Download - AAPM TG-51 SUMMARY
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