1. Measurement of Absorbed Dose (6)
CALIBRATION OF MEGAVOLTAGE BEAMS: TG-21 PROTOCOL
A protocol for the determination of absorbed dose from high energy photon and electron beams

2. TG-21 Protocol (Med Phys 10:741-771, 1983)
Exposure calibration factor (Nx)
Cavity Gas calibration factor (Ngas)
(In Co-60 beam)
Part I
(any radiation quality and energy)
med
med
water x x x
gas
Dose to cavity gas Dgas
dose to medium Dmed
Dose to medium Dmed
dose to water Dwater
Part II
Part III

3. Part I Exposure calibration factor (Nx) (In Co-60 beam)
Cavity Gas calibration factor (Ngas)

4. Nx : 60Co Exposure Calibration Factor of a Chamber
60Co source
M is the charge collected, corrected for temperature and pressure
X is the exposure (in Roentgen) at the point of measurement × X
M (22C, 760 mm Hg)
NX is R/C (or R/reading), obtained in a primary or secondary standard laboratory.

5. Ngas : Cavity Gas Calibration Factor of a Chamber
60Co source
Dgas, dose to cavity gas × X
M (22C, 760 mm Hg)
Q and M are, respectively, the charge produced and collected, corrected for temperature and pressure. They are related by the collection efficiency Q=M/Aion

6. Characteristics of Ngas
Ngas only depends on the volume or the mass of the air in the chamber cavity. It is independent of the type or energy of the incident radiation. Once obtained, its value remains the same unless the physical properties are changed.
If we know the volume (or mass) of the air in the chamber cavity, then we know Ngas. Unfortunately, the chamber volume cannot be accurately measured. Instead, Ngas is calculated from Nx.

7. Conversion from Nx to Ngas For commonly used ionization chambers
TG-21: Table XVII

8. Conversion from NX to Ngas
60Co source
Photon
energy fluence
electron fluence × X
Dwall
Dgas
k = 2.58x10-4 C/kg-R
Slide #9

9. Conversion from NX to Ngas (cont’d)
Slide #8
Slide #10

10. Conversion from NX to Ngas (cont’d)
cap
wall
Slide #9
is the fraction of ionization due to electrons from the wall.
(1-) is the fraction of ionization due to electrons from the cap.

11. Conversion from NX to Ngas (cont’d)
k = 2.58x10-4 C/kg-R
bwall = 1.005
If wall-material = cap-material, a = 1.0,
If wall-material ≠ cap-material, obtain a from TG-21: Fig.1

12. Conversion from NX to Ngas (cont’d)
use TG-21: Table I

13. Conversion from NX to Ngas (cont’d)
Aion provided by the standard lab
Awall use TG-21: Table II or Table III

14. Conversion from air-kerma calibration factor Nk to
exposure calibration factor Nx x

15. Part II
med
med x x
gas
Dose to cavity gas Dgas
dose to medium Dmed

16. Photons & electrons any energy
Dose-to-cavity gas
Photons & electrons any energy
Dgas
Chamber cavity gas calibration factor
Chamber reading needs corrections

17. Corrections to Chamber Reading
Reading for an unsealed chamber generally needs to be corrected for temperature, pressure, polarity effect, and ion-recombination correction factor Pion(the inverse of collection efficiency Aion).
Pion can be obtained using the two-voltage method:
Q1 is the charge collected at voltage V1.
Q2 is the charge collected at voltage V2= V1 /2.
Pion can be obtained through the ratio Q1/ Q2. (TG-21: Fig.4)

18. TG-21: Fig.4

19. Conversion from Dose-to-cavity gas to Dose-to-medium
Photons & electrons any energy
(thin wall, or wall material same as medium) ×
Dgas
Dmed

20. Conversion from Dose-to-air to Dose-to-medium (cont’d)
Dependent on beam quality & energy

21. Ratios of mean, restricted mass collision stopping powers
Photon beams
(TG-21: Fig.2, Table IV)

22. Characterization of Beam Quality (photons)
For photon beams, energy is characterized by the ionization ratio of a 10x10 cm field:
d = 20 cm
d = 10 cm
I20
I10 OR

23. Electron beams (TG-21: Table V-VII for water/air, polystyrene/air, acrylic/air)
Ratios of mean, restricted mass collision stopping powers

24. Characterization of Beam Quality (electrons)
For electron beams, energy is characterized by:
100%
ionization
50%
Depth in water (cm)
d50

25. Replacement correction factor: Prepl
For parallel-plate chambers: Prepl = 1.00
point of measurement x x
gas
med
med

26. Replacement correction factor: Prepl (Photon beams)
For cylindrical chambers: d x x
gas
med
med
point of measurement
Photon beams (gradient corrections)
If d = dmax, Prepl = 1.0
If d > dmax, use TG-21: fig.5

