Fibromatosis of the breast represents <0.02% of all breast tumors. Typically, fibromatosis is non-metastasizing but locally aggressive and is treated with surgical excision. We report a case of a nonagenarian requiring mastectomy for an epidermal inclusion cyst that diagnostically mimicked fibromatosis of the breast. She underwent diagnostic breast imaging for a period of almost two years to monitor the size of her breast tumor. However, in a six-month timeframe, her mass dramatically increased in size and was becoming symptomatic to the patient. The rapidly growing tumor was concerning for occult breast malignancy.