Purpose: A commercial shielding company (Veritas Medical Solutions) provides shielding options for radiotherapy treatment rooms using modular, high-iron content interlinked blocks (VeriShieldᵀᴹ). Monte Carlo simulations were used to calculate shielding properties and compare against density-scaled shielding parameters from the NCRP151 report.
Methods: The simulation model utilized the Tool for Particle Simulation (TOPAS) code for radiation transport calculations where the realistic in-air model consisted of a photon source, concrete wall with varying thickness, and a detector. Vendor-provided (Varian Medical Systems) phase-space files for TrueBeamᵀᴹ 6X, 6FFF, 10X, 10FFF, and 15X beams were used as sources for deriving primary tenth-value layer (TVL) values for ordinary concrete (V150) as well as concretes having higher iron content (V250 and V300). The concrete wall thickness varied between 0-100cm, with the source and wall exterior distance generally being 600cm. Circular-shaped detectors with 3cm radial bins and 0.05MeV-energy binning were placed 30cm from the wall exterior to score photon fluence. Subsequently, collisional kerma and primary shielding TVL values (1st TVL₁ and equilibrium TVLₑ) were calculated with consideration for detector volume-averaging.
Results: Reproducing the NCRP151 geometry verified the primary TVL values for 6X in conventional concrete within 2%. Using the 6X phase-space files, TVL values of concrete were reduced by 2cm with (TVL₁, TVLₑ) values in V150, V250, and V300 being (33.4cm, 31.2cm), (21.0cm, 19.4cm), and (16.7cm, 16.0cm) respectively. The 6FFF TVL values were (30.4cm, 30.4cm), (19.2cm, 18.9cm), and (15.3cm, 14.6cm) respectively. Similar trends were observed for 10X, 10FFF, and 15X beams. Sensitivity to angular volume-averaging was assessed towards providing reference values. Statistical uncertainties for TVLₑ were typically < 1%.
Conclusion: Radiological simulations to derive shielding parameters for VeriShieldᵀᴹ modular blocks indicated substantial differences between density-scaled results towards providing improved accuracy for vault shielding. Comprehensive simulations for remaining shielding parameters such as leakage and scatter are underway.