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SU‐GG‐T‐247: An Investigation On the Cross‐Calibration Value of the Plane‐Parallel Chamber Against the Cylindrical Chamber for Different High‐Energy Electron Beams with TG‐51 Protocol
Author(s) -
Yu M,
Gladstone D
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2961999
Subject(s) - ionization chamber , atomic physics , calibration , electron , physics , ionization , beam (structure) , cathode ray , ion , nuclear physics , optics , quantum mechanics
Purpose: To investigate the cross‐calibration value of the plane‐parallel chamber for different high‐energy electron beams. Method and Materials: An Exradin parallel‐plate ion chamber (P11) and a calibrated cylindrical ion chamber (PTW N30006) were used for measurements. R 50 of electron beams was calculated from Eq.(16) of the AAPM TG‐51 protocol with depth‐ionization curves measured by the cylindrical chamber. Since 2 cm ⩽ R 50 ⩽ 20 cm for electron beams 9, 12, 15 and 18 MeV, ( K ′ R50 ) pp , the electron beam quality conversion factor, was calculated from Eq.(20) of the protocol. Dose per MU at d ref using the cylindrical chamber was determined with worksheet B according to the protocol. The parallel‐plate chamber was used with the TG‐51 worksheet C to determine the cross‐calibration value, K c , of the plane‐parallel chamber against the cylindrical chamber for the above electron beams. Measurements were repeated several times. Results: K c should be constant for the same parallel‐plate chamber for different energy electron beams since it only depends on chamber's construction, however, our measurements show K c increases with beam energy. The average cross‐calibration value for 18 MeV is approximately 0.8 % larger than that for 9 MeV. K c is a function of ( K ′ R50 ) pp which depends on beam energy via Eq.(20). If Eq.(20) doesn't describe such dependence accurately, K c will vary with beam energies. When we fit our measurement data with a different set of parameters in Eq.(20), K c no longer varies with beam energies. Conclusion: Eq.(20) in TG‐51 protocol may not accurately describe the dependence of ( K ′ R50 ) pp as a function of electron beam energies. Variations of the cross‐calibration value for different beam qualities for the same parallel‐plate chamber are observed. It may be necessary to use different parameters in Eq.(20) to more accurately describe the above dependence.