Magnetic resonance imaging defined mucinous rectal carcinoma is an independent imaging biomarker for poor prognosis and poor response to preoperative chemoradiotherapy.

Introduction: Mucinous adenocarcinomas represent a potentially poor prognostic subgroup identifiable by imaging. We compared outcomes between magnetic resonance imaging (MRI) detected rectal mucinous carcinoma and adenocarcinomas. The diagnostic performance of MRI compared with initial biopsy in detecting mucinous adenocarcinoma was also assessed.

Methods: The proportion of patients downstaged in the mrMucinous and adenocarcinoma groups was compared. Cox proportional hazard models were used to test independence of mucinous status and baseline MRI and clinical variables on survival. Differences in survival for mucinous versus non-mucinous tumours were tested for significance using the Mantel-Cox log rank test.

Results: 60/330 (18%) patients were correctly diagnosed with mucinous rectal cancer based on pre treatment MRI compared with 15/330 (5%) on initial biopsy (diagnostic odds ratio=4.67, p<0.05). All 60 (100%) patients undergoing surgery for mrMucinous tumours were confirmed as such on final histopathology. Significantly fewer mrMucinous tumours showed ypT downstaging when compared with non-mucinous tumours (14/60 (23%) versus 111/270 (40%), p=0.01). Three-year survival outcomes for patients for MRI detected mucinous tumours were significantly worse: disease free survival (DFS) was 48% versus 71%, p=0.006 and OS was 69% versus 79% p=0.04. MRI Mucin was an independent variable for poor DFS (hazard ratios (HR)) 0.58 95% Confidence interval (CI) 0.38-0.89).

Conclusions: MRI diagnosis of mucinous adenocarcinoma is diagnostically superior to preoperative biopsy and occurs in up to 20% of rectal cancer patients. It is an independent imaging biomarker for response to preoperative chemoradiotherapy (CRT) and prognosis. MRI documentation of mucinous status will enable future pursuit of treatment strategies in this poor prognostic subgroup.