Impact of Mucin Proportion in the Pretreatment MRI on the Outcomes of Rectal Cancer Patients Undergoing Neoadjuvant Chemoradiotherapy

PURPOSE: The purpose of this study was to evaluate treatment response to neoadjuvant chemoradiotherapy (CRT) with regard to mucin status in pathology and pretreatment magnetic resonance imaging (MRI) in locally advanced rectal cancer. MATERIALS AND METHODS: Between 2003 and 2011, 306 patients with locally advanced rectal cancer received neoadjuvant CRT followed by surgery, and mucinous adenocarcinoma (MAC) was found in 27 (8.8%). All MAC patients had MRI before and after CRT and mucin proportion at MRI was measured. Therapeutic response was assessed by pathology after total mesorectal excision. To determine the optimal cut-off for mucin proportion in predicting good CRT response (near total or total regression) and negative circumferential resection margin (CRM), the receiver-operating characteristic analysis was performed. RESULTS: After neoadjuvant CRT, overall downstaging occurred in 44.4% of MAC and 72.4% of non-MAC (p=0.001), and positive CRM (≤1 mm) was observed more frequently in MAC (p<0.001). The optimal threshold for treatment response was 30% for mucin proportion, and there are nine with low mucin proportion (<30%) and 18 with high mucin proportion (≥30%) in pretreatment MRI. Negative CRM and tumor downstaging occurred more common in patients with mucin <30%, although statistically insignificant (p=0.071 and p=0.072, respectively). Regarding oncologic outcomes, lower mucin proportion in pretreatment MRI was associated with better disease-free and overall survival in MAC group (p=0.092 and 0.056, respectively), but the difference did not reach statistical significance. CONCLUSION: Poor treatment outcome with neoadjuvant CRT was observed in patients with MAC, especially those with high mucin proportion at pretreatment MRI.