Watch and Wait in Rectal Cancer Patients with Residual Mucin on MRI following Neoadjuvant Therapy

Background Neoadjuvant therapy (NAT) leads to a clinical complete response (cCR) in a significant proportion of patients with locally advanced rectal cancer (LARC), allowing for possible nonoperative management. The presence of mucin on MRI after NAT leads to uncertainty about residual disease and appropriateness of a watch-and-wait (WW) strategy in patients with no evidence of disease on proctoscopy (endoscopic cCR). Methods MRI reports for LARC patients seen between July 2016 and January 2020 at Memorial Sloan Kettering Cancer Center were queried for presence of mucin in the tumor bed on MRI following NAT. Clinicodemographic, pathologic, and outcome data were compiled and analyzed. Results Of 71 patients with mucin on post-treatment MRI, 20 had a cCR and 51 had abnormalities on endoscopy and/or physical exam. One patient with a cCR opted out of WW; thus, 19 patients (27%) entered WW and 52 patients (73%) were planned for surgery (Non-WW). Of the 19 WW patients, 15 (79%) have had no local regrowth with median follow-up of 50 months (range, 29-76 months), while 4 (21%) experienced regrowth between 9 and 29 months after neoadjuvant therapy. Of the 52 patients who were planned to have surgery (Non-WW), 49 underwent resection while 3 developed metastatic disease that precluded curative-intent surgery. Five (10%) of the 49 patients who underwent surgery, including the one with an endoscopic cCR, had a pathologic complete response. Conclusions The presence of mucin after NAT for LARC does not preclude WW management in otherwise appropriate candidates who achieve an endoscopic cCR.