Extranodal Rosai–Dorfman Disease of the Breast

nomas can be challenging as they may resemble fat necrosis, xanthogranulomatous mastitis, granular cell tumor, lipoblastoma, and histiocytoid carcinoma. Here, we illustrate a pure typical mammary hibernoma in a postmenopause woman with a soft mass in the upper-outer quadrant of the left breast. No other relevant clinical conditions were present. The lesion measured 4.3 cm in greatest dimension and was composed of an abnormal population of monomorphic multivacuolated S100-positive, CD68-negative cells with no atypia that resembled normal brown adipose tissue (Fig. 1). Surgical margins were clean and after 23 months of follow-up the patient is disease-free. To date, only five pure mammary hibernomas have been reported in literature, therefore, their pathogenesis is poorly understood. Despite hibernoma can arise in any site where brown fat persists, such as posterior neck and upper back and shoulders, the most common primary site is represented by the thigh, an abnormal location for brown fat. For this reason, it has been hypothesized that mesenchymal precursors have the potential to differentiate into brown adipose cells in extra-brown fat locations, as in the breast. Largecohort molecular studies, coupled with follow-up data, are needed to start approaching the biology underpinning mammary hibernoma and to address the operational implications of these uncommon diagnosis for surgeons and pathologists.