Large pools of extracellular mucin are present, lined by malignant glandular epithelium.
The malignancy epithelium consists of atypical appearing pseudostratified hyperchromatic cells. Some of the mucin pools have an attenuated or absent epithelium. The mucin appears to dissect with relative ease into the stromal tissues.
While there is some debris within the mucin, there are no signet ring cells. The cells floating in the mucin are consistent with macrophages.
The WHO classification recognizes a subset of tumors that have >50% extracellular mucin as a mucinous subtype. If there is prominent extracellular mucin but it comprises >10% but less than 50% of the tumor volume, the term "adenocarcinoma with mucinous features" is most appropriate. When more than 50% of the tumor contains signet ring cells, then the diagnosis is specified as "signet ring cell carcinoma".
Mucinous adenocarcinoma accounts for about 10% of all colon cancers. DNA mismatch repair defects and MSI-H are seen more often in this subtype (Chiang) and p53 is less frequently positive.
Patients with HNPCC (Lynch syndrome) and younger patients are more likely to be affected. These tumors arise with equal frequency in the right and the left colon.
This tumor subtype has been reported to carry a worse prognosis than the comventional type of adenocarcinoma. Some suggestions for this observation relate to the mucin, which may (1) facilitate dissection of tissue planes resulting in more extensive growth or (2) prevention of the immunological recognition of tumor cells by interfering with inflammatory responses.
Tumors in at least one study showed significantly better survival in those with proximal location and those with MSI (Chiang).
and MAC tumor with MSI
Chiang JM, et al. Mucinous adenocarcinoma showing clinicopathological and molecular characteristics in relation to different colorectal cancer subgroups. Int J Colorectal Dis. 2010 Aug;25(8):941-7.