Evaluation of solid phantom materials as water substitutes in high-energy photon beam radiotherapy: A comprehensive dosimetric analysis using GATE/Geant4 simulation

To evaluate the dosimetric equivalence of six solid phantom materials as water substitutes for radiotherapy quality assurance using validated Monte Carlo simulation. A 6 MV Varian TrueBeam linac was modeled using GATE/Geant4 with IAEA-compliant phase space files. After rigorous validation against experimental measurements (>99 % gamma pass rate, 2 %/2 mm), percentage depth dose (PDD) and lateral profiles were computed for RW3, Nylon, PMMA, Polyester, Polyethylene, and Original Plastic Water, then compared to water using statistical analysis and 3D dose mapping. Original Plastic Water showed superior equivalence (PDD: 0.164 %, RMSE: 1.480 %), followed by RW3 (PDD: −0.653 %, RMSE: 1.891 %). Polyethylene exhibited positive bias (PDD: 3.978 %). PMMA, Polyester, and Nylon demonstrated substantial negative deviations (PDD: −11.304 %, −11.313 %, −8.922 %; RMSE: 3.710 %, 3.600 %, 2.758 %, respectively), particularly at greater depths. Original Plastic Water and RW3 are validated as clinically acceptable water substitutes for high-precision radiotherapy QA. PMMA, Polyester, and Nylon require correction factors for accurate dosimetry. This framework provides robust methodology for evidence-based phantom material selection.