Nodular fasciitis of the breast: clinicopathologic and molecular characterization with identification of novel USP6 fusion partners.

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that can mimic malignancy due to its rapid growth, cellularity, and mitotic activity. Involvement of the breast is rare and diagnosis on biopsy can be challenging. In this largest series to date, we examined the clinicopathologic and molecular characteristics of 12 cases of nodular fasciitis involving the breast/axilla. All patients were female, with a median age of 32 years (range 15-61). The lesions were 0.4 to 5.8 cm in size (median 0.8). All cases presented as palpable masses, and two patients had overlying skin retraction. Microscopically, lesions were relatively well-circumscribed nodular masses of bland myofibroblastic spindle cells within a variably myxoid stroma. Infiltrative growth into adipose tissue or breast epithelium was frequent. Mitotic figures were present in all cases, ranging from 1 to 12 per 10 high-power fields (median 3). Immunohistochemically, all cases expressed smooth muscle actin and were negative for pan-cytokeratin, p63, desmin, CD34, and nuclear beta-catenin. Targeted RNA sequencing performed on 11 cases identified USP6 gene fusions in eight; one additional case was positive by break-apart fluorescence in situ hybridization. The common MYH9-USP6 rearrangement was detected in four cases; another case had a rare alternative fusion with CTNNB1. Three cases harbored novel USP6 gene fusions involving NACA, SLFN11, or LDHA. All fusions juxtaposed the promoter region of the 5' partner gene with the full-length coding sequence of USP6. Outcome data were available for eight patients; none developed recurrence or metastasis. Five patients elected for observation without immediate excision, and self-resolution of the lesions was reported in three cases. Albeit uncommon, nodular fasciitis should be considered in the differential diagnosis of breast spindle cell lesions. A broad immunohistochemical panel to exclude histologic mimics, including metaplastic carcinoma, is important. Confirmatory detection of USP6 rearrangements can aid in classification, with potential therapeutic implications.