Nodular Fasciitis of the Vulva: Clinicopathologic Features and Diagnostic Pitfalls With Molecular Confirmation.

Nodular fasciitis (NF) is a benign, self-limited myofibroblastic proliferation that typically arises in the subcutaneous tissues of the extremities and trunk, while vulvar involvement is distinctly rare. Owing to its rapid growth and histologic features, NF may mimic both benign and malignant lesions, creating significant diagnostic challenges. We report a case of a 34-yr-old woman presenting with a rapidly enlarging, painless vulvar mass initially suspected to represent a Bartholin gland cyst. Gross examination revealed a 2.5 cm, well-defined lesion with a tan, glistening cut surface. Microscopically, the lesion was unencapsulated and composed of spindle cells arranged in a storiform pattern with alternating cellularity, set within a myxoid and collagenous stroma. Extravasated erythrocytes and occasional osteoclast-like giant cells were present, while cytologic atypia and necrosis were absent. Mitotic activity was low. Immunohistochemically, the lesion showed diffuse smooth muscle actin expression and was negative for CD34, desmin, ALK, cytokeratin, estrogen receptor, progesterone receptor, and STAT6 with retained RB1 expression and membranous β-catenin staining. Next-generation sequencing identified a MYH9:::USP6 fusion, confirming the diagnosis. This case highlights the importance of recognizing nodular fasciitis in the vulva as a potential diagnostic pitfall, particularly in lesions clinically interpreted as Bartholin gland cysts. Integration of morphologic, immunohistochemical, and molecular findings is essential to avoid misdiagnosis and overtreatment.