Abstract

SMARCA4-deficient undifferentiated malignant neoplasm (SD-UMN) is a rare and recently described entity characterized by the loss of expression of the SMARCA4 (BRG-1) and/or SMARCA2 (BRM) proteins that are involved in chromatin remodeling. SD-UMN presents a diagnostic challenge due to its rarity and unique histological and immunohistochemical features. In this report, we present a case of primary cutaneous SD-UMN in a 67-year-old man who presented with a rapidly growing, ulcerated, and bleeding nodule on his right cheek. Histopathological examination revealed a highly cellular dermal tumor consisting of pleomorphic epithelioid cells with prominent mitotic activity and necrosis, lacking any morphological evidence of differentiation. Immunohistochemical analysis showed a complete loss of SMARCA4 and SMARCA2 expression, while INI-1 expression remained intact. Additionally, a range of markers, including high molecular weight cytokeratin, p63, NKX2.2, CD99, CDX2, CD56, ERG, NUT, desmin, androgen, chromogranin, CD34, and CD43 were all negative. To date, only two cases of primary SMARCA4-deficient undifferentiated skin neoplasm have been reported in the literature. Therefore, this case report adds to the limited body of knowledge on the clinical and histopathological features of this novel entity. The report highlights the importance of considering SD-UMN in the differential diagnosis of undifferentiated cutaneous tumors.