Estimation of backscatter factor in ISO-recommended phantoms for photon beams of different beam sizes using Monte...

Backscatter factor (BSF) is a critical correction parameter in radiation dosimetry, accounting for dose enhancement due to photon scattering within irradiated media; however, systematic evaluations beyond the ISO slab phantom remain limited. In this work, BSFs were comprehensively estimated for all ISO-recommended phantoms-slab, cylindrical, pillar, and rod-over a photon energy range of 10 keV-2 MeV and multiple field sizes using the FLUKA Monte Carlo code with an air-kerma-based formalism. The results show a strong coupled dependence of BSF on photon energy and beam size, with a characteristic peak in the intermediate energy range (∼40-80 keV) and geometry-driven variations and field-size sensitivity-maximized in the slab phantom and minimized in the rod due to scattering volume constraints, while cylindrical and pillar phantoms exhibit intermediate behaviour. To extend these results to realistic irradiation conditions, a Python-based standalone tool was developed using PCHIP interpolation and fluence-weighted averaging to compute spectrum-averaged BSFs for ISO 4037 beams, diagnostic and therapeutic spectra, and user-defined spectra, demonstrating deviation within 0%-2.3% with published data. The study provides a BSF dataset across all ISO phantoms along with a validated computational framework, enabling improved accuracy in dosimeter calibration and determination of operational quantities such asH_p(3) andH_p(0.07). The Python-based standalone executable of the BSF software, compatible with both Windows and Linux platforms, along with its complete source code, is provided as open-access supplementary data.