A new approach to electron-beam reference dosimetry.

Abstract

A new approach is proposed for electron-beam dosimetry under reference conditions and data necessary to use this approach are presented. The approach has the following features; it uses ion chambers and starts from an absorbed-dose calibration factor for 60Co to be consistent with the present proposal for the new AAPM photon-beam protocol; it uses R50 to specify the beam quality and the reference depth, dref = 0.6R50 - 0.1 (all quantities in cm), recommended by Burns et al. [Med. Phys. 23, 383-388 (1996)]; it has a formalism which is parallel to the kQ formalism for photon-beam dosimetry; it fully accounts for the impact on stopping-power ratios of realistic electron beams; it allows an easy transition to using primary standards for absorbed dose to water in electron beams when these are available. The equation for dose to water under reference conditions is; DWQ = MPionPgrQk'R50kecalND,w60Co. The term PgrQ is not needed with plane-parallel chambers but corrects for gradient effects with cylindrical chambers and is measured in the user's beam. The parameter kecal is associated with converting the 60Co absorbed-dose calibration factor into one for an electron beam of quality Qe and contains most of the chamber to chamber variation. Calculated values of kecal are presented as well as Monte Carlo calculated Pwall values for plane-parallel chambers in a water phantom irradiated by a 60Co beam since these are needed to calculate kecal. The factor k'R50 is a function of R50 and converts the absorbed-dose calibration factor to that for the electron-beam quality of interest. Two analytical expressions are presented which are close to universal expressions for all cylindrical Farmer-like chambers and for well-guarded plane-parallel chambers respectively. Calculated values are presented graphically for electron beams with energies between 5 and 50 MeV.