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PURPOSE Prospective data on the efficacy of a watch-and-wait strategy to achieve organ preservation in patients with locally advanced rectal cancer treated with total neoadjuvant therapy are limited. METHODS In this prospective, randomized phase II trial, we assessed the outcomes of 324 patients with stage II or III rectal adenocarcinoma treated with induction chemotherapy followed by chemoradiotherapy (INCT-CRT) or chemoradiotherapy followed by consolidation chemotherapy (CRT-CNCT) and either total mesorectal excision (TME) or watch-and-wait on the basis of tumor response. Patients in both groups received 4 months of infusional fluorouracil-leucovorin-oxaliplatin or capecitabine-oxaliplatin and 5,000 to 5,600 cGy of radiation combined with either continuous infusion fluorouracil or capecitabine during radiotherapy. The trial was designed as two stand-alone studies with disease-free survival (DFS) as the primary end point for both groups, with a comparison to a null hypothesis on the basis of historical data. The secondary end point was TME-free survival. RESULTS Median follow-up was 3 years. Three-year DFS was 76% (95% CI, 69 to 84) for the INCT-CRT group and 76% (95% CI, 69 to 83) for the CRT-CNCT group, in line with the 3-year DFS rate (75%) observed historically. Three-year TME-free survival was 41% (95% CI, 33 to 50) in the INCT-CRT group and 53% (95% CI, 45 to 62) in the CRT-CNCT group. No differences were found between groups in local recurrence-free survival, distant metastasis-free survival, or overall survival. Patients who underwent TME after restaging and patients who underwent TME after regrowth had similar DFS rates. CONCLUSION Organ preservation is achievable in half of the patients with rectal cancer treated with total neoadjuvant therapy, without an apparent detriment in survival, compared with historical controls treated with chemoradiotherapy, TME, and postoperative chemotherapy.

10 Background: Interest in organ preservation (OP) strategies for rectal cancer (RC) patients persists. The efficacy of long course chemoradiation (LCRT) vs. short course radiation therapy (SCRT) relative to OP is unknown. We compared OP rates between SCRT and LCRT total neoadjuvant therapy (TNT) strategies. Methods: During the COVID-19 pandemic we established an institutional SCRT mandate with no exceptions. For comparison, we identified RC patients treated with LCRT immediately before and after the mandate period. After completion of TNT, patients were restaged by clinical exam, endoscopy, and MRI. A watch and wait (WW) approach was recommended for patients with a clinical complete response (cCR), defined by the MSK regression schema. Total mesorectal excision (TME) was recommended for non-cCR patients. OP was defined as alive, TME-free, and with no evidence of disease in the pelvis. We performed survival analysis for: local regrowth rate, OP, disease-free survival (DFS), and overall survival (OS). Results: We identified 563 consecutive patients with RC treated with TNT, of whom 231 were excluded due to either metastatic disease, synchronous/metachronous malignancies, or non-adenocarcinoma histology (Jan. 2018-Jan. 2021). Patient and tumor characteristics were similar in the LCRT (n = 256) and SCRT (n = 76) cohorts. No significant differences in high-risk features were noted. Most patients had clinical stage III disease (82% in LCRT vs. 83% in SCRT). Induction chemotherapy followed by consolidative radiation was the most common treatment order (78% (LCRT) vs. 70% (SCRT)). The median interval from end of TNT to clinical restaging was 8 weeks (LCRT) and 9 weeks (SCRT). The cCR rate was 46% in both cohorts. The cCR rate was numerically higher in patients treated with radiation first, as compared to chemotherapy first (53% vs. 44% (LCRT) and 52% vs. 43% (SCRT)). Among patients with a cCR, the likelihood of WW management was similar (98% (LCRT) vs. 94% (SCRT)). From start of TNT, the median follow-up was 32 and 28 months respectively for LCRT and SCRT. The 2-year OS (95% vs. 92%), DFS (78% vs 70%), and distant recurrence (20% vs. 21%) rates were similar. Among all patients, the 2-year OP rate was 40% (95% CI 35-47%) for LCRT and 29% (95% CI 20-42%) with SCRT. In those patients managed by WW, the 2-year local regrowth rate was 20% (95% CI 12-27%) with LCRT vs. 36% (95% CI 16-52%) with SCRT. Conclusions: In this nonrandomized comparison, while cCR rates were similar, we observed a numerically higher OP rate with LCRT-TNT than with SCRT-TNT. The ongoing ACO/ARO/AIO-18.1 trial, hypothesizing that LCRT-TNT will increase OP rates relative to SCRT-TNT, should definitively answer this question.

