Long term patient-reported dyspareunia following definitive chemoradiation for anal cancer: utilizing the anterior vaginal wall as an organ-at-risk to define an actionable dosimetric goal

Introduction Chemoradiation (CRT) is the standard treatment for squamous cell carcinoma of the anus (SCCA). This study aimed to investigate the relationship between vaginal dosimetry and long-term patient-reported dyspareunia after treatment. We further aimed to utilize the anterior vaginal wall (AVW) as an organ-at-risk (OAR) to define an actionable dosimetric clinical goal to decrease the risk of patient-reported dyspareunia. Methods Women with SCCA treated with intensity-modulated radiation-based chemoradiation were surveyed at least two years after successfully completing therapy. The Female Sexual Function Index (FSFI) pain subscore ≤4 was used to define dyspareunia. Dosimetric parameters were calculated for both the full vaginal canal and AVW. Multivariable linear regression models were created to identify predictors of FSFI pain subscore using backward selection to identify final variables include in the models. An actionable dosimetric predictor for dyspareunia was established using the Youden index method for cutoff optimization. Results Of 184 women who were contacted, 90 (49%) returned completed surveys. Of those who completed surveys, 51 (56.7%) reported being sexually active and 47 had dosimetric data available for review. Of sexually active respondents, 32 (68%) had a FSFI pain subscore ≤4. Multiple regression models were generated using the full vaginal canal and AVW as OARs, and both models showed similar predictive relationships with volumetric dose parameters emerging as the best dosimetric predictors for dysparenuia. Age over 65 years was also associated with higher FSFI pain subscores (e.g. less pain with intercourse) in both models. AVW V35Gy<60% was identified as the optimal cutoff to reduce the risk of patient-reported dyspareunia. Conclusion Increased dose to the vaginal canal is significantly associated with worse patient-reported dyspareunia following CRT for SCCA. Minimizing dose to the AVW to V35Gy<60% may reduce the risk of this quality of life-limiting toxicity. Further prospective evaluation is needed to validate these findings.