Colonic adenomatous polyps involving submucosal lymphoglandular complexes

Lymphoglandular complexes (LGCs) are lymphoid nodules containing intestinal mucosa, present in close apposition to muscularis mucosae or submucosa. Rarely, colorectal adenomas involve submucosal LGCs, simulating invasive adenocarcinoma with associated submucosal lymphoid aggregates, and presenting a diagnostic pitfall. We aimed to identify distinctive histologic features between submucosal LGCs and true invasion. Seven adenomas (tubular/tubulovillous adenomas [n=6], including 4 with high-grade dysplasia and 1 with focal intramucosal adenocarcinoma, and sessile serrated adenoma [n=1]) were in the right (n=5) and left colon (n=2). Seven adenocarcinomas were in the right (n=3), left (n=2), and rectum/rectosigmoid colon (n=2). Adenomatous glands involving submucosal LGCs were invested in lamina propria, showed continuity with surface adenoma, were well rounded and contained within lymphoid tissue, and predominantly lacked classic features of "pseudoinvasion." One case showed a herniation pattern carrying muscularis mucosae. Adenocarcinomas had at least one of the following features: Infiltrating single cells/small clusters (n=5), poorly formed, fused, and irregular glands (n=2), solid tumor nests (n=1), desmoplastic reaction (n=5), intraluminal necrosis (n=3), or lymphovascular invasion (n=1). In contrast, no adenoma had these features. Adenocarcinomas showed no herniation, but connection to surface tumor (n=5) was seen. Five invasive adenocarcinomas extended into the submucosa beyond the lymphoid aggregate. In conclusion, adenomas involving LGCs are a rare, clinicopathologically distinct form of pseudoinvasion that mimics invasive adenocarcinoma; histologic features that distinguish them are a well-rounded contour contained within the lymphoid tissue, and lack of infiltrating single cells/small clusters, poorly formed, fused, and irregular glands, solid tumor nests, desmoplastic reaction, and lymphovascular invasion.