The ultrasound study was unremarkable. Mammotome guided biopsy showed Masson's tumor. Masson was a pathologist who worked in Montreal. His original article appeared in 1923 " papillary endothelial hyperplasia" he called the lesion. The AFIP described 17 breast Masson tumors. All were unilateral.  The differential is angiosarcoma of the breast. The Masson tumors are less than 2 cm and angiosarcomas are usually larger than 2 cm. 

The tumor has been described in fingers, the neck, tongue, muscle and retroperitoneum. 

Not typically the first diagnosis I make in breast masses.