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Atypical teratoid/rhabdoid tumors (AT/RTs) are highly aggressive brain tumors of early childhood poorly responding to therapy. The majority of cases show inactivation of SMARCB1 (INI1, hSNF5, BAF47), a core member of the adenosine triphosphate (ATP)-dependent SWI/SNF chromatin-remodeling complex. We here report the case of a supratentorial AT/RT in a 9-month-old boy, which showed retained SMARCB1 staining on immunohistochemistry and lacked genetic alterations of SMARCB1. Instead, the tumor showed loss of protein expression of another SWI/SNF chromatin-remodeling complex member, the ATPase subunit SMARCA4 (BRG1) due to a homozygous SMARCA4 mutation [c.2032C>T (p.Q678X)]. Our findings highlight the role of SMARCA4 in the pathogenesis of SMARCB1-positive AT/RT and the usefulness of antibodies directed against SMARCA4 in this diagnostic setting.

Malignant change in a benign vascular tumor is exceedingly rare, and there have been only five previously reported convincing cases. Four new cases of angiosarcoma (AS) arising in a hemangioma/vascular malformation (HVM) are described. All patients were in the 6th or 7th decade of life (two female, two male). Development of an enlarging deep-seated mass was the main presenting symptom. MRI disclosed the presence of two separate soft tissue masses in both thighs in one patient. No patient had a history of prior radiotherapy at the same site. Preoperative duration, known in three cases, ranged from 1 to 24 months (median 12 months). Three tumors were located in the lower extremities (thigh and buttock), one in the retroperitoneum, and one in the parotid region. Three patients were treated by marginal excision; in one case only a biopsy was performed. Radiotherapy/chemotherapy was given in all cases. Two patients were disease free 2 and 14 months after surgery and two developed metastases. Grossly, the tumors were described as frankly hemorrhagic masses or as firm, whitish areas with hemorrhagic nodules and were centered in skeletal muscle in three cases. Size ranged between 2.2 cm and 8 cm (median 4.3 cm). Histologically, all the tumors had two distinct components. In three cases the benign and the malignant components were variably intermixed, whereas in one case the HVM was mainly located at the edge of the malignant tumor. The benign component showed features of an arteriovenous hemangioma (three cases) or intramuscular capillary hemangioma. AS showed epithelioid morphology in three cases and a well-differentiated dissecting pattern in one case. An imperceptible transition between the two components was noted in two cases. The two anatomically separate masses excised from one patient appeared almost identical. All cases were positive for at least two endothelial markers (CD31, CD34, VWF) and negative for the epithelial markers (EMA, AE1/AE3, Pan-keratin). Possible mechanisms for this exceptional phenomenon are discussed.