Large pseudoangiomatous stromal hyperplasia complicated with gynecomastia and lobular differentiation in a male breast

Pseudoangiomatous stromal hyperplasia (PASH) is a benign lesion often observed in parts of the mammary stroma in a variety of breast diseases. It is characterized by stromal myofibroblastic proliferation that possesses irregularly anastomosing slit-like pseudovascular spaces lined by a layer of spindle cells. PASH commonly occurs in premenopausal women; however, it has also been observed in men with gynecomastia. Although tumor-forming lesions are rare, we report on a case with a large PASH in a male breast complicated with gynecomastia. Imaging showed a tumor with a diameter of >10 cm in the left breast, and aspiration cytology revealed a benign lesion. Since the tumor was large and gradually increased in size, a simple mastectomy was performed. The tumorous lesion was diagnosed as PASH based on the pathological and immunohistological findings. It was complicated by gynecomastia with acinar and lobular formation, which resembled female mammary gland secretory activity that is observed during lactation.