of the TG-51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG-51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy.

IV. Conclusions (con‘ t)

54

It is recommended that a first-time-TG-51-user should obtain both the absorbed dose and air-kerma calibration actors on the same chamber, and perform an initial comparison as described in this study to estimate the inherent discrepancy expected from the implementation of the TG-51 protocol

IV. Conclusions (con‘ t)