44-year-old woman with left-sided breast mass
Fibromatosis of the breast is an uncommon fibroblastic proliferation that is considered low-grade but can mimic invasive cancers on imaging and histology. Some cases are associated with familial adenomatous polyposis (FAP), including Gardner and Turcot syndrome. FAP is caused by germline mutations in the APC gene, which regulates the beta-catenin protein. Most cases are not part of systemic fibromatosis syndromes. A small percentage is associated with prior trauma (like nodular fasciitis) and surgery. It can occur within breast parenchyma but frequently arises from pectoralis fascia and extends into the breast.
Fibromatosis is rare, constituting about 0.2% of all breast tumors. Most patients are women in their 40s; however, any age can be affected.
On mammograms, irregular dense masses are seen. Calcifications are not typically identified. On ultrasound, irregular masses with a thick echogenic rim are present. On magnetic resonance imaging (MRI), mass-forming lesions with irregular margins are found.
In a typical histological presentation, long, sweeping fascicles of spindle cells are seen that infiltrate around normal ducts and lobules. Spindle cells consist of fibroblasts and myofibroblasts. Mitoses are absent or rare. Lymphocyte aggregates can be seen at the periphery of these lesions. Borders of the lesion are usually irregular. Diagnosis of fibromatosis is challenging on core needle biopsies.
Immunohistochemically, the tumor cells express nuclear beta-catenin in 60%-80% cases while muscle markers (desmin, actin, and calponin) show 40%-75% positivity. Cytokeratins are negative and CD34 is negative. The only other breast stromal lesion negative for CD34 is nodular fasciitis (which is beta-catenin negative, can have circumscribed or irregular margins, and usually presents as a rapidly growing painful mass). Most cases are also negative for hormone receptors.