Abstract

Fibromatoses encompass a broad group of histopathologically similar fibroblastic/myofibroblastic proliferations with divergent clinical manifestations and behavior. Deep (desmoid-type) fibromatoses are typically large, rapidly growing, and locally aggressive tumors that occur in the abdominal wall, mesentery, and extraabdominal soft tissue, principally the musculature of the trunk and extremities. Most sporadic cases of desmoid fibromatosis harbor inactivating mutations in CTNNB1, the gene encoding beta-catenin. Tumors occurring in the context of familial adenomatous polyposis and Gardner syndrome bear inactivating mutations in APC. By contrast, mutations in CTNNB1 or APC have not been identified in cases of superficial fibromatosis. Here, we present a case of a 78-year-old male who developed a 1.5-cm nodule on the right medial calf that had grown over six months. Incisional biopsy and subsequent excision demonstrated a dermal-based proliferation of bland spindle-shaped and stellate cells arranged in elongated fascicles with multifocal extension into the subcutaneous tissue. The lesional cells diffusely expressed cytoplasmic smooth muscle actin and nuclear beta catenin by immunohistochemistry, while desmin was negative. A targeted next generation sequencing panel revealed a pathogenic mutation in CTNNB1 (c.110C>T, p.Ser37Phe, variant allele frequency 9%) and a variant of unknown significance in APC (c.3206G>A, p.Arg1069Lys, variant allele frequency 11%). The histopathologic and molecular findings confirmed a diagnosis of cutaneous desmoid-type fibromatosis. Although exceedingly rare, recognition that desmoid fibromatosis can arise in the skin is clinically important, as these tumors have the capacity for aggressive locoregional behavior and a high rate of recurrence.

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