QUANTIFICATION OF MEAN ENERGY AND PHOTON CONTAMINATION FOR ACCURATE DOSIMETRY OF HIGH-ENERGY ELECTRON-BEAMS

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface ((E) over bar(0)) f rom the half-value depth (R-50) has been studied. The influence of ene rgy, angular spread and range straggling on the shape of the depth dos e distribution and the R-50 and R-p ranges is described using the simp le Gaussian range straggling model. The relation between the R-50 and R-p ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the re lation between (E) over bar(0) and different range concepts, such as R -50 and the maximum dose and the surface dose related mean energy depo sition ranges, (R) over bar(m) and (R) over bar(0), is analysed. In pa rticular the influence of multiple electron scatter and phantom genera ted bremsstrahlung on R-50 is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-v alue depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in wa ter for energies between 2 and 50 MeV. Simple linear relations describ ing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon bac kground at R-p, the treatment head generated bremsstrahlung distributi on can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the differe nce in stopping power ratios between a clean electron beam and the pho ton contamination. The absorbed dose determined using ionization chamb ers in heavily photon contaminated (10%) electron beams may be too low -by as much as 1.5%-without correction.