Signet ring cell colorectal carcinoma: a distinct subset of mucin-poor microsatellite-stable signet ring cell carcinoma associated with dismal prognosis.

We evaluated a consecutive series of signet ring cell colorectal carcinomas in an attempt to correlate the histopathologic pattern of infiltration with molecular alterations and prognosis. Of the 4760 primary colorectal carcinomas surgically resected between the years 2002 and 2012, 53 (1%) were composed of >50% signet ring cells. Of the 53 signet ring cell carcinomas, 40 (75%) were composed of >50% extracellular mucin with signet ring cells floating within pools of mucin and were subclassified as mucin-rich signet ring cell carcinomas. Thirteen (25%) carcinomas were characterized by diffusely infiltrating carcinomas with minimal to no extracellular mucin and were subclassified as mucin-poor signet ring cell carcinomas. All 13 mucin-poor signet ring cell carcinomas were either stage III or IV, whereas many cases of mucin-rich signet ring cell carcinoma were stage I or II (17 cases) (P=0.005). Compared with mucin-rich tumors, mucin-poor signet ring cell carcinomas more frequently demonstrated adverse histologic features such as lymphatic invasion (13/13, 100% vs. 22/40, 55%; P=0.002), venous invasion (6/13, 46% vs. 3/40, 8%; P=0.004), and perineural invasion (11/13, 85% vs. 9/40, 23%; P=0.0001). Twenty-three of 53 (43%) signet ring cell carcinomas demonstrated high levels of microsatellite instability (MSI-H). Twenty-two of 23 (96%) MSI-H signet ring cell carcinomas were mucin rich; only 1 MSI-H signet ring carcinoma was mucin poor (P=0.0033). Mucin-poor signet ring cell carcinoma had significantly reduced overall and recurrence-free survival compared with mucin-rich signet ring cell carcinomas (P=0.0035 and 0.0001, respectively), even when adjusting for tumor stage. Mucin-poor signet ring cell carcinoma had a higher propensity for peritoneal dissemination (5/13, 38%) compared with mucin-rich signet ring cell carcinoma (5/40, 12.5%), although this was not statistically significant (P=0.052). Finally, MSI-H and microsatellite-stable signet ring cell carcinomas had similar overall and recurrence-free survival (P=0.2266 and 0.1055, respectively), even when adjusting for tumor stage. In conclusion, we identified a unique subset of signet ring cell colorectal carcinoma with diffuse infiltration and minimal to no extracellular mucin (mucin-poor signet ring cell carcinoma), which lacks MSI-H and has a dismal prognosis with an aggressive clinical course often with peritoneal dissemination. Further, our results confirm that MSI does not affect survival in colorectal signet ring cell carcinomas.