Outcomes of Definitive Radiation Therapy for Inoperable Endometrial Cancer and Dosimetric Comparison Study of HDR Brachytherapy vs. SBRT Boost

Abstract

To report outcomes in patients with inoperable endometrial cancer (ECa) treated with definitive radiotherapy and perform a dosimetric comparison of organs at risk (OARs) for HDR brachytherapy (BT) vs SBRT boost in those who received combined external beam RT (EBRT)+BT treatment.An institutional retrospective analysis of ECa patients treated with definitive RT from 2008-2020 was performed. EQD2 doses for target D90 and pelvic OAR D0.1 cc, D2 cc were collected. The Kaplan-Meier method was used to estimate overall survival (OS), local, regional and distant failure rates. For patients with EBRT+BT boost, comparative SBRT boost plans were generated using CT scans and structures from a representative BT fraction and prescribed an identical dose. SBRT planning was optimized for target coverage and respect of ABS OAR constraint guidelines.32 patients were included. Median age was 67 years (range: 50-90). Median follow-up time was 25.3 months (range: 1.5-113.2). Stage was 66% FIGO I, 16% FIGO II, 16% FIGO III, and 3% FIGO IV. 81% (26/32) of tumors were endometrioid adenocarcinoma, 6% carcinosarcoma, and 12% UPSC. Median EBRT dose was 45 Gy in 25 fractions. 11 patients received sequential EBRT boost to gross disease, median 13.8 Gy (range: 5.4-19.6 Gy). BT doses ranged from 22.5 Gy in 3 fractions to 30 Gy in 5 fractions, with either double or single tandem applicators. Median total EQD2 for all patients was 75.6 Gy (40-104.9 Gy). Overall, 2-year LC, LRC, and DC rates were 83.1% (95% CI 60.7-93.3%), 78.2% (95% CI 54.9-90.4%), and 77.4% (95% CI 55.8-89.4%), respectively. 2-year OS was 73.3% (95% CI 51.6-86.4%). Single modality (EBRT or BT) use and total EQD2 < 70 Gy were significantly associated with decreased LRC; 45% when < 70 Gy vs 93.8% when > 70 Gy (P = 0.014). 18 patients received BT boost after EBRT. Dosimetric comparison of BT and SBRT boost yielded median total EQD2 D2cc of 73.8 Gy and 76.3 Gy for bladder (P = 0.83); 57.1 Gy and 58.1 Gy for rectum (P = 0.03); 67.3 Gy and 70.8 Gy for sigmoid (P = 0.011); and 61.2 Gy and 66.8 Gy for small bowel (P = 0.003), respectively. Dosimetric comparison details are listed in Table 1.We observed favorable control rates for inoperable ECa treated with definitive RT. Dosimetric comparison of BT vs SBRT boost in this setting showed comparable target coverage and slightly higher OAR doses with SBRT. These findings warrant caution and need for further study into dose heterogeneity, tumor control, and risk of normal tissue injury.