Study and considerations on the determination of attenuation curves in concrete for radiotherapy beams

Abstract

This study addresses the reliance on outdated Tenth Value Layer (TVL) data for radiotherapy beams in concrete, which results in inconsistencies in reported values. Attenuation curves were determined for various radiotherapy accelerator spectra below 10 MV in concrete. The aim is to provide more accurate data with approximately 1% statistical uncertainty, thereby correcting the reliance on outdated TVL data and establishing a more accurate framework for future research.

Numerical simulations were carried out using Geant4, which also allowed for the evaluation of statistical uncertainties. A divergent isotropic point source beam of X-rays, matching radiotherapy accelerator spectra, was directed toward concrete barriers of varying thicknesses. An ICRU-tissue equivalent detector, simulating the thorax of the MIRD Phantom, was positioned to measure the transmitted dose. The findings indicate that some results aligned with current TVL values, thereby validating the methodology. Nevertheless, many new data points revealed significant differences in shielding thickness requirements based on accelerator spectra and concrete density, along with minor differences attributable to concrete compositions. Discrepancies in shielding thickness from established standards like IAEA SRS47 and NCRP 151 were unraveled by introducing new data alongside the parameters used to determine them.

This research highlights the need for refined data in radiotherapy accelerator shielding calculations. By presenting new insights, it advocates for improving current practices and emphasizes the importance of reporting accelerator spectra and associated uncertainties. This study contributes by helping users understand which variables are relevant for reporting new data and which are less critical.