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Case Reports

Abstract

A 28-year-old female presented with a two-year history of an enlarging, painful vulvar lesion. Physical examination revealed a 4 cm mass arising from the labia majora. Excisional biopsy demonstrated an exophytic dermal nodule composed of spindle cells with moderate cellularity, haphazard growth, and frequent interspersed lymphocytes, histiocytes, and occasional eosinophils.  Cytologically, the tumor cells showed expanded cytoplasm, occasional mitotic activity, and monomorphous growth with occasional pleomorphism and multinucleation.  The tumor cells showed weak cytoplasmic staining with SMA, cytokeratin AE1/AE3, and EMA. Ki67 demonstrated modestly increased proliferative activity. The tumor cells were negative for ALK1, desmin, CD31, CD34, STAT6, CK5/6, CAM5.2, S100, SOX10, HMB45, p63, p16, and calponin. The tumor cells showed retained nuclear expression of INI1(SMARCB1) and BRG1 (SMARCA4). CyclinD1 showed overexpression in tumor cells. Estrogen and progesterone receptor immunostains show positive expression. The cytologic features raised consideration of a fusion-driven tumor and a solid tumor NGS fusion panel was performed, demonstrating a THBS1::ERBB2 rearrangement. This is the first case of a low-grade vulvar spindle cell neoplasm with THBS1::ERBB2 fusion to be reported in the literature. Further, mesenchymal tumors with ERBB2 (HER2) fusion have rarely been reported in the literature. Morphologically, the tumor resembles inflammatory myofibroblastic tumor but lacks the characteristic ALK1 fusion. Interestingly, inflammatory myofibroblastic tumors of the female genital tract with THBS1::ALK1 rearrangement have been reported, raising consideration for this tumor to have a similar biologic classification. We present this case to bring awareness to this novel translocation with ERBB2 identified in a vulvar low-grade spindle cell neoplasm.