A Homogeneous Water-Equivalent Anthropomorphic Phantom for Dosimetric Verification of Radiotherapy Plans

Water is treated as radiological equivalent to human tissue. While this seems justified, there is neither mathematical proof nor sufficient experimental evidence that a water phantom can be treated as equivalent to human tissue. The aim of this work is to simulate and validate a water phantom that is tissue equivalent in terms of the dosimetric characteristics of both water and human tissue Dynamic, intensity-modulated radiotherapy plans for two head and neck, one brain, one pelvis, and three lung/mediastinum cases were chosen for this study. Using a treatment planning system (TPS) (Eclipse, Varian Medical System, Polo Alto, CA, USA) and Anisotropic Analytic Algorithm in a grid resolution of 5 mm × 5 mm, a patient-equivalent water phantom was calculated from all rays in the isocentric plane as an array of water equivalent depths (d_WE). These rays were plotted versus isocentric separation and ray-tracing direction. Planar doses were compared between the isocentric plane in the patient computed tomography and the water equivalent phantom using gamma criteria of 2%-2 mm and 3%-3 mm. Except in one lung case, >95% gamma agreement was seen when using 3%-3 mm and >90% pass rate was seen when using 2%-2 mm. For head and neck cases, gamma-fail was restricted to the periphery. For mediastinum cases, gamma-fail was restricted to the lungs. This study demonstrates that a heterogeneous patient can be converted to a water phantom with comparable dosimetric characteristics and disagreements restricted to the lung area for both modulated and open beams. Potential sources of error include the d_WE calculation and TPS dose computation.