Rectal tonsil mimicking a rectal neuroendocrine tumor.

Rectal tonsil is an uncommon reactive lymphoid proliferation that may mimic a rectal neuroendocrine tumor endoscopically and a low-grade lymphoid neoplasm histologically. A 43-year-old woman underwent colonoscopy for abdominal pain, which revealed a smooth, hemispheric subepithelial lesion measuring approximately 22 mm in the lower rectum, 8 cm from the anal verge. Endoscopic ultrasonography demonstrated a well-demarcated hypoechoic lesion predominantly confined to the submucosal layer, with preservation of the muscularis propria. Because a rectal neuroendocrine tumor was the leading preoperative diagnosis and superficial biopsy might not preserve the required submucosal architecture, en bloc endoscopic submucosal dissection was performed. Histology demonstrated reactive lymphoid hyperplasia with tonsil-like architecture. Immunohistochemistry showed preserved B- and T-cell compartmentalization, BCL6-positive reactive germinal centers, relative absence of BCL2 expression within germinal centers, and germinal-center-predominant Ki-67 activity. No morphological or immunophenotypic evidence of lymphoma was identified; molecular clonality testing was not performed. The findings were compatible with rectal tonsil.