3509 Background: Clinical response following neoadjuvant therapy is paramount to identifying locally advanced rectal cancer (LARC) patients suitable for Watch and Wait (WW). A 3-tier schema was devised to stratify clinical response. Patients with a complete clinical response (cCR) are considered for WW, while those with an incomplete clinical response (iCR) are recommended for total mesorectal excision (TME). A near complete response (nCR) tier captures patients with significant, but not complete, response to be considered for WW. This schema’s efficacy has yet to be validated. We investigated survival and organ preservation (OP) rates based on this 3-tier clinical response assessment in patients with LARC who underwent total neoadjuvant therapy (TNT) in a prospective, multi-center clinical trial. Methods: Patients with MRI stage II and III rectal adenocarcinoma were randomized to either induction chemotherapy (FOLFOX or CAPEOX) followed by chemoradiation or chemoradiation followed by consolidation chemotherapy (FOLFOX or CAPEOX). At 8+/-4 weeks following TNT, response on digital rectal and endoscopic examinations was evaluated by the 3-tier schema. The date of this restaging clinical response assessment was used as time zero. The endpoints of rate of OP, disease-free survival (DFS), TME-free DFS, and overall survival (OS) were evaluated using the Kaplan-Meier method with differences analyzed by the log-rank test. Results: Clinical response assessments were available for 294 patients. The median time to assessment after neoadjuvant therapy was 7.9 weeks. Based on the 3-tier schema, 124 patients were categorized as cCR, 113 as nCR, and 57 as iCR. Baseline age, sex, average distance from the anal verge, clinical T classification, and clinical N classification were similar between the response groups. The table shows the 3-year rates of OP, DFS, TME-free DFS, and OS. The median follow-up was 2.36 years. Of the patients with a nCR, the 3-year TME rate was 48% compared with 21% in the cCR group. Conclusions: The 3-tier clinical response assessment has prognostic implications for OP and DFS in patients with LARC who underwent TNT. In patients with a nCR, more than half achieved OP at 3 years. This information should be utilized to counsel patients regarding their expected outcomes. Clinical trial information: NCT02008656. [Table: see text]

4008 Background: Organ preservation (OP) with a watch and wait strategy (WW) and total neoadjuvant therapy (TNT) are new treatment paradigms for patients with locally advanced rectal cancer. The safety and efficacy of WW and of TNT have not been studied prospectively. Methods: Patients with MRI stage II and III rectal adenocarcinoma were randomized to 4 months of FOLFOX or CAPEOX before (Induction) or after (Consolidation) fluorouracil or capecitabine based chemoradiotherapy (CRT). Patients were re-staged 8-12 weeks after finishing TNT with digital rectal exam, flexible sigmoidoscopy and MRI. Patients with complete or near-complete clinical response were offered WW. Those with incomplete response had total mesorectal excision. The trial was designed so that each arm served as its own single-stage study that discriminates between 3-year disease-free survival (DFS) rates of 75% (historical null) and 85%, with 86% power, and a two-sided type I error of 5%. Secondary objectives included comparing DFS, OP, and distant metastasis-free survival (DMFS) rates between the two arms using the log-rank test. Results: Of 324 patients enrolled, 307 (152 I, 155 C) are currently evaluable for the time-to-event analysis as of 2/1/2020. Median follow-up is 2.1 years; 52 DFS events were observed. Patient demographics and tumor characteristics were generally balanced across the two arms. Full compliance with systemic chemotherapy was 82% and 81% for the I- and C-arms, respectively. The median radiation dose was 5400 cGy for both arms. Table shows 3-y DFS, DMFS, and OP rates. Conclusions: A WW strategy for patients with locally advanced rectal cancer that achieve a clinical complete response to TNT results in organ preservation for a high proportion of patients without compromising survival. Up-front CRT followed by consolidation chemotherapy resulted in a numerically higher WW rate compared to induction chemotherapy followed by CRT. Clinical trial information: NCT02008656 . [Table: see text]