Monte carlo assessment of advanced shielding materials for space-efficient radiotherapy vault design

Conventional radiotherapy vault shielding typically employs 2 to 3 meters of ordinary concrete, consistent with NCRP Report 151 reference data. While NCRP 151 specifies dose-based design goals rather than mandating particular materials, the practical default of concrete results in vault footprints of 150 to 200 m² per linac, which can constrain treatment capacity within fixed departmental space. Recent advances in high-density composite shielding materials offer the potential to achieve the same dose-based design goals with reduced barrier thickness.

In this study, we investigate advanced shielding materials, including specialty concretes and high-density polymer composites, as potential alternatives to conventional concrete barriers, aiming to demonstrate that modern materials could safely enable more compact vault designs while maintaining regulatory compliance.

TOPAS Monte Carlo simulations evaluated a 6 MV and an 18 MV linear accelerator (LINAC) vault comparing conventional concrete against five candidate shielding materials. We modeled a typical 4-vault radiotherapy department to demonstrate potential capacity increase. Simulations calculated linear attenuation coefficients (μ), tenth-value layers (TVLs), and dose distributions at regulatory measurement points. Spatial analyses projected the clinical impact of implementing optimized shielding.

While concrete required 165.8 m2 floor area per vault, candidate materials achieved 17%-32% reductions for 6 MV beams and 22%-34% reductions for 18 MV beams. Specifically, Steel-magnetite and Datolite-Galena concretes showed great space-saving potentials with footprint reductions of 28% and 31% for 18 MV beams, respectively, while maintaining refined doses at 2.40 × 10-2 and 2.42 × 10-2 mSv/week, well below the 0.1 mSv/week NCRP limit.

This study demonstrates that high-density composite shielding may reduce vault footprint while maintaining dose levels below established shielding design limits. These findings suggest that systematic evaluation of alternative shielding materials may provide useful insights for space-efficient radiotherapy vault design while remaining within established dose-based shielding frameworks.