AAPM TG-51 IAEA TRS 398

Samir Laoui

University of California, Irvine

Introduction

Methodology for clinical reference dosimetry

Uses Dosimeters Traceable to national Standards

National protocol for calculation of dose at a reference point

Based on an absorbed dose to water instead of exposure or air-Kerma

Scope

TG-51: Protocol for clinical reference dosimetry for external beam dosimetry

Photons: Co-60 50 MV

Electrons: 4 MeV 50 MeV

Uses water phantom

Uses ion chambers calibrated in terms of adsorbed-dose-to-water in Co-60 beam

Summary: Photon Beams

I. Select a Chamber

II. 𝑁𝐷,𝑊𝐶𝑜−60 : From an Accredited Dosimetry

Calibration Laboratory (ADCL)

I. 𝐾𝑄 vs. Q: Need to measure beam quality

II. Measure M, the charge reading

III. Ready to measure the absorbed dose to water at the reference depth

𝐷𝑊𝑄

= 𝑀𝐾𝑄𝑁𝐷,𝑊𝐶𝑜−60

Summary: Electron Beams

I. Select a Chamber

II. 𝑁𝐷,𝑊𝐶𝑜−60 :ADCL

III. TG-51 provides calculated values of photon-electron conversion factor Kecal (Table lookup)

IV. K’R50 vs. R50: Measure Beam Quality

V. Measure 𝑃𝑔𝑟𝑄

VI. Measure M

VII.Ready to measure the absorbed dose

to water at the reference depth

Co

WDecalR

Q

gr

Q

W NKKMPD60

50 ,

'

TG-51 Based formalism

For a known: 𝑁𝐷,𝑊𝑄

𝐷𝑊𝑄

= 𝑀𝑁𝐷,𝑊𝑄

For any user quality Q:

𝐷𝑊𝑄

= 𝑀𝐾𝑄𝑁𝐷,𝑊𝐶𝑜−60

KQ is the correction factor that accounts for the differences between the Co-60 and the user beam and is chamber specific

Theoretical determination of KQ

Is the ratio of beam Quality Q to Co-60 of the water to air stopping power ratios including various perturbation factors

Valid for cylindrical and plane-parallel chambers

Co

celgrflwall

water

air

Q

celgrflwall

water

air

Q

PPPPL

PPPPL

k

60

Photon Beams

Electron Beams

Co

WDQ

Q

W NKMD60

,

Co

WDecalR

Q

gr

Q

W NKKMPD60

50 ,

'

KQ

AAPM TG-51: Reference Conditions

Photons: Beam Quality: %dd at 10 cm (no electron contamination)

Reference depth: 10 cm, 10x10 cm2 field size, in either an 100 cm SSD at phantom surface or 100 cm SAD at the depth of the detector.

Electrons: Beam Quality: R50

Reference depth: At dref = 0.6 R50 – 0.1 cm

Performed with a field size ≥ 10x10 cm2,

≥ 20x20cm2 for R50 > 8.5 cm

M is the fully corrected ion chamber reading 𝑀 = 𝑃𝑖𝑜𝑛𝑃𝑇𝑃𝑃𝐸𝑙𝑒𝑐𝑃𝑝𝑜𝑙𝑀𝑟𝑎𝑤

𝑃𝑖𝑜𝑛: Corrects for ion recombination (Ion-chamber collection inefficiency correction factor)

𝑃𝑇𝑃: Corrects for temperature and pressure

𝑃𝐸𝑙𝑒𝑐:Corrects for inaccuracy if the electrometer is calibrated separately

𝑃𝑝𝑜𝑙: Corrects for chamber polarity effects

𝑀𝑟𝑎𝑤: is the uncorrected ion chamber reading

Correction factors

1.003 ~ 0.997

2

)(

raw

rawrawpol

M

MMP

radiation continuousfor 2

radiation pulsedfor 1

where,

051

1

X

.

V

V

M

M

V

V

PX

L

H

L

raw

H

raw

X

L

H

ion

(kPa)

33101

0.222.273

)C(2273

o

P

. T .PTP

Photons: KQ for cylindrical chambers 𝐾𝑄 is the quality conversion factor which converts the calibration factor

for a Co-60 beam to that of a beam of quality Q

Photons: KQ for cylindrical chambers

Photon Beam Quality Specification

Photon beam quality is specified by %dd(10)x in water phantom due to photons only. 10x10 cm2 at phantom surface at an SSD of 100 cm.

