The effects of cutouts on output, mean energy and percentage depth dose of 12 and 14 MeV electrons.

Electron field-shaping cerrobend cutouts on the linear accelerator applicator have some effects on the output and percentage depth dose. These effects which arise from the lateral scatter nonequilibrium are particularly evident in higher energies and in cutouts with smaller radius. Dose measurements for circular, square, and triangular cutouts as well as open field was performed in a 10 × 10 cm applicator, using plane parallel type ion chamber with a 100 cm source surface distance. The Percentage Depth Doses curves were drawn and the outputs were measured for each of these cutouts. The output factors, normalized to open 10 × 10 cm field, varied between 0.891 and 0.996 depending on the energy, cutout shape, and cavity area. With the use of cutouts, R(100) shifted toward the surface. The shifts ranged from 9 to 0 mm and from 13 to 0 mm for 12 and 14 MeV, respectively, depending on the shape and cavity area. For R(90), R(80), and R(50) the ranges for observed shifts narrowed down and practically no shifts were observed for R(20). We present these changes in the form of predictive formulas, which would be useful in clinical applications.