Desmoid-type fibromatosis of the breast mimicking cancer

Summary

A 42-year-old woman presented to our hospital with an enlarging mass in the right breast. A clinical examination of the breast showed a palpable lump of 3 cm in the right upper inner quadrant, which was firm and easily movable under the skin. Mammography showed a lobular, high-density, spiculated mass in the right breast. On ultrasound, there was an irregular-shaped low-echoic mass with posterior echo attenuation and interruption of the anterior border of the mammary gland. Magnetic resonance imaging of the breast showed a lobulated mass with a spiculated margin. The mass was hypointense on a T1-weighted image, and hyperintense on a