SU‐E‐T‐686: Monte Carlo Study of Material Approximation Introduced Dose Difference in Electron Conformal Therapy Planning

Abstract

Purpose: When Monte Carlo (MC) is used in dose calculation with CT data, materials are often assigned based on the local Houndsfield units if the density threshold method is applied. When bolus is used to compensate for body surface irregularities and/or to modulate the therapeutic depth in bolus electron conformal therapy (bolusECT), the bolus material, which is machineable wax, is treated as soft tissue, since the density of bolus overlaps with soft tissue. This study investigates the dose errors introduced by this material misassignment.Methods: BEAMnrc was used to simulate electron beams from a Trilogy accelerator. SPRRZnrc was used to calculate stopping power ratios (SPR) of tissue to wax, SPR(tissue/wax), and tissue to water, SPR(tissue/water), for 12 and 15 MeV electron beams which are the most commonly used energies in bolusECT. DOSXYZnrc was applied in dose distribution calculation in tissue with wax slabs of various thicknesses and a wedge‐bolus on top respectively. The MC dose distributions calculated using different bolus materials were compared.Results: With the same density, the SPR(tissue/wax) for 12 and 15 MeV beams ranges from 0.934 to 0.944, while the SPR(tissue/water) values are between 0.990 and 0.991, indicating higher stopping powers of wax. The 12 MeV beam range is 6.5 cm in wax and 6.7 cm in tissue. For a 12 MeV beam, the dose in tissue immediate under the bolus is overestimated by about 2.5% of maximum dose at 3 cm bolus thickness if wax bolus is treated as tissue. For 15 MeV beams, the error is about 1.4% Conclusions: Due to stopping power differences, dose differences are observed if the bolus material is misassigned in bolusECT dose calculations. Large dose overestimations occur at thick bolus region. However, the error is mostly within clinical tolerance, especially for thin bolus regions.