Abstract

SMARCB1 gene is a member of the SWI/SNF complex, acting as a tumor suppressor gene. The loss of function of both alleles results in complete loss of INI1/BAF47 immunohistochemical nuclear expression. INI1 loss characterizes the expanding group of SMARCB1-deficient neoplasms from various organ-systems, including the recently described primary cutaneous SMARCB1-deficient carcinoma (PCSDC). We report a case of a 79-year-old female who presented with a 4.0 cm ill-defined, alopecic right scalp mass and right cervical lymphadenopathy. PET/CT scan revealed extensive hypermetabolic activity in the abovementioned lesional areas, without additional PET-avid locations. Biopsy revealed a poorly differentiated carcinoma not further characterized. The patient underwent resection of the scalp lesion with right neck dissection. An infiltrative, high-grade basaloid neoplasm diffusely involved the dermis and subcutis, without epidermal involvement. Tumor cells had focal plasmacytoid appearance and formed sheets, nests and cords with extensive angiolymphatic and perineural invasion and nodal involvement (22/28 positive lymph nodes). By immunohistochemistry, tumor cells expressed AE1/AE3, CAM5.2, CK20 (focal), p40, p63, GATA3, MOC31 (focal), with scattered INSM1-positive cells. Tumor cells were negative for CK7, Merkel Cell Polyomavirus, SMA, S100 and CD34. Loss of INI1 expression in tumor cells confirmed the diagnosis of PCSDC. PCSDC has a poor prognosis and typically presents at an advanced stage, without UV mutation signature. The diagnosis of PCSDS may be challenging on biopsies given the overlapping immunophenotype with other cutaneous tumors, and may be mistaken for a metastatic process. INI1 expression loss and the absence of other primary sites are helpful to establish the diagnosis.

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