The effect of oblique electron beams to the surface dose under the bolus.

The aim of this study is to determine the effect of bolus to the surface dose in oblique electron incidences. Irradiations with 4.5, 6, 7.5, 9, and 12-MeV electron beams were made for the incidence angles of 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees and using 3 different bolus setups: (1) unbolused (no bolus), (2) 5-mm bolus, and (3) 10-mm bolus. A set of EBT gafchromic film pieces placed on the phantom surface was irradiated with a 400-cGy dose at D(max) for each setup. Whereas surface dose increased with increasing incidence degrees in the absence of a bolus, it was seen that there was a large surface dose decreasing in the presence of a bolus with increasing incidence angles. For 60 degrees incidence angle, the relative surface doses with unbolused setup were: 88.10%, 90.06%, 89.35%, 90.25%, and 97.10%; with 5-mm bolus: 66.45%, 81.20%, 99.78%, 124.43%, and 116.07%; and with 10-mm bolus: 22.65%, 45.20%, 55.20%, 65.82%, and 90.27% for 4.5, 6, 7.5, 9, and 12 MeV, respectively. The use of bolus in the treatment of highly oblique surfaces with low-energy electron beams significantly decreases the surface dose.