Ulcerated nodular fasciitis of the finger mimicking soft tissue sarcoma: a case report.

Nodular fasciitis (NF) is a benign, self-limiting myofibroblastic proliferation that often mimics soft tissue sarcoma because of its rapid growth, hypercellularity, mitotic activity, and local infiltration. NF typically arises in the forearm, trunk, or upper arm, whereas finger involvement is extremely rare, accounting for 0-2% of cases. Ulceration or protrusion is even more unusual and may further resemble aggressive malignancy. We report a rare case of ulcerated NF of the finger, clinically mimicking a fungating sarcoma. A 26-year-old Japanese woman presented with a rapidly enlarging ulcerated mass on the ulnar side of the right middle finger. MRI revealed an isointense lesion on T1-weighted images and hyperintense signal on T2-weighted and T2 fat-suppressed images. Core needle biopsy showed spindle-cell proliferation in a loosely storiform pattern within a fibromyxoid stroma, with vascular fibrinoid necrosis and neutrophilic infiltration. Immunohistochemistry demonstrated smooth muscle actin positivity and negativity for CD34 and S100. Fluorescence in situ hybridization confirmed USP6 gene rearrangement, supporting the diagnosis of NF. Because of progressive growth after biopsy, marginal excision with full-thickness skin grafting was performed under local anesthesia, achieving complete removal and functional preservation. Histopathological findings were identical to those of the biopsy specimen. The postoperative course was uneventful, and there was no recurrence at the final follow-up, with full finger mobility maintained. This case highlights a rare manifestation of NF with ulceration in the finger, an entity that can closely mimic high-grade sarcoma. Awareness of this presentation and the use of molecular confirmation of USP6 gene rearrangement are crucial for accurate diagnosis and avoidance of unnecessary radical surgery, particularly in functionally and cosmetically critical locations such as the hand.