A NEW APPROACH TO ELECTRON-BEAM REFERENCE DOSIMETRY

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 Co-60 to be consistent wi th the: present proposal for the new AAPM photon-beam protocol; it use s R-50 to specify the beam quality and the reference depth. d(ref)=0.6 R(50)-0.1 (all quantities in cm), recommended by Burns pr nl. [Med, Ph ys. 23, 383-388 (1996)]; it has a formalism which is parallel to the k (Q) formalism for photon-beam dosimetry; it fully accounts for the imp act 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: D-w(Q)=MP(ion)P(gr)(Q)k(R50)'k(ec al)N(D,w)(60Co). The term P-gr(Q) is not needed with plane-parallel ch ambers but corrects for gradient effects with cylindrical chambers and is measured in the user's beam. The parameter k(ecal) is associated w ith converting the Co-60 absorbed-dose calibration factor into one for an electron beam of quality Q(e), and contains most of the chamber to chamber variation, Calculated values of k(ecal) are presented as well as Monte Carlo calculated P-wall values for plane-parallel chambers i n a water phantom irradiated by a Co-60 beam since these are needed to calculate k(ecal), The factor k(R50)' is a function of R-50 and conve rts the absorbed-dose calibration factor to that for the electron-beam quality of interest. Two analytical expressions an presented which ar e close to universal expressions for all cylindrical Farmer-like chamb ers and for well-guarded plane-parallel chambers respectively. Calcula ted values are presented graphically for electron beams with energies between 5 and 50 MeV. (C) 1998 American Association of Physicists in M edicine.