Solitary fibrous tumor of the breast: report of a case with emphasis on diagnostic role of STAT6 immunostaining

We herein report the clinical, radiological, and pathological findings of a rare case of a solitary fibrous tumor (SFT) occurring in the breast parenchyma of a 62-year-old female. The tumor was incidentally detected at a mammographic screening, and, ultrasonographically, presented as a single, well-circumscribed nodule. On needle core biopsy, the diagnosis of SFT was suggested based on a proliferation of CD34-positive spindly cells set in a fibrous stroma containing medium-sized blood vessels with hyalinization of their walls and branching configuration. The diagnosis was confirmed in the excised specimen, which exhibited a tumor with an immunohistochemical profile consistent with SFT, including diffuse expression of CD34, CD99 and bcl2. As STAT6 nuclear immunoexpression is the result of the inv12(q13q13)-derived NAB2–STAT6 fusion, which characterizes SFT, we analyzed immunohistochemically our case with a commercially available anti-STAT6 antibody. We showed that mammary SFT exhibits a diffuse nuclear STAT6 immunoreactivty, suggesting its potential diagnostic role. The present case emphasizes that the diagnosis of SFT can be confidentially rendered on needle core biopsy. Although SFT is suspected on characteristic morphologic features, immunohistochemistry, revealing immunoreactivity for CD34, bcl-2, CD99 and STAT6, is crucial in the differential diagnosis of potential benign and malignant mimics.