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.