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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.

793 Background: A growing body of literature suggests that 25-hydroxyvitamin D [25(OH)D] levels are inversely related to the risk of developing CRC and that deficiency is associated with CRC-specific mortality. Due to the unique racial-ethnic diversity and UV exposure patterns of the San Francisco Bay Area, we aimed to evaluate vitamin D levels among our CRC patients at time of diagnosis and during treatment. Methods: Permanent residents of the SF Bay Area with a new diagnosis of CRC of any stage were recruited between 2011 and 2015 prior to initiation of therapy. Self-reported data on sun exposure, diet, and exercise patterns were collected. Clinical data including disease stage and primary tumor location were abstracted from charts. Serum 25(OH)D levels at time of diagnosis and at 6-month follow-up were batched and measured using the Liaison XL assay (Heartland Assays). Supplement use was not restricted. Kruskal-Wallis and Pearson correlation tests were used for categorical or continuous variables, respectively, to evaluate the associations of patient characteristics with 25(OH)D levels. Results: Among 94 patients with a new diagnosis of CRC, median 25(OH)D level at baseline was 27.0 ng/mL (range 7.2-59.0); 26% had deficient levels (

3514 Background: A pathologic complete response (pCR) to pre-operative chemoradiation (CRT) is associated with improved prognosis in rectal cancer patients treated with total mesorectal excision (T...