1. AAPM TG-51 Protocol
(Med Phys 26: , 1999)
A protocol for clinical reference dosimetry of high-energy photon and electron beams
亞東紀念醫院 放射腫瘤科
蕭安成

2. For clinical reference dosimetry of external beam radiation therapy
AAPM TG-51 Protocol
For clinical reference dosimetry of external beam radiation therapy
photon beams with nominal energies between 60Co and 50 MV
electron beams with nominal energies between 4 and 50 MeV.
The protocol uses ion chambers with absorbed-dose-to-water calibration factors,
traceable to national primary standards
absorbed dose to water
The formalism is simpler than the TG-21, but it is applicable in water only.

3. For photon beams, with beam of quality Q,
General Formalism
For photon beams, with beam of quality Q,
M is corrected
For 60Co, kQ = 1,
only cylindrical chamber allowed at present
Converts beam quality (energy) from Co-60 to Q.

4. For electron beams, with beam of quality Q (R50),
General Formalism
For electron beams, with beam of quality Q (R50),
, only for cylindrical chambers, to correct for gradient effects.
, photon-electron conversion factor, chamber dependent,
, Converts electron energy from ecal to R50

5. Obtaining An Absorbed-dose To Water Calibration Factor
60Co source
d = 5 cm D
d = 5 cm
M (corrected) 
( Gy/C or Gy/rdg ), traceable to national
primary standards

6. Obtaining An Absorbed-dose To Water Calibration Factor

7. Obtaining An Absorbed-dose To Water Calibration Factor

8. Obtaining An Absorbed-dose To Water Calibration Factor
Chamber waterproofing
equivalent waterproofing techniques must be used for measurements in the user’s beam and in the calibration laboratory.
inherently waterproof
For nonwaterproof chambers
a waterproofing sleeve which minimizes air gaps near the chamber wall (< 0.2 mm) and it should be made of polymethylmethacrylate (PMMA) < 1 mm thick.

9. Measurement Phantoms Charge Measurement
Water phantom with dimensions of at least 303030 cm3.
Charge Measurement
fully corrected charge reading, M
M = PionPTPPelecPpolMraw (C or rdg),
Polarity corrections , should be  or 1.003

10. Electrometer correction factor
It is common practice in the US to calibrate ion chambers and electrometers separately.
If the electrometer is calibrated separately from the ion chamber, Pelec , corrects the electrometer reading to true coulombs.
Pelec = 1.00 if the electrometer and ion chamber are calibrated as a unit.

11. Corrections for ion-chamber collection inefficiency
Corrected for lack of complete collection efficiency.
Pion , must be  1.05
Voltages should not be increased above normal operating voltages just to reduce Pion
For continuous (i.e., 60Co)
For pulsed or pulsed-swept beams (i.e., Linac)

12. Beam Quality Specification
For cylindrical and spherical chambers the shift is taken
0.6rcav for photon beams
0.5rcav for electron beams
rcav : radius of the air cavity in a cylindrical ion chamber.

13. Point of Measurement and Effective Point of Measurement r
point of measurement
rcav
cylindrical
parallel plate
Photon: r = 0.6 rcav
electron: r = 0.5 rcav

14. Beam Quality Specification
For photon beams the depth-ionization curve is treated as a depth-dose curve
For electron beams, the depth-ionization curve must be further corrected for the significant change in the stopping-power ratio with depth to determine depth-dose curves

15. Beam Quality Specification
For photon beams, curve I : raw data, curve II: shifted data.
For electron beams, curve I : raw data, curve II: shifted data. curve II must be further corrected to obtain the percentage depth-dose curve.

16. Beam Quality Specification
For measurements of absolute dose at the reference depth in both electron and photon beams,
a cylindrical chamber’s point of measurement (center of the chamber)
The gradient effects are included implicitly in the beam quality conversion factor kQ for photons and
explicitly by the term for electrons.

17. Beam-quality specification for photon beams
%dd(10)x : PDD at 10 cm depth in water, FS = 1010 cm2, SSD = 100 cm, includes photon component only.
%dd(10) : PDD at 10 cm depth in water, FS = 1010 cm2, SSD = 100 cm, measured photon beam includes the effects of electron contamination in the beam.
%dd(10)pb : Same as %dd(10) except that a 1 mm lead foil is in place below the accelerator at about 50 cm from the phantom surface ( or 30 cm if 50 cm clearance is not available)

18. Beam-quality specification for photon beams
1010 cm2
100 cm
d =10 cm
dmax
%dd(10)
1010 cm2
100 cm
d =10 cm
dmax
%dd(10)pb
1 mm pb
50 cm
%dd(10)x = %dd(10), E < 10MV
%dd(10)x = [ %dd(10)pb] %dd(10)pb,
E ≥ 10MV, foil at 50 cm

19. Beam quality specification for electron beams
Beam quality in electron beams is specified by R50
Field size on the phantom surface 1010 cm2
(  2020 cm2 for R50  8.5 cm, i.e., E  20 MeV)
(cm)
2  I50  10 cm
I50  10 cm
I50 , ionization curve falls to 50% of its maximum

20. Either an SSD or an SAD setup can be used
Photon Beam Dosimetry
Reference conditions
1010 cm2
100 cm
d =10 cm
SAD setup
1010 cm2
100 cm
d =10 cm
SSD setup
Either an SSD or an SAD setup can be used

21. Photon Beam Dosimetry
Absorbed dose to water in clinical photon beams

22. Photon Beam Dosimetry
Absorbed dose to water in clinical photon beams

23. Electron Beam Dosimetry
Reference conditions
open beam
reference depth
dref = 0.6R (cm)
Field size
≥ 1010 cm2, R50  8.5 cm ( E  20 MeV )
≥ 2020 cm2, higher-energy FS
SSD=100 cm
dref
SSD setup

24. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams

25. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams

26. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams

27. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams
For Farmer-like cylindrical chambers,
2  R50  9 cm .
error  0.2%:

28. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams
For well-guarded plane-parallel chambers,
2  R50  20 cm .

29. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams
gradient effects, , for plane-parallel chambers
(for cylindrical chambers)

30. Electron Beam Dosimetry
Absorbed dose to water in clinical electron beams
Use of plane-parallel chambers
SSD
SSD FS FS
dref
dref
MCyl
Mpp

31. Summary - photons get a traceable
measure %dd(10)Pb with lead foil (shift depth if necessary)
deduce %dd(10)x for open beam from %dd(10)Pb
measure Mraw at 10 cm depth in water (no depth shift !!!)
M = PionPTPPelecPpol Mraw
lookup kQ for your chamber

32. Summary - electrons get a traceable
measure I50 to give R50 (shift depth if necessary)
deduce dref = 0.6 R cm (approx. at dmax)
measure Mraw at dref (no depth shift !!!)
M = PionPTPPelecPpol Mraw
lookup kecal for your chamber
determine (fig, formula)
establish (Mraw 2 depths)