Facial Nodular fasciitis: A clinicopathologic challenge

Summary: A 59-year-old male presented with a 3.3 cm ulcerating and enlarging nodule on the left cheek for 7 months. An incisional biopsy demonstrated a highly cellular proliferation of atypical stellate spindled cells and dilated capillaries within myxoid stroma emulating a tissue culture appearance. Immunohistochemical stains were positive for CD10 and SMA in the lesional cells. The cells were negative for P40, CK AE1/AE3, and SOX10 making a spindle squamous cell carcinoma and melanoma less likely. Gene rearrangement for USP6 showed a MYH9::USP6 fusion confirming the diagnosis of nodular fasciitis. Facial nodular fasciitis is a benign myofibroblastic neoplasm, however due to its rapid growth and unusual location, it can clinically mimic a malignant process. Morphologically, lesions with increased cellularity and mitotic rate complicate the diagnosis. If suspicion is high, USP6 gene rearrangement can be confirmatory. MYH9::USP6 fusions are the most common gene rearrangement in nodular fasciitis. This fusion is also detected in fibroma of tendon sheath and aneurysmal bone cysts. Surgical intervention is the standard treatment for facial nodular fasciitis, however intralesional triamcinolone and spontaneous regression have been reported. Rare cases of a malignant nodular fasciitis have been described and harbor other USP6 fusion partners such as CALD1::USP6 and PPP6R3::USP6.