Watch and wait in rectal cancer patients with residual mucin on magnetic resonance imaging following neoadjuvant therapy

Neoadjuvant therapy leads to a clinical complete response in a considerable proportion of patients with locally advanced rectal cancer, allowing for possible nonoperative management. The presence of mucin on magnetic resonance imaging (MRI) after neoadjuvant therapy leads to uncertainty about residual disease and appropriateness of a watch-and-wait strategy in patients with no evidence of disease on proctoscopy (endoscopic clinical complete response). MRI reports for locally advanced rectal cancer 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 neoadjuvant therapy. Clinicodemographic, pathologic, and outcome data were compiled and analyzed. Of 71 patients with mucin on posttreatment MRI, 20 had a clinical complete response, and 51 had abnormalities on endoscopy and/or physical exam. One patient with a clinical complete response opted out of watch-and-wait; thus, 19 (27%) patients entered watch-and-wait, and 52 (73%) patients were planned for surgery (non-watch-and-wait). Of the 19 watch-and-wait 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-watch-and-wait), 49 underwent resection while 3 developed metastatic disease that precluded curative-intent surgery. Of the 49 patients who underwent surgery, 5 (10%) had a pathologic complete response (including the patient with an endoscopic clinical complete response). The presence of mucin after neoadjuvant therapy for locally advanced rectal cancer does not preclude watch-and-wait management in otherwise appropriate candidates who achieve an endoscopic clinical complete response.