Surface Dose Enhancement Using Low-Z Electron/Photon Beams

Abstract

Advances in linear accelerator technology has lead to the development of a linac which incorporates an in-air target assembly, making it possible to investigate novel treatments using multiple target designs. One such novel treatment uses multiple low-Z targets to enhance surface dose, replacing the use of synthetic tissue equivalent material (bolus). This treatment technique will decrease the common dosimetric and set up errors prevalent in using physical treatment accessories like bolus. A standard Monte Carlo model of a Varian Clinac linear accelerator was modified to incorporate a low-Z target in the electron beam. Simulations were performed using beryllium, carbon, aluminum, and copper as potential low-Z targets, in which they were placed in the secondary target position. The energies used were 2.5 and 4 MeV. The results determined carbon to be the target material of choice. Simulations of 15, 30 and 60% RCSDA carbon beams were propagated through slab phantoms. The resulting PDDs were weighted and combined with a standard 6 MV treatment beam to assess feasibility of surface dose enhancement. Using combinations of 15, 30, 60% RCSDA beams operated at 2.5 and 4 MeV in combination with a standard 6 MV treatment beam the surface dose was shown to be enhanced to within 80-100% the dose at dmax. Physical versions of the experimental targets were installed into a 2100C Varian Clinac (Varian Medical Systems Inc., Palo Alto, CA). The bending magnet shunt current was adjusted with the 4 MeV scattering foil in the beam-line, allowing mean energy selections of 2.5 and 4 MeV with the 2100C. Electron energy was determined through comparison of Monte Carlo modeled depth dose curves with experimentally measured data. The modeled low-Z beams were successfully validated using machined versions of the targets. Water phantom measurements and slab phantom simulations show excellent correlation to within 5%. This study presents a viable option for surface dose enhancement without the use of an external modifier.