Comparison of effective source-surface distances for electron beams derived from measurements made under different...

The purpose of this paper is to investigate the effect of scatter conditions around the chamber on the effective source-surface distance (SSDeff) calculation. Three setups were considered, viz with the chamber: in air, in solid water at the surface, and in solid water at dmax. Ionization measurements for electron beams from a Varian Clinac-210 degrees C were recorded from 100 cm to 120 cm nominal SSD in 5 cm increments, using a parallel-plate Markus chamber. Two electron energies, 6 and 12 MeV, were investigated with a range of electron applicators (Series-III, 10 x 10 to 25 x 25 cm2) and different cerrobend inserts (square and rectangular). The study was undertaken by placing the chamber in air and in solid water, respectively. SSDeff was calculated from the ionisation chamber measurements using the method of Khan (1984). The results are summarised in the table 1. In almost all cases the SSDeff calculated using the in-air data is less. This is likely to be due to a larger proportion of scatter off the applicators reaching the chamber when it is in air than when it is at dmax in the phantom. The results also show the difference is reduced for a larger applicator, probably because the applicator scatter component reaching the in-air measurement point is reduced.