27. Replacement correction factor: Prepl (Electron Beams)
For cylindrical chambers:
dmax
point of measurement x x
gas
Electron beams (fluence corrections)
med
med
d = dmax, use TG-21: Table VIII

28. Conversion from Dose-to-air to Dose-to-medium
Photons (Wall material different from medium)
Thick wall:
All ionization produced by electrons arising in the wall
wall
wall × ×
Dgas
Dwall
Dmed
med
med
med

29. Conversion from Dose-to-air to Dose-to-medium
Photons (Wall material different from medium)
a is the fraction of total ionization produced by electrons arising in the wall, (1- a) the fraction arising in the medium

30. Wall Correction Factor: Pwall
For electron beams: Pwall = 1.00
For photon beams:
‘a’ is the fraction of total ionization produced by electrons arising in the wall, (1- a) the fraction arising in the medium.
use TG-21, Fig.7

31. Wall Correction Factor: Pwall
For electron beams: Pwall = 1.00
For photon beams:

32. (note: chamber is not involved in this part.)
Part III x x
med
water
Dose to medium Dmed
dose to water Dwater
(note: chamber is not involved in this part.)

33. Conversion from Dose-to-medium to Dose-to-water
(photons)
Same source-to-detector distance,
same field size,
But scale depth by: SF = dmed/dwater
(TG-21: Table XIII)
dwater
dmed × ×
Dmed
Dwater
med
water
Under conditions of electron equilibrium:

34. Conversion from Dose-to-medium to Dose-to-water
(photons)

35. Ratios of mass energy absorption coefficients (photons)
Use TG-21: Table XII

36. Excess scatter correction (ESC) (photons)
for polystyrene: ESC = 1.0
for acrylic: use TG-21, Table XIV

37. Dose to water at dmax: Dwater(dmax)
(photons)
SSD setup
SAD setup d
isocenter
dmax ×
dmax
isocenter × d
water ×
water

38. Conversion from Dose-to-medium to Dose-to-water (electrons)
dmax
dmax × ×
Dmed
Dwater
plastic
water

39. Conversion from Dose-to-medium to Dose-to-water (electrons)

40. Ratios of mean, unrestricted collision mass stopping powers
(electrons)
Use TG-21: Table XV
Ratios of electron fluences at dmax:
For acrylic:
For polystyrene:
Use TG-21: Table XVI

41. TG-21 Summary photon electron Dwater(dmax) dose to water Dgas
dose to chamber cavity gas
Dmed
dose to phantom medium
Dwater
dose to water
1.00 (polystyrene) Table XIV (acrylic)
Table XII
photon
Fig.5 (photon) Table VIII (electron)
electron
Fig.7, fig.2 or table IV, table IX (photon) 1.00 (electron)
Fig.2, Table IV (photon) Tables V-VII (electron)
Table XVI (polystyrene) 1.00 (acrylic)
1.030 (polystyrene) (acrylic)
Fig.4

42. Miscellaneous

43. For Output Calibration
point of measurement
Inner surface of the front plate of Parallel-plate chamber
Center of cylindrical chamber
(the effect of displacement is included in Prepl)

44. Equipment Needed
Ion chamber and electrometer, need calibration every 2 years.
calibration traceable to standard laboratory (primary standard lab: NIST, secondary standard labs).
cylindrical chamber for photons of all energies and electrons with energies  10 MeV.
plane-parallel chamber for electrons of all energies (mandatory for energies < 10 MeV).
system to measure air pressure and water temperature

45. 8.4 The Bragg-Gray Cavity Theory (C- effective point of measurement)
For parallel plate chambers, the effective point of measurement is at the inner face of the front plate
Effective point of measurement
For cylindrical chambers, the effective point of measurement is displaced 0.85r from the center
Effective point of measurement
0.85r

46. 8.4 The Bragg-Gray Cavity Theory (C- effective point of measurement)
Tracklength of each electron entering through ds  amount of ionization produced
Number of electrons entering the circle through ds
Total amount of ionization produced due to electrons entering through ds
Area perpendicular to electron fluence ds  r x d 2x
Xeff = 8r/3p = 0.85r

47. Depth Dose measurement with a cylindrical chamber
100
Before shifting 80 60
dose
After shifting 40 20 5 10 15 20
Depth in water (cm)

48. Depth Dose measurement with chamber
For photon beam, depth-ionization = depth-dose
Photon energy spectrum does not change much with depth. Consequently, the energy spectrum of the secondary electrons (generated by the photons) also does not change much with depth. Thus, the stopping power ratio remains (nearly) the same with depth.
For electron beam, depth-ionization ≠ depth-dose
Electron energy decreases with depth in water ( ~ 2 MeV/cm ), therefore the stopping power ratio changes with depth. Stopping power ratio at different energy/depth needs to be applied to convert ionization to dose. (note: if diode is used for electron depth-dose measurement, no stopping power ratio correction is needed, because does not change with energy)