SMARCB1 (INI1)-negative rhabdoid carcinomas of the gastrointestinal tract: clinicopathologic and molecular study of a highly aggressive variant with literature review.

Gastrointestinal (GI) neoplasms with rhabdoid features have been reported since 1989 under diverse names (giant cell carcinoma, pleomorphic carcinoma, malignant rhabdoid tumor, adenocarcinoma with rhabdoid features/phenotype, anaplastic carcinoma, etc.), but their clinicopathologic spectrum, SMARCB1 (INI1) status and relationship to common GI carcinomas have not been clarified yet. We describe 2 carcinomas from the stomach and the cecum with exclusive rhabdoid morphology. The patients died of disease at 6 and 10 months, respectively. The tumors coexpressed vimentin, pancytokeratin, and EMA. Both showed complete loss of nuclear SMARCB1/INI1. Molecular analysis (KRAS, EGFR, BRAF, PIK3CA, and microsatellite studies) revealed a CpG-island methylator phenotype in the cecal tumor (CIMP(+)/MLH1(-)/BRAF(V600E)/MSI-H), confirming epithelial origin. The gastric tumor showed poorly differentiated adenocarcinoma in regional nodes, again confirming epithelial derivation. Other genes tested were wild type in both cases. Review of reported cases (total: 39) revealed a glandular component in 33%. Affected sites are: stomach (13), colon (11), small bowel (10), and distal esophagus (5). Of the 34 patients with follow-up ≥12 months, 29 (85%) died within 1 year (mean: 4 mo). Molecular tests were performed in 8/39 cases. A CIMP(+)/BRAF(V600E)/MLH1(-) phenotype was found in 3/4 right colon tumors. Loss of nuclear SMARCB1 protein was noted in 3/6 cases tested. This study highlights the heterogeneity of rhabdoid GI neoplasms and supports their epithelial derivation. Rhabdoid phenotype likely represents a common pathway of dedifferentiation with frequent loss of SMARCB1 and highly aggressive course. The CIMP phenotype represents a novel subset of rhabdoid GI carcinomas. This rare variant should be distinguished from proximal-type epithelioid sarcoma and other SMARCB1-deficient mimics.