Modeling scatter through sides of island blocks used for intensity‐modulated bolus electron conformal therapy

Purpose Passive Radiotherapy Intensity Modulators for Electrons (PRIME) devices are comprised of cylindrical tungsten island blocks imbedded in a machinable foam slab within the patient's cutout. Intensity‐modulated bolus electron conformal therapy (IM‐BECT) uses PRIME devices to reduce dose heterogeneity caused by the irregular bolus surface. Heretofore, IM‐BECT dose calculations used the pencil beam redefinition algorithm (PBRA) assuming perfect collimation. This study investigates modeling electron scatter into and out the sides of island blocks. Methods Dose distributions were measured in a water phantom at 7, 13, and 20 MeV for devices having nominal intensity reduction factors of 1.000 (foam only), 0.937, 0.812, and 0.688, corresponding to nominal island block diameters (dnom) of 0.158, 0.273, and 0.352 cm, respectively. Pencil beam theory derived an effective diameter (dIS) to account for in‐scattered electrons as a function of dnom and beam energy (Ep,0). However, for out‐scattered electrons, an effective diameter (dmod) was estimated by best fitting measured data. Results In the modulated region (under island blocks, depth