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The TG-51 protocol (Med Phys 26, 1847-70, 1999)

The 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 .

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The TG-51 protocol (Med Phys 26, 1847-70, 1999)

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  1. The 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.

  2. Equipment Needed • Ion chamber and electrometer • calibration traceable to standard laboratory • waterproofing for ion chamber ( if needed) <1mm PMMA • water phantom (at least 30x30x30 cm3) • lead foil for photons 10MV and above • 1 mm  20% • system to measure air pressure and water temperature

  3. Obtain an Absorbed-dose to Water Calibration Factor 60Co source dmax  D M (corrected) Dose to water per unit charge (reading)

  4. Quality Conversion Factor Ideally, for a given chamber individual calibration factor should be obtained for each beam quality Q used in the clinic. So that: This is impractical, as the standard laboratory may not have the particular beam quality Q available, thus a quality conversion factor kQ is introduced to convert the calibration factor for Co-60 to that for the beam quality Q.

  5. General Formalism In a Co-60 beam: In any other photon beam Q: (only cylindrical chamber allowed at present) Converts beam quality (energy) from Co-60 to Q. In any electron beam R50: (both cylindrical and parallel-plate chambers allowed) Converts modality from photon to electron. Gradient correction for cylindrical chambers Converts electron energy from ecal to R50.

  6. Charge measurement Polarity correction Press/temperature Pelec Electrometer correction Ion recombination correction

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

  8. Determination of Photon Beam Quality Q TG-21: photon beam quality (energy) determined by the ionization ratio (d=20cm to d=10cm) TG-51: photon beam quality (energy) determined by %dd(10)x, the percent depth dose at d=10cm due to photons only.

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

  10. Percent Depth-Dose (ionization) for photon beams Percent depth dose to be measured at SSD = 100 cm for a 1010 cm2 field size. Parallel-plate chamber: measured curve II. Cylindrical chamber: measured curve I, needs to be shifted by 0.6 rcav to get curve II. Curve II is the percent dose (percent ionization) curve, including contaminated electrons. 100 II I 80 % depth-dose (ionization) 60 %dd(10) 40 20 5 10 15 20 Depth in water (cm)

  11. 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. %dd(10)x= [0.8905+0.00150%dd(10)pb] %dd(10)pb [foil at 50 cm, %dd(10)pb>73%]

  12. Percent depth dose measured at SSD = 100 cm for a 1010 cm2 field size with a 1mm Pb filter placed at ~50 cm from the surface. 1mm Pb filter 100 II I (cylindrical chamber) 80 (% dd)Pb 60 %dd(10)Pb 40 20 ~50 cm 5 10 15 20 Depth in water (cm) Curve II is the (%dd)Pb curve, with the contaminated electrons in the original beam removed, but generates its own contaminated electrons. %dd(10)x= [0.8905+0.00150%dd(10)pb] %dd(10)pb

  13. Reference conditions for Photon Beam Calibration photon source 100 cm 10 cm 1010 cm2 1010 cm2 10 cm water water SSD setup SAD setup

  14. Photon Beam Dosimetry where M is the fully corrected (temperature, pressure, polarity, recombination) chamber reading, kQis the quality conversion factor, is the absorbed dose to water chamber calibration factor

  15. Reference conditions for Electron Beams Electron source Depth: dref = 0.6 R50 - 0.1 cm where R50 is the depth in water at which the dose is 50% of the maximum dose. dref is approx. at dmax Field size:(why different field sizes for different energies?) > 10x10 cm2 on surface R50<8.5 cm > 20x20 cm2 on surface R50>8.5 cm SSD: as used in clinic between 90 cm and 110 cm (typically 100 cm) SSD dref water SSD setup

  16. Percent Depth-ionization for electron beams 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. 100 II I 80 % depth-ionization 60 40 I50 20 R50 = 1.029I50 – 0.06 (cm) for 2I50 10 cm R50 = 1.059I50 – 0.37 (cm) for I50 >10 cm 2 4 6 8 Depth in water (cm)

  17. Electron Beam Dosimetry ( ) depends on user’s beam must be measured in clinic. parallel plate chamber: 100 80 dref+0.5rcav % depth-dose 60 cylindrical chamber: dref 40 20 Same as shifting the point of measurement downstream by 0.5rcav. 2 4 6 8 Depth in water (cm)

  18. Electron Beam Dosimetry (Kecal) Kecal is the photon-to-electron conversion factor, for an arbitrary electron beam quality Qecal, taken as R50 = 7.5 cm. The values of Kecal are available in the TG-51 protocol. Parallel-plate chambers: cylindrical chambers:

  19. 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:

  20. k’R50 for Cylindrical Chambers NRC-CNRC

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

  22. 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. kecalis the photon to electron conversion factor, fixed for a given chamber model is the absorbed dose to water chamber calibration factor

  23. 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 Mrawat 10 cm depth in water (no depth shift !!!) • M = PionPTPPelecPpol Mraw • lookupkQfor your chamber

  24. Summary - electrons • get a traceable • measure I50 to give R50 (shift depth if necessary) • deduce dref = 0.6 R50 -0.1 cm (approx. at dmax) • measure Mrawat dref (no depth shift !!!) • M = PionPTPPelecPpol Mraw • lookup kecal for your chamber • determine (fig, formula) • establish (Mraw 2 depths)

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