Lipomatous (Fat-Forming) Solitary Fibrous Tumor of the Breast: A Case Report of an Uncommon Variant of a Rare Clinical Entity.

Solitary fibrous tumor (SFT) is an uncommon tumor of mesenchymal origin, which can arise at any anatomic location and can exhibit versatile histological features and a clinical course ranging from benign to frankly malignant. Lipomatous (fat-forming) SFT is a morphological variant of SFT characterized by an adipose tissue component. Breast SFT is an extremely rare clinical entity, and the literature review yielded only 28 previously reported cases. However, lipomatous (fat-forming) SFT is much less common than conventional tumors and, to our knowledge, it has never been reported in the breast. We describe a case of a 54-year-old postmenopausal woman who presented with a palpable mass on her right breast. No other associated features such as nipple discharge, skin changes, or axillary lymphadenopathy were present. The clinical differential diagnosis included fibroadenoma, phyllodes tumor, and mammary hamartoma. A ultrasound scan examination demonstrated a large, oval, well-circumscribed lesion with indeterminate features, but suspicious of malignancy. However, a needle core biopsy was performed and histological examination with ancillary immunohistochemical staining confirmed the diagnosis of SFT, a lipomatous variant. The lesion was excised with clear margins and histological examination confirmed SFT with low-risk features and follow-up was planned. Careful histological evaluation with diffuse and strong nuclear expression of STAT6 helped to distinguish lipomatous SFTs from other mimics. Here, we describe the first case of a lipomatous variant of a SFT involving the breast.