Dosimetric evaluation and energy optimization in proton therapy for Hodgkin’s lymphoma using a GATE-based Monte Carlo framework

Proton therapy offers superior dose conformity because of the Bragg peak, which makes it particularly attractive for treating Hodgkin’s lymphoma (HL) in young patients. In this work, we aimed to (i) determine the optimal proton energy for a simplified spherical target (37 mm diameter, 5–42 mm depth) in a homogeneous water phantom consistent with clinical commissioning practice, and (ii) quantify the relative out-of-field doses to organs at risk (OARs) using a standardized anthropomorphic CIRS ATOM phantom. We used the GATE Monte Carlo (MC) toolkit to generate depth-dose curves in a water phantom for energies from 60 to 90 MeV. We selected the optimal energy by maximizing a weighted Target Coverage & Sparing (TCS) metric that accounts for both target dose coverage and sparing of normal tissue distal to the target. The 70 MeV beam maximized this metric for the investigated target geometry (5–42 mm depth), with a TCS value of 0.876 and 93.8% target coverage. We then calculated absorbed doses to OARs using a voxelized CIRS ATOM phantom. Relative organ doses showed an approximately exponential decrease with distance, ranging from 39.9 mGy (left lung) to 0.079 mGy (thyroid) per Gray of physical dose to the target. All simulations had statistical uncertainties below 2% in the target volume and below 3.2% in organs at risk. This methodological framework provides a standardized reference for comparing proton therapy plans and assessing secondary cancer risks in a standardized geometry for HL patients, upon further validation with patient-specific data.