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DOSIMETRY PROTOCOLS

DOSIMETRY PROTOCOLS. I. Introduction. First generation: SCRAD(1971), ICRU14, 21 (1969, 1972) Exposure calibration factor , N x for ion chambers in 60 Co beam Look up dose conversion factors vs. Nominal energy(c λ ,C E )

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DOSIMETRY PROTOCOLS

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  1. DOSIMETRY PROTOCOLS

  2. I. Introduction • First generation: SCRAD(1971), ICRU14, 21 (1969, 1972) • Exposurecalibration factor, Nx for ion chambers in 60Co beam • Look up dose conversion factors vs. Nominal energy(cλ,CE) • No special considerations in either the typeof chamber used or the actual qualityof 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. (C or rdg) Charge Measurement • Polarity corrections, Ppol • Electrometer correction factor, Pelec

  13. (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. (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) • 60Copulse/swept beam

  15. Point of Measurement & Effective Point of Measurement  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= [0.8905+0.00150%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- 0.1cm • 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. 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 Electron Beam Dosimetry

  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 arbitraryelectron beam quality, taken as R50 = 7.5 cm. The values of Kecalfor 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 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 arein 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 betweenabsorb-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 betweenabsorb-dose and air-kerma standards (con‘t) kQ=1.0 Ppol~1.0

  36. Relation betweenabsorb-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 · The equations used to determine absorbed dose for photons are as follows: TG-51 TG-21

  40. Electron Equations · 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 rcavshift to effective point of measurement). · Place chamber at 10cm + 0.6 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 - 300 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.

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