Modeling and validation of a system for measurement of linear attenuation coefficients of tissue substitutes for internal dosimetry phantoms using PHITS Monte Carlo code.

According to ICRU, any material used to simulate the physical characteristics of a body tissue could be called a tissue substitute. The linear attenuation coefficient (μ) of a material is an essential parameter for the characterization of a tissue substitute. This coefficient, (μ), measures the loss of radiation intensity due to absorption and scattering as it traverses a material. The Monte Carlo (MC) method simulates radiation interactions by creating geometries built with various materials and transporting particles and photons across these geometrical models. Studies have demonstrated the effectiveness of MC codes in evaluating materials' absorption and scattering characteristics, showing strong compatibility with NIST data. PHITS is a Monte Carlo code that efficiently handles particle transport across wide energy ranges. This study aims to simulate an experimental setup developed to measure the attenuation coefficients of tissue substitute samples using the Monte Carlo PHITS code and validate the developed computational model using the linear attenuation coefficients experimentally obtained and theoretical NIST XCOM data. The experimental configuration included ballistic gel tissue substitute samples (10x10 × 2 cm) positioned between a 3" × 3" NaI(Tl) detector and a Ra-226 source. The detector was shielded using 5 cm thickness lead blocks, and the source was collimated with an 8 mm aperture. This apparatus was simulated to record photon interactions. Ra-226 photons with energies of 186.1, 241.9, 295.2, 351.9, 609.3, 1759, and 2204.1 keV were evaluated. The PHITS simulation yielded relative errors below 1.7 % for spectra and 1 % for monoenergetic energies based on 1E+07 source particle histories. The μ values were calculated, showing a high correlation with experimental data. Discrepancies were below 5 % for most energies, except for 186.1 keV (12 %). Monoenergetic photon simulations exhibited differences below 4 % compared to experimental data and below 1 % relative to NIST data. This study highlights the methodology's efficacy and PHITS's applicability for evaluating tissue substitute materials.