Design and evaluation of electron beam energy degraders for breast boost irradiation

Abstract

Background: For breast cancer patients who require electron boost energies between 6 and 9MeV, an energy degraders (ED) in the 9MeV beamline was specially designed and manufactured to increase the skin dose of 6MeV and to reduce the penetration depth of 9MeV beams. Methods: We used Monte Carlo (MC) techniques as a guide in the design of ED for use with linear accelerators. In order to satisfy percent depth dose (PDD) characteristics and dose profile uniformity in water, the shape and thickness of Lucite® ED in the 9MeV beamline was iteratively optimized and then manufactured. The ED geometry consists of a truncated cone attached on top of a plane plate, with total central thickness of 1.0cm. The ED was placed on the lower most scraper of the electron applicator. The PDDs, profiles, and output factors were measured in water to validate the MC-based design. Results: Skin doses with the EDs increased by 8-9%, compared to those of the 9MeV beam. The outputs with the EDs were 0.882 and 0.972 for 10×10 and 15×15cm2 cones, respectively, as compared to that of a conventional 9MeV beam for a 10×10cm2 cone. The X-ray contamination remained less than 1.5%. In-vivo measurements were also performed for three breast boost patients and showed close agreement with expected values. Conclusions: The optimally designed ED in the 9MeV beamline provides breast conserving patients with a new energy option of 7MeV for boost of the shallow tumor bed. It would be an alternative to bolus and thus eliminate inconvenience and concern about the daily variation of bolus setup.