<b>Monte Carlo Dosimetric Evaluation of Bismuth Oxide/Barium Sulfate–Epoxy Grid Block Collimators for Spatially Fractionated Radiotherapy</b>

Abstract

This study evaluates the dosimetric performance of Bi₂O₃/BaSO₄–epoxy composite grid block collimators for Spatially Fractionated Radiotherapy (SFRT) as lightweight, non-toxic alternatives to conventional brass and Cerrobend grids in LINAC-based treatments. Epoxy composites reinforced with Bi₂O₃ and BaSO₄ were synthesized and machined into clinically realistic grid geometries. Experimental linear and mass attenuation coefficients were measured at therapeutic photon energies and validated against the NIST XCOM database. Monte Carlo simulations were performed using TOPAS to model a 6 MV TrueBeam LINAC with a 22 × 22 × 7.5 cm³ grid block containing hexagonally arranged divergent circular apertures. Percentage Depth Dose (PDD), lateral dose profiles, surface dose, and Peak-to-Valley Dose Ratio (PVDR) were evaluated in a water phantom and compared with brass and Cerrobend grids. The composite reproduced PDD within ±5% of BJR 25 reference data, with dmax​ ≈ 1.5 cm for all materials. Surface dose was slightly higher than Cerrobend (+2.53%) and brass (+1.23%) but remained clinically acceptable. At 10 cm depth, PVDR values were 4.04 (composite), 4.13 (brass), and 4.78 (Cerrobend). Beam profiles were symmetric, and measured attenuation coefficients showed strong agreement with XCOM predictions (R² > 0.99). These results indicate that Bi₂O₃/BaSO₄–epoxy composite grid blocks achieve dosimetric performance comparable to conventional metallic grids and are suitable for SFRT applications.