Electron beam therapy at extended SSD: A Monte Carlo investigation

Abstract

Electron beam therapy is used to treat superficial tumours at a standard 100 cm source-to-surface distance. Certain clinical situations however, require the use of an extended SSD. Monte Carlo methods were used to investigate clinical electron beams, at standard and non-standard SSD, from a Siemens Oncor Avant Garde linear accelerator. The linac treatment head was modelled in BEAMnrc for 5 cm diameter, 10 × 10 cm², 15 × 15 cm² and 20 × 20 cm² electron fields, 6, 9 and 12 MeV and 100, 110 and 120 cm SSD. The DOSXYZnrc code was used to calculate extended SSD factors and dose contributions from various parts of the treatment head.The main effects of extended SSD on water phantom dose distributions have been verified by Monte Carlo methods. Monte Carlo calculated and measured extended SSD factors showed an average difference of ± 1.8 %. For the 5 cm diameter field, the relative output at extended SSD reduced more rapidly than for the larger fields. An investigation of output contributions showed this was mainly due to a rapid loss of scatter dose reaching the d_max point from the lower scrapers of the electron applicator. The 5 cm diameter field showed a reduction in dose contributions, while the larger fields generally showed an increased contribution from these scrapers with increase in SSD. BEAMDP derived angular distributions of applicator scattered electrons have shown a large number of acute angle electron tracks contributing to the output for the larger field sizes, explaining the shallow output reduction.