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)

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)

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

5. II. TG-21 protocol (Med Phys, 1983)
TG-21 protocol has two major components:
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

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.

7. II. TG-51 protocol (con‘t)
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.

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)

9. General Formalism In a Co-60 beam: In any other photon beam:
(only cylindrical chamber allowed at present)
In any electron beam:
(both cylindrical and parallel-plate chambers allowed)

10. kQ values for cylindrical chambers in photon beams
NRC-CNRC

11. Measurement Phantoms
A water phantom with dimensions of at least 303030 cm3

12. Charge Measurement (C or rdg) Polarity corrections, Ppol
Electrometer correction factor, Pelec

13. Charge Measurement(con‘t)
(C or rdg)
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.

14. Charge Measurement(con‘t)
(C or rdg)
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

15. Effective point of measurement r
point of measurement
rcav
cylindrical
parallel plate
Photon: r = 0.6 rcav
electron: r = 0.5 rcav

16. Beam Quality Specification (Photons)
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. (505cm or 301cm)
%dd(10)x = [ %dd(10)pb] %dd(10)pb
[foil at 50 cm, %dd(10)pb>73%]

17. Beam Quality Specification (electrons)
Percent depth ionization to be measured at SSD = 100 cm for field size  1010 cm2 (or 2020 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 2I50  10 cm
R50 = 1.059I50 – 0.37 (cm) for I50 > 10 cm

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-CNRC

21. kQ values for cylindrical chambers in photon beams

22. Reference conditions e- dref = 0.6 R50 - 0.1 cm photon dref = 10 cm
photon source

23. Photon Beam Dosimetry where M is the fully corrected chamber reading,
kQ is the quality conversion factor,
is the absorbed dose to water chamber calibration factor

24. Electron Beam Dosimetry
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

25. Electron Beam Dosimetry ( )
depends on user’s beam must be measured in clinic.
Cylindrical chamber:
Same as shifting the point of measurement upstream by 0.5rcav.
parallel plate chamber:

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.

27. Electron Beam Dosimetry ( )
is the electron quality conversion factor converting from Qecal to Q.
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:
Parallel-plate:

28. k’R50 for Cylindrical Chambers
NRC-CNRC

29. k’R50 for Parallel Plate Chambers
NRC-CNRC

30. Worksheet Sample 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

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

32. 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.

33. 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

34. Relation between absorb-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

35. Relation between absorb-dose and air-kerma standards (con‘t)
kQ =1.0 Ppol~1.0

36. Relation between absorb-dose and air-kerma standards (con‘t)
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
· The equation used to determine ND,W/NK is as follows:
- where Prepl, Pwall and L/ρ values were taken from TG-21 data and Ngas/Nx comes from Gastorf et al.

38. TG-51
TG-21

39. Photon Equations TG-51 TG-21
· The equations used to determine absorbed dose for photons are as follows:
TG-51
TG-21

40. Electron Equations TG-51 TG-21
· The equations used to determine dose for electrons are as follows:
Both values are absorbed dose at dref.
TG-51
TG-21

41. TG-51 Photon Measurements
· Search for dmax (apply a 0.6 rcav shift to effective point of measurement).
· Place chamber at 10cm 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 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. RESULTS
Comparison Between Absorbed Dose and Air Kerma Calibration

44. RESULTS Comparison Between Absorbed Dose and Air Kerma Calibration
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.

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

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

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

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

49. 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. IV. Conclusions (con‘t)
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.

53. IV. Conclusions (con‘t)
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.

54. IV. Conclusions (con‘t)
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