Characteristics of Megavoltage Electron Beams Directed through Silicone for Bolus Electron Therapy

Abstract

In high energy electron beam therapy, a bolus can be used for the conformal dose to target and achieve the appropriate treatment quality. To evaluate the use of a customized silicone bolus, we analyzed the dosimetric properties of the electron beam according to the silicone bolus thickness. Eight silicone slits were fabricated using an acrylic mold. The size of each slit was 150 × 150 × 5 m3 (width × length × height). Degassing was performed to eliminate bubbles in the slits during the silicone casting process. Depth dose measurements were performed for a 6 MeV and 9 MeV electron beams with 10 × 10 cm2, 6 × 6 cm2 and 5 cm diameter circular fields with various silicone thickness and with the conventional bolus. For the depth dose measurement, the film was placed between two 6 cm solid water phantoms, and the electron beam was delivered parallel to the film. The Rando phantom dose with customized bolus were evaluated with the 10 × 10 cm2 6 MeV electron beam. As the slab thickness increased, the depth dose in the solid water phantom decreased after reaching the maximum dose depth (dmax). Depending on the silicone thickness, the surface dose of the phantom was increased until reaching the electron beam dmax and was decreased after dmax. The coefficient to convert the thickness of the silicone to water were 1.06 in 6 MeV and 1.07 in 9 MeV, respectively. We measured the electron beam dose properties for making patient-specific silicone boluses with optimal thickness. The results of this study can be used to predict the silicone bolus thickness required in order to deliver the appropriate dose to the target.