The role of image-guided volume-adapted high-dose-rate endorectal brachytherapy boost in total neoadjuvant treatment of distal locally advanced rectal cancer

3611 Background: Endorectal brachytherapy has been used as palliative or preoperative treatment for advanced or locally advanced rectal cancer, with moderate radiation induced toxicity. This study aimed to explore the efficacy of high-dose-rate 192 Ir brachytherapy combined with external beam radiotherapy (EBRT) in total neoajuvant treatment (TNT) of distal rectal cancer. Methods: From January 2017 to December 2022, eligible rectal adenocarcinoma patients were assessed as clinical stage II -III, with primary tumor located in distal rectum (≤5 cm from anal verge). The local staging was determined using endorectal ultrasound and magnetic resonance imaging (MRI). In addition, cases included in this study underwent preoperative semiweekly endorectal brachytherapy of 8-12Gy/2-3F/1-1.5w, followed by EBRT of 45-50.4Gy/25-28F, fluorouracil-based concurrent and consolidation chemotherapy. The brachytherapy was image-guided volume-adapted, and delivered by multi-channel 192 Ir source applicators with different diameters. Radiation proctitis was graded from 0 to 4 using Radiation Therapy Oncology Group (RTOG) criteria. Results: A total of 40 cases were enrolled in this retrospective study, biopsy-proven and diagnosed as clinical stage II (n = 8), stage III (n = 32). The rates of T4 and positive circumferential resection margin (CRM) were 35% and 87.5% respectively. The median value of the interval from completion of radiotherapy to operation was 86 days (interquartile range: 75-98 days). After neoadjuvant treatment, 2 cases were undergoing the watch-and-wait strategy with sustained clinical complete response (cCR), 38 patients received operation and R0 resection rate was 97.4%. The pathological complete response (pCR) rate was 28.9%, and major pathological regression rate was 67.5%. The theoretical rate of anus preservation was 62.5%, including 6 of 21 cases underwent abdominoperineal resection but achieved pCR, 17 cases underwent low anterior resection or intersphincteric resection, and 2 cases with sustained cCR. After a median follow-up of 37 months (range 12-68 months), the 3-year disease-free survival, local-regional reccurence-free survival and overall survival was 81.1% (95%CI: 64.3%-91.2%), 94.9% (95%CI: 80.6%-98.5%) and 96.9% (95%CI: 82.1%-99.4%), respectively. No patients experienced Grade ≥3 radiation proctitis. The median Wexner fecal incontinence score of anus-preserving cases was 1 (range 0-8). Subgroup analysis showed that the complete response rate (pCR or sustained cCR) was lower in cases received 2 fractions brachytherapy than those received 3 fractions (27.8% vs 36.4%). Conclusions: The addition of image-guided volume-adapted high-dose-rate 192 Ir endorectal brachytherapy boost to TNT could improve the pathological tumor response and anus preservation rate in distal locally advanced rectal cancer.