Monte Carlo-based assessment of solid phantom equivalence to water in radiation therapy quality assurance

Abstract

Accurate dosimetry is crucial for optimizing radiotherapy outcomes, balancing tumor control with healthy tissue sparing. Water phantoms represent the gold standard due to their tissue-equivalent properties, yet practical limitations such as setup complexity and portability drive the need for solid alternatives. This study employs Monte Carlo simulations using the GATE platform to comprehensively evaluate the dosimetric equivalence of common solid phantoms; RW3, Nylon, PMMA, Polyester, Polyethylene, Polystyrene, Original Plastic Water, and Virtual Water; against water for a 6 MV flattened photon beam from a Varian TrueBeam linear accelerator. Dosimetric parameters, including percentage depth dose (PDD) curves, lateral dose profiles at depth of 10 cm, and 3D volumetric dose distributions, were assessed under identical beam configurations (10 × 10 cm² field size, 100 cm SSD). Validation against experimental data acquired using a PTW Semiflex ionization chamber (0.125 cm³) in a motorized water phantom confirmed simulation fidelity, with gamma pass rates exceeding 99% (2%/2 mm criterion). Results ranked RW3 as the superior water substitute (mean PDD deviation: 0.65%; lateral: 4.68%; RMSE: 5.62%), followed by Plastic and Virtual phantoms, while Nylon, and Polyester exhibited unacceptable deviations (>10%). Polyethylene showed anomalous positive bias (+3.98% PDD). These findings, benchmarked against IAEA and AAPM tolerance criteria (±2% for reference dosimetry, ±3% for routine QA), highlight material-specific radiological properties influencing photon interactions, underscoring RW3's utility for clinical quality assurance while identifying materials requiring correction factors (Polystyrene, Polyethylene) or unsuitable for clinical use without extensive adjustments (Nylon, Polyester exceeding 10% deviation).