Dosimetric verification of HDR vaginal cylinder applications using Gafchromic film and Monte Carlo simulation with a custom phantom.

INTRODUCTION

High-dose-rate brachytherapy (HDR-BT) in vaginal cylinder application is a common and effective treatment for gynecological malignancies. However, ensuring accurate dose delivery to the target while minimizing exposure to surrounding tissues is critical. Verification of the planned dose distribution using independent dosimetric methods is essential for quality assurance.

AIM: This study aimed to verify prescription doses in HDR vaginal cylinder applications using Gafchromic EBT3 film dosimetry with a custom in-house phantom, complemented by independent Monte Carlo (MC) simulations for validation.

METHODS

A custom PMMA phantom ("Brachy Sarbo") was developed to position EBT3 films at fixed radial distances. Treatment plans for 4, 5 and 6 Gy were created in SagiPlan® TPS. Dose distributions were validated using Geant4-based MC simulations and film measurements with LINAC-based reference for film response.

RESULTS

MC simulations closely matched prescribed doses, with 0.06 % error. Film dosimetry captured trends but showed deviations between -3.99 % and 4.45 %. Gamma analysis (3 %/3 mm) revealed better spatial agreement at higher doses, with MC vs. LINAC achieving 87.5 % pass rate at 5 Gy. MC vs. BT showed lower agreement at 4 Gy (15.8 %) but improved notably at 5 Gy and 6 Gy, reflecting better consistency with increasing dose.

CONCLUSION

This study demonstrates that EBT3 film dosimetry using an in-house phantom offers a practical and reliable method for verifying prescription doses in HDR vaginal cylinder treatments. MC simulations provide a strong theoretical complement and together, this approach strengthens confidence in dose accuracy and support robust QA in HDR-BT for gynecological cancers.