Design and Implementation of a Monte Carlo Framework for Assessment of Spoiler Applications in Abutting Electron Fields

BACKGROUND: Field matching problems in abutting electron fields can be managed by using spoilers. OBJECTIVE: The aim of this study was to design a Monte Carlo framework for the assessment of spoiler application in abutting electron fields. MATERIAL AND METHODS: In this experimental study, a Siemens Primus treatment head was simulated for a 5 MeV electron beam using BEAMnrc, DOSXYZnrc and EGSnrc user codes. Validation of beam model was done by measurement using a MP3-M water tank and a Semi-flex Chamber-31010 (PTW, Freiburg, Germany). An in-house routine was developed to calculate the combined isodose curves resulting from simulated adjacent fields. The developed framework was analyzed using PMMA and chromium spoilers. RESULTS: The penumbra width increased from 27.5 mm for open fields to 42 mm for PMMA and 40 mm for chromium. The maximum junction dose reduced from 115% for open fields to 107% for PMMA and 108% for chromium. R(90) reduced about 6 mm for PMMA and 3 mm for chromium. Uniformity index reduced from 93% to 77% for both spoilers. Surface dose increased from 79% to 89% for PMMA and 88% for chromium. CONCLUSION: Using spoilers, penumbra width at the surface was increased, size and depth of hot spots as well as the therapeutic range were reduced and dose homogeneity at the junction of abutting electron fields was improved. For both spoilers, the uniformity index reduced, and surface percent dose increased. The results of this research can be used to optimize dose distribution in electron beam treatment using abutting fields.