Low-grade periductal stromal sarcoma of the breast with myxoid features: Immunohistochemistry

A 52-year-old woman was admitted with a painful right breast tumor measuring more than 20 cm in largest diameter, which ulcerated the overlying skin. The lesion had appeared 4 years previously but the patient hesitated to seek medical care due to 'fear of cancer'. Microscopically, the tumor was composed of spindle cells that formed cuffs around multiple open tubules and ducts set in an abundant, myxoid stroma. The spindle cells had significant atypia with nuclear pleomorphism, occasional cytoplasmic vacuolation and moderate mitotic activity. The ducts and lobules surrounded by the proliferating tumor cells had minimal distortion, with a pericanalicular growth pattern devoid of the phyllodes pattern. The tumor had a multinodular growth pattern with coalesced and individual tumor nodules, the latter being found mostly at the periphery of the lesion. On immunohistochemistry the tumor cells were positive for smooth muscle actin, CD34, and vimentin, and focally positive for CD10. A diagnosis of low-grade periductal stromal sarcoma (PDSS) with myxoid features was established. PDSS is a distinct low-grade breast sarcoma, the appropriate diagnosis of which requires extensive tumor sampling and additional broad immunohistochemistry. PDSS should not be confused with other spindle cell breast tumors because they require different treatment.