Phantom-based dosimetric verification of a high dose rate ⁶⁰Co brachytherapy for vaginal cuff treatment using EBT3 film and Geant4 Monte Carlo simulations

Introduction: High dose rate (HDR) vaginal cuff brachytherapy is a standard adjuvant treatment for endometrial and early cervical cancers in the post operative setting. However, uncertainties in dose delivery may lead to local recurrence. Aim: This study aims to perform phantom-based dosimetric verification of HDR ⁶⁰Co brachytherapy using radiochromic films (EBT3) and Geant4 Monte Carlo (MC) simulations to cross-validate dose distributions calculated by the treatment planning system (TPS). Methods: A custom-designed perspex slab phantom was used to simulate vaginal cuff anatomy. Prescribed doses of 4 Gy, 5 Gy and 6 Gy were delivered using a single channel vaginal applicator. EBT3 film dosimetry was used to measure absorbed doses, while MC simulations replicated the experimental setup for computational dose prediction. Comparative analysis was performed among EBT3, MC and TPS data. Results: Good agreement was observed among the three methods, with deviations within ±10%. However, the most prominent discrepancies were observed at the vaginal apex, a clinically significant region. Deviations observed with EBT3 film and MC methods highlighted potential discrepancies in TPS dose estimates. Conclusion: Independent dosimetric verification using EBT3 films and MC methods confirms the accuracy of delivered dose and reveals limitations in TPS-based dose predictions, particularly in anatomically sensitive regions such as the vaginal cuff apex. These findings support the integration of film and MC-based verification into clinical quality assurance (QA) protocols for HDR-BT to enhance treatment reliability and patient safety.