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To assess the proportion of patients with rectal adenocarcinoma achieving clinical complete response after neoadjuvant chemoradiation (NACTRT) and MR guided endorectal brachytherapy boost (MR-ERBT) Patients with rectal cancers (T2-T4/N0-N+) treated with concurrent chemoradiotherapy (50Gy/25# with Capecitabine) between June-2017 to April-2022. Post RT, patients having residual non-circumferential lesions <8cm in length were administered escalated-dose MR-ERBT with Ir192 HDR source. A median dose of 12Gy (8-15Gy) in 3 (2-3) fractions at 3-5 day intervals was delivered using MR-ERBT after external radiation. Data on near complete/complete clinical response (nCR/cCR) rates, local regrowth rates and clinical outcome were collected for analysis. Of the 145 patients who received MR-ERBT, majority were staged as T3(78.7%) and N1-2 (77.5%) rectal cancers. Median tumor length was 4cm and 123 (85%) of the tumors were located in the lower rectum (0-5cm from anal verge). Seventy-six (52%) patients achieved cCR or nCR (37 cCR, 39 nCR) and were advised observation or watch and wait (WW) management. The 69 patients having partial response were advised surgery. The patients having nCR 16 (29%) underwent resection, of these 10 (62%) had pathological complete response (pCR). The patients with partial or poor response, 57 underwent resection and of these 11 (19%) had pCR, 12 patients refused surgery due to fear of permanent stoma and continued to be on follow up. Of the 79 patients undergoing resection, 36 (45.5%) had sphincter preserving surgeries. At the median follow up of 30 months, local regrowth was seen in 8 (10.5%) of patients on WW and 6 were surgically salvaged while other 2 had synchronous metastatic relapse. Thus, 56 (41%) achieved organ preservation and continued to be on WW management. Twelve (8,2%) patients developed distant metastasis in the entire cohort, 3 in the WW group and 9 in the resected group. There were no pelvic recurrences seen in the resected patients. The disease-free survival at 3 years were (96.1% vs 89% Observation vs. resected (p_0.05) respectively. The overall survival at 3 years were (93% vs 98% Observation vs. resected (p_0.44) respectively. Late rectal toxicity was observed in 16(11%) patients on observation Dose Escalated MR-EBRT is an effective and safe method to enhance complete clinical response, thus improving the rate of organ preservation for distal rectal cancers. [ABSTRACT FROM AUTHOR]

20-30% of the locally advanced rectal cancer patients undergoing neoadjuvant chemoradiation (NACRT) have complete response (CR) and are suitable for wait and watch approach. This tumor response can be enhanced by radiotherapy dose escalation such as brachytherapy which is typically given at the end of NACRT. However, there is no data so as to which patients are likely to benefit by brachytherapy to achieve CR. We aimed to identify decrease in tumor length following CTRT, as predictor of CR, and eventual organ preservation. Patients diagnosed with T2-T4, N0-N2 distal rectal cancer treated from December 2017 to March 2021 were retrospectively reviewed. All were treated with radiation dose of 50 Gray (Gy) in 25 fractions (#) with concurrent capecitabine. All patients received escalated dose of radiation by high dose rate endorectal brachytherapy of 8-12 Gy in 2-3 #s within 2 to 3 weeks post CTRT. Baseline tumor length (MRI) and residual tumor length after CTRT during brachytherapy (assessed with planning MRI) were compared for response. Statistical analysis was done to determine decrease in tumor length as predictor of CR. One hundred patients records were reviewed. Disease and treatment characteristics are provided in table 1. Median length of tumor at baseline was 4.9 cm (3.8-6 cm). 39 patients had >50% decrement in tumor length (median length 3 cm) from baseline post CTRT, whereas 61 patients had <50% response. Out of 39 patients with>50% response, 26 patients achieved CR (66.7%) while 13 patients had partial or no response (33.3%), while out of 61 patients with <50% decrease in tumor length, 27 achieved CR (44.2%) and 34 had partial or no response (55.8%). The patients who had >50% reduction in tumor length had higher probability of achieving CR than the patients with <50% reduction (66% vs 44% p=0.029). Response assessment at the end of CTRT with respect to decrease in length of tumor may predict CR and thus can lead to better selection of patients for escalated radiation dose via endorectal brachytherapy leading to higher rate of organ preservation. [ABSTRACT FROM AUTHOR]