Mucinous Degeneration on MRI After Neoadjuvant Therapy in Patients With Rectal Adenocarcinoma: Frequency and Association With Clinical Outcomes.

Background: Patients with nonmucinous rectal adenocarcinoma may develop mucinous changes after neoadjuvant chemotherapy, described as mucinous degeneration. The finding's significance in earlier studies has varied. Objective: To assess the frequency of mucinous degeneration on MRI after neoadjuvant therapy for rectal adenocarcinoma and to compare outcomes among patients with nonmucinous tumor, mucinous tumor, and mucinous degeneration on MRI. Methods: This retrospective study included 201 patients (mean age, 61.8±2.2 years; 81 women, 118 men) with rectal adenocarcinoma who underwent neoadjuvant therapy followed by total mesorectal excision from October 2011 to November 2015, and who underwent baseline and restaging rectal MRI examinations and had at least 2 years of follow-up. Two radiologists independently evaluated MRI examinations for mucin content, defined as T2 hyperintensity in the tumor or tumor bed, and resolved differences by consensus. Patients were classified into three groups based on mucin status: nonmucinous tumor (≤50% mucin on baseline and restaging examinations), mucinous tumor (>50% mucin on baseline and restaging examinations), and mucinous degeneration (≤50% mucin on baseline examination, >50% mucin on restaging examination). The three groups were compared. Results: Interreader agreement for mucin content, expressed as kappa, on baseline MRI was 0.893, and on restaging MRI was 0.890. A total of 156/201 (77.6%) patients had nonmucinous tumor, 34/201 (16.9%) mucinous tumor, and 11/201 (5.5%) mucinous degeneration. Mucin status was not significantly associated with complete pathologic response (P=.41), or local or distant recurrence (both P>.05). Death rate during follow-up was not significantly different (P=.06) between patients with nonmucinous tumor (23.1%), mucinous tumor (29.4%), and mucinous degeneration (9.1%). In adjusted Cox-regression analysis, using mucinous degeneration as reference, HR for overall survival rate for mucinous tumor group was 4.7 (95% CI, 0.6-38.3; P=.14), and for nonmucinous tumor was 8.0 (95% CI, 0.9-59.9; P=.06). On histopathologic assessment, all 11 patients with mucinous degeneration showed acellular mucin, yet 10/11 patients showed viable tumor (i.e., in tumors' nonmucinous portions). Conclusion: Mucinous degeneration on MRI is not significantly associated with pathologic complete response, recurrence, or survival. Clinical impact: Mucinous degeneration on MRI is uncommon and should not be deemed an indicator of pathologic complete response.