3531 - Achievable Dosimetry for the Anterior Vaginal Wall and Bulboclitorus with Vaginal Dilator Use in Pelvic Radiotherapy for Anal and Rectal Cancers
Presenter(s)
A. Shimomura1, M. Oorloff2, S. C. Zhang3, J. Steers2, M. Kamrava2, and K. M. Atkins2; 1Cedars-Sinai Department of Radiation Oncology, Los Angeles, CA, 2Department of Radiation Oncology, Cedars-Sinai Medical Center, Los Angeles, CA, 3Cedars-Sinai Medical Center, Los Angeles, CA
Purpose/Objective(s): Vaginal radiation exposure is associated with sexual dysfunction after pelvic radiotherapy (RT). Recently, the anterior vaginal wall (AVW) volume (V) receiving 35 Gy (V35) >60% has been associated with patient-reported dyspareunia. The use of a daily vaginal dilator (VD) during RT allows for AVW displacement, enhancing vaginal dose-sparing. Further, the bulboclitorus (BC) has been defined as a putative organ at risk (OAR) with feasibility in BC-sparing during anal cancer RT, specifically a BC V30Gy <50% and V40Gy <35%. However, there is a paucity of data on real-world achievability of these constraints in the setting of daily VD for anal and rectal cancers, which was studied herein.
Materials/Methods: Retrospective study of 26 adult women with anal or rectal cancer treated with definitive-intent pelvic intensity-modulated RT (IMRT) with daily VD use between 2021-2025. Standard institutional practice included delineation and constraint of the external genitalia (V20Gy <50%, V30Gy <35%, V40Gy <5%), while AVW (V35Gy <60%, dose [D] 50% <48 Gy) and BC constraints (V30Gy <50%, V40Gy <35%) were implemented in August 2024. The AVW and BC were manually contoured and dose-volume data extracted for cases where it wasn’t delineated. Logistic regression (LR) predicting AVW V35Gy >60% was performed.
Results: The median age was 60 years (interquartile range [IQR] 53-66), 54% (14/26) had anal/rectal squamous cell carcinoma (SCC) and 28% (10/26) had rectal adenocarcinoma. Among anal SCC, 21% (3/14) were cT3, 7% (1/14) cT4, and 43% cN1. Among rectal adenocarcinoma, 60% (6/10) were cT3, 20% (2/10) cT4, 50% cN2, including 40% (4/10) with external iliac and 10% (1/10) with inguinal nodal coverage. The most prescribed regimens were 50-50.4 Gy in 25-28 fractions (50%, 13/26) and 54-55 Gy in 25-30 fractions (50%, 13/26). The rate of achieving an AVW V35Gy <60% was higher in patients for whom the structure was intentionally spared vs not spared (100% [4/4] vs 32% [7/22], p=0.022), with a median V35 Gy of 46% (IQR 18-58%) vs 75% (IQR 52-83%), respectively. The rate of achieving BC constraints was high regardless of sparing intent (24/26 V30Gy <50%, 25/26 BC V40Gy <35%). On multivariable LR, only intentional sparing of AVW/BC structures remained significantly associated with the odds of AVW V35Gy <60% (odds ratio 0.02, 95% confidence interval 0.01-0.70; p=0.030), while VD diameter, external or inguinal LN regions, and adenocarcinoma histology were not (p>0.05).
Conclusion: In a heterogenous, real-world cohort of women treated with pelvic RT for anorectal cancers using daily VD, AVW and BC sparing was feasible. BC sparing was high, likely due to existing practice of intentionally sparing external genitalia. An AVW V35Gy <60% was significantly more likely to be achieved if intentionally spared, supporting daily VD use during anorectal RT with intentional sparing as best practice.