Incomplete Resection Is Twice as Likely in Locally Advanced Mucinous Compared to Nonmucinous Rectal Adenocarcinoma: A National Propensity-Matched Analysis.

Background and objectives: Rectal mucinous adenocarcinoma (MA) has poor response to neoadjuvant chemoradiation (NCR) and higher involved radial surgical margin rates than nonmucinous rectal adenocarcinoma (NMA).

Methods: The National Cancer Database (2010-2018) was queried for adult patients with clinical stage II and III rectal cancer. Patients with MA and NMA treated with NCR and total mesorectal excision (TME) were identified. We sought to evaluate differences in pathologic downstaging and completeness of resection between MA and NMA rectal adenocarcinoma.

Results: We identified 13 294 patients, 12 655 (95.2%) NMA and 639 (4.8%) MA. After 3:1 propensity score matching for pathologic outcomes, 1707 NMA and 569 MA patients were included. MA patients had more involved distal (2.1% vs. 1.1%, p = 0.047) and radial (29% vs. 15%, p < 0.001) margins, and less pathologic downstaging (49% vs. 55%, p = 0.015). Among MA patients, minimally invasive resection had higher distal (2.1% vs. 1.4%, p = 0.65) and radial margin involvement (35% vs. 26%, p = 0.09) compared to open resection. Incomplete resection rates were similar between low anterior and abdominoperineal resection.

Conclusions: TME of locally advanced rectal MA treated with NCR resulted in nearly double the involved radial surgical margins compared to NMA. Operative approach should be carefully considered. Extended or extra-mesorectal resection may reduce the risk of incomplete resection.