Surface and buildup dose characteristics for 6, 10, and 18 MV photons from an Elekta Precise linear accelerator.

Abstract

Understanding head scatter characteristics of photon beams is vital to properly commission treatment planning (TP) algorithms. Simultaneously, having definitive surface and buildup region dosimetry is important to optimize bolus. The Elekta Precise linacs have unique beam flattening filter configurations for each photon beam (6, 10, and 18 MV) in terms of material and location. We performed a comprehensive set of surface and buildup dose measurements with a thin window parallel‐plate (PP) chamber to examine effects of field size (FS), source‐to‐skin distance (SSD), and attenuating media. Relative ionization data were converted to fractional depth dose (FDD) after correcting for bias effects and using the Gerbi method to account for chamber characteristics. Data were compared with a similar vintage Varian linac. At short SSDs the surface and buildup dose characteristics were similar to published data for Varian and Elekta accelerators. The FDD at surface (FDD0) for 6, 10, and 18 MV photons was 0.171, 0.159, and 0.199, respectively, for a 15×15 cm2, 100 cm SSD field. A blocking tray increased FDD0 to 0.200, 0.200, and 0.256, while the universal wedge decreased FDD0 to 0.107, 0.124, and 0.176. FDD0 increased linearly with FS (~1.16%/cm). FDD0 decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV FDD0 actually increased slightly with increasing SSD. This is likely due to the unique distal flattening filter for 6 MV The measured buildup curves have been used to optimize TP calculations and guide bolus decisions. Overall the FDD0 and buildup doses were very similar to published data. Of interest were the relatively low 10 MV surface doses, and the 6 MV FDD0's dependence on SSD. © 2003 American College of Medical Physics. PACS number(s): 87.53.–j, 87.66.–a