GSOR18 Presentation Time: 11:55 AM: Obesity and Skin D2cc Predict for Radiation Necrosis After Interstitial Brachytherapy Boost in Locally Advanced Vulvar Cancer

Abstract

<h3>Background</h3> Interstitial brachytherapy (ISBT) has been increasingly utilized in treating locally advanced as well as locally recurrent vulvar cancer. However, the risk of BT-associated grade 3 or 4 skin necrosis is largely underreported, owing to the rarity of the disease. The purpose of this investigation was to evaluate therapeutic benefit of BT boost and risk of necrosis in a relatively large subset of patients with locally advanced vulvar cancer. <h3>Methods</h3> Patients with vulvar cancer treated with radiation therapy at Upstate Medical University between January 1, 2014 and July 1, 2021 were retrospectively analyzed. Demographics, oncologic history, external beam and BT dosimetry details, clinical outcomes, and CTCAE graded skin toxicities were evaluated and compared across individuals receiving external beam radiation (EBRT) only versus EBRT+brachytherapy (EBRT+BT). <h3>Results</h3> Of the 33 patients with vulvar cancer included, the median age of the cohort was 65.5 years; 97% were non-Hispanic Caucasian. Median follow-up was 31.1 months (2 patients were lost to follow-up and 11 patients were deceased at the time of analysis). The majority of patients had FIGO stage III and IV disease (24/33, 73%); however, stage IV was more frequent among patients treated with EBRT+BT (9/15, 60%; p=0.03). A total of 6 patients were being treated for recurrent disease and 27 with definitive intent. In this cohort, 15 patients received only EBRT and 18 received EBRT+BT. There was a greater proportion of patients with history of smoking in the EBRT+BT group (87% vs. 56%, p=0.05); and the percentage of active smokers was numerically greater in the EBRT+BT group (54% vs. 20%, p=0.09). The median dose of EBRT alone was 6120 cGy (4500-7000 cGy) and the median dose of EBRT for patients receiving BT was 5040 cGy (4500-6750 cGy). Median BT prescription was 2500 cGy over 5 fractions twice daily (1600-3000 cGy). Overall (n=31), there were 6 cases of grade 3 (19%) and 7 of grade 4 (23%) radiation-related skin toxicity including ulceration and/or necrosis. Grade 4 toxicities were exclusively cited among patients treated with BT (7/7, 100%, p=0.014). There were 10 (33%) cases of necrosis: 3 in the EBRT group and 7 in the EBRT+BT group (50%, p=0.07). The odds of developing necrosis were greater among women treated with BT, OR=4.3, CI (0.85-22.23). Obese patients (BMI ≥ 30 kg/m2) had 15 times the odds of developing necrosis from BT as compared to non-obese patients, OR=15.0, 95% CI (1.03-218.3). The risk of BT-related necrosis was even greater in obese smokers, OR=20.0, (1.4-282.5). Patients with grade 4 skin necrosis had a mean skin D2cc of 139%, whereas those with grade 1 or 2 toxicity had a mean D2cc of 79.8% (p=0.001). Overall response to treatment, either partial or complete, appeared to be greater in the BT group (92% vs. 81%, p=0.14). Clinically, a greater proportion of patients receiving BT achieved complete response (77% vs. 65%, p=0.312). Complete responders had a significantly lower BMI (30.3 kg/m²) than non-responders (36.2 kg/m²) and partial responders (37.2 kg/m²) (p=0.05). <h3>Conclusion</h3> ISBT for locally advanced vulvar cancer appears to provide an improvement in overall response to therapy. Toxicity appears greatest in obese patients who are active smokers. Additionally, skin D2cc greater than 130% of prescribed dose appears to correlate with increased risk of grade 4 skin necrosis. These clinical and dosimetric factors should be taken into account when utilizing ISBT for vulvar cancer.