Microcystic/reticular schwannoma of the proximal sigmoid colon: case report with review of literature.

We report a case of microcystic/reticular schwannoma of the proximal sigmoid colon in a 61-year-old man. A 12-mm polyp was detected while the patient was undergoing screening for colorectal neoplasm. This rare variant of schwannoma was initially described in 2008 and shows a predilection for the visceral organs, predominantly the gastrointestinal tract. We also review 11 other reported cases of microcystic/reticular schwannomas in the gastrointestinal tract. Unlike conventional gastrointestinal schwannomas, which are more common in the stomach, this variant appears to be more common in the large intestine. Histologic examination of this polyp showed predominant lipoblast-like vacuolated cells within a myxoid stroma with focal spindle cell areas. Features suggestive of malignancy, like nuclear pleomorphism, mitosis, or necrosis, were absent. Immunohistochemistry for S100 protein showed strong nuclear and cytoplasmic positivity, whereas cytokeratin and CD117 stains were negative. It is important to entertain microcystic/reticular schwannoma in the differential diagnosis of a signet ring cell adenocarcinoma or a myxoid gastrointestinal stromal tumor, particularly on small biopsy specimens.