Assessment of dose to vaginal mucosa for gynecologic template interstitial high-dose-rate brachytherapy using Monte Carlo simulation

PURPOSE: This study investigated reliable vaginal mucosa dose-volume histogram (DVH) metrics in gynecologic template interstitial high-dose-rate brachytherapy (HDR-BT) for the purpose of standardized dose reporting. MATERIAL AND METHODS: Gynecologic template (Syed/Neblett) interstitial HDR-BT patients treated from September 2016 to November 2022 at the study institute were included in the cohort. Each patient implant included a vaginal mucosa contour defined by a 5 mm expansion from vaginal cylinder, then another volume with clinical target volume subtracted. DVH metrics were investigated between D(0.1cc) to D(4cc). Clinical plans were re-calculated using Monte Carlo (MC) simulations both in heterogeneous material and in water. RESULTS: The patient cohort included 61 patients with clinical plans using conventional homogeneous dose calculation (TG43). Heterogeneous vs. water MC dose differences were between –1.1% and –1.4% for all metrics investigated. DVH metrics D(1cc) and smaller resulted in > 5% discrepancies between TG43 and MC dose (to water) calculation due to the proximity of source positions in/nearby the vaginal mucosa. Reliability improved when DVH metric volume was larger (D(2cc) and D(4cc)). Both D(2cc) and D(4cc) presented very high linear correlation between TG43 and MC reported doses for the vagina, and average ± standard deviation dose difference was 4.6 ±2.9% and –3.0 ±1.9%, respectively. Dose differences decreased when the clinical target volume was removed: –1.5 ±3.5% and –0.8 ±2.1% for D(2cc) and D(4cc), respectively. CONCLUSIONS: For perineal template gynecologic HDR-BT procedures, the 2 cc volume is the smallest representative volume that reliably reports vaginal dose and at minimum should be reported to establish dose and outcome evaluation.