Woo Cheal Cho MD1, Fabiola Balarezo, MD1 UNIQUE MORPHOLOGIC AND IMMUNOPHENOTYPIC FEATURES OF MALIGNANT RHABDOID TUMOR OF LIVER MIMICKING PROXIMAL-TYPE EPITHELIOID SARCOMA Woo Cheal Cho MD1, Fabiola Balarezo, MD1 1Department of Pathology and Laboratory Medicine, Hartford Hospital, Hartford, Connecticut INTRODUCTION MICROSCOPIC FINDINGS Malignant rhabdoid tumor (MRT) is a rare but highly aggressive pediatric neoplasm that occurs most commonly in the kidney. Extra-renal MRT is even rarer and has been reported in the central nervous system (atypical teratoid/rhabdoid tumor) and other sites including the liver. To date, there are fewer than 50 cases of MRT of the liver reported in the literature. Here, we report a case of MRT of the liver with unique morphologic and immunophenotypic features in a 1-year-old male infant. CASE PRESENTATION A B C A 1-year-old male presented with a right-sided non-tender abdominal mass. Imaging studies revealed a heterogeneously enhancing mass (approximately 10.0 x 10.0 x 7.9 cm) within the liver, suspicious for hepatoblastoma. Liver biopsy showed a proliferation of malignant epithelioid cells with clear to amphophilic cytoplasm and small nucleoli (Figures A-C). The tumor focally exhibited an organoid/trabecular growth pattern, and a rim of compressed non-neoplastic liver parenchyma was seen at the periphery of the tumor (Figure A). Necrosis was present (Figure B) and numerous mitotic figures (7 mitoses/10 HPF) (Figure C) were identified. No definite rhabdoid cells were seen. Immunohistochemically, the tumor was diffusely positive for CK19, CKMNF116, vimentin, and beta-catenin (membranous) (Figure D) and focally positive for EMA, glypican-3, and CD34 (Figure E). The tumor exhibited negative immunoreactivity for CK7, CK20, Hep Par-1, arginase-1, AFP, desmin, myogenin, HMB45, melan-A, OCT3/4, CEA, chromogranin, synaptophysin, calponin, SMA, S100, and SOX10. There was also loss of nuclear INI1 expression (Figure F). The diagnosis of INI1-negative malignant neoplasm was entertained with differential diagnoses that include extra-renal MRT and proximal-type epithelioid sarcoma (ES). Although the tumor showed positive expression of CD34 and beta-catenin and failed to demonstrate classic rhabdoid cells, the patient’s age and tumor location favored extra-renal MRT over ES. D E F DISCUSSIONS Extra-renal MRT and proximal-type ES share similar morphologic and immunophenotypic features, including characteristic rhabdoid cells and loss of nuclear INI1 expression. CD34 and beta-catenin have been suggested as useful markers for distinguishing extra-renal MRT from proximal-type ES. Proximal-type ES often shows positive expression of CD34 and beta-catenin while extra-renal MRT typically lacks immunoreactivity for these markers. The presented case is unique in that the tumor lacked classic morphologic features of rhabdoid cells and showed an overlapping immunoprofile with proximal-type ES. To the best of our knowledge, this is the first reported case of extra-renal MRT with positive expression of CD34 and beta-catenin. REFERENCES Oda Y et al. Extrarenal rhabdoid tumors of soft tissue: clinicopathological and molecular genetic review and distinction from other soft-tissue sarcomas with rhabdoid features. Pathol Int 2006;56:287-95. Oita S et al. Malignant rhabdoid tumor of the liver: a case report and literature review. Pediatr Rep 2015;7:5578.