Monte Carlo Study of Dose Difference in Electron Conformal Therapy Planning Introduced by Bolus Material Approximation

Abstract

When Monte Carlo (MC) method is used for dose calculations on a patient CT dataset, the materials in the CT images are often assigned based on the local Houndsfield units if the density threshold method is applied. In bolus electron conformal therapy (bolusECT), the bolus material, machinable wax, is therefore treated as soft tissue. This study investigates the dose errors introduced by this material misassignment. BEAMnrc was used to simulate electron beams from a Varian Trilogy accelerator. SPRRZnrc code was used to calculate stopping power ratios of tissue to wax, $SPR^{tissue}_{wax}$, and tissue to water, $SPR^{tissue}_{water}$, for 12 and 15 MeV electron beams. DOSXYZnrc was applied to calculate dose in a tissue phantom with the rectangular wax slabs of various thicknesses or a wedge-shaped bolus on top. The $SPR^{tissue}_{wax}$ values for 12 and 15 MeV beams were calculated to be 0.935~0.945, while the $SPR^{tissue}_{water}$ values were 0.990~0.991. The dose in tissue immediately under the bolus is overestimated by about 2.5% of maximum dose with the 3 cm bolus thickness if wax bolus is treated as tissue for 12 MeV beams. For 15 MeV beams, the error is about 1.4%. A left chest wall electron treatment case was simulated using wax and soft tissue as bolus materials in turn. Dose differences are observed in MC simulations if the bolus material is misassigned as tissue in bolusECT dose calculations. However, the error is mostly within clinical tolerance, especially for thin bolus regions which are more likely to be encountered in practice.