For cylindrical ion chambers, the % depth-ionization should be shifted by 0.6 of the cavity radius

No shift is required for plane-parallel ion chambers

Electron Beam Quality Specification

Is specified by R50

To determine R50, measure the PDD at 100 SSD. In a cylindrical chamber, correct for gradient effect by shifting the curve by 0.5 radius

No shift is required if a plane-parallel chamber

is used

cm) 10(for (cm) 37.0 059.1

cm) 10 2(for (cm) 06.0 029.1

5050

5050

50

II

II

R

Electron: Absorbed dose to water

𝐷𝑊𝑄

= 𝑀𝐾𝑄𝑁𝐷,𝑊𝐶𝑜−60

Where

𝐾𝑄 = 𝑃𝑔𝑟𝑄

𝐾𝑅50 And 𝐾𝑅50

= 𝐾𝑅50

′ 𝐾𝑒𝑐𝑎𝑙

For Electron beams with 𝑅50 ≤4.3 cm (10 MeV or less), and higher energies, a plane-parallel chambers are preferred

For beams with 𝑅50 ≤ 2.6 cm (6 MeV or less), plane-parallel chambers must be used

Electrons conversion factors

𝐾𝑄 = 𝑃𝑔𝑟𝑄

𝐾𝑅50 = 𝑃𝑔𝑟

𝑄 𝐾𝑅50

′ 𝐾𝑒𝑐𝑎𝑙

𝐾𝑅50

′ : Electron Quality conversion factor

𝐾𝑒𝑐𝑎𝑙: Photon-electron conversion factor

𝑃𝑔𝑟𝑄

= Correction gradient at the point of calibration (dref), is necessary

only for cylindrical chambers.

chamber parallel-planefor 1

chamber lcylindricafor )(

) 5.0(

refraw

cavrefraw

Q

gr

dM

rdM

P

Cylindrical Chambers: K’ R50

Figure 5: K’R50 at low electron energies for

cylindrical chambers

Figure 7: K’R50 at high electron energies for

cylindrical chambers

Parallel Plate Chambers: K’ R50

Figure 6: K’R50 at low electron energies for

plane-parallel chambers

Figure 8: K’R50 at high electron energies for

plane-parallel chambers

Rationale for an update to TG-51

The majority of chambers available today do not have KQ

factors listed in TG-51

User can obtain Linac-based absorbed dose calibration coefficients for ion chambers from ADCL

Monte Carlo radiation transport algorithms now allow accurate modelling for ion chamber geometries

New beam parameters were introduced (e.g., FFF)

Addendum to TG-51 outline

Pertains to Megavoltage photon beams

Calculated KQ factors for new chambers are presented

Comparison of calculated and measured KQ factors

Uncertainty analysis for implementation of TG-51 (Tables II and III)

Recommendation of implementation

Comparison between

AAPM TG-51 & IAEA TRS-398

Comparison between

AAPM TG-51 & IAEA TRS-398

Q

Co

WD

Q

W KNMD 60

,

TG-51 (Eq. 3):

IAEA (Eq. 2):

TG-51 (Eq. 8):

IAEA (Pg. 82 & Pg. 108):

ionpolelecTPraw PPPPMM

For continuous radiation:

IAEA (Eq. 16):

2

2

)/(/

)/(1)(

LH

L

raw

H

raw

LHHion

VVMM

VVVP

TG-51 (Eq. 11):

For pulsed radiation: Same if Pion~ 1

IAEA (Eq. 15):

LH

L

raw

H

raw

LHHion

VVMM

VVVP

//

/1)(

TG-51 (Eq. 12):

Photon & Electron Beam Quality Q

IAEA (Pg. 68):

TG-51 (Eqs 13 & 14):a

xdd )10(%

TPR20,10

Photons Electrons

TG-51 (Eqs 16 & 17):

50R

R50

IAEA (Pg. 86 & Eq. 23):

Beam Quality Conversion Factor

KQ – Photon Beams

IAEA (Table 14):

Table Lookup to find KQ based on TPR20,10

TG-51 (Table I):

Table Lookup to find KQ based on %dd(10)x

Beam quality specification Per IAEA-398

TPR20,10

The ratio of doses on the beam central

axis at the depths of z=20 cm and z=10

cm in water obtained at an SAD of 100

cm and 10 x10 cm2

Comparison TG-51 vs. TRS-398

TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams

TRS-398: The code of practice includes dosimetry recommendations and standardized procedures for the dosimetry of therapeutic beams of low-and medium-energy x-rays, 60Co gamma-rays, high-energy photons, electrons, protons and heavy ions. It also includes all the various ionization chamber calibration possibilities available in different national standards laboratories, from 60Co to direct calibrations in high energy photon and electron beams

AAPM TG-51 protocol is used predominantly in North America, it is expected that the IAEA TRS-398 code of practice will predominantly be used throughout the rest of the world, notably in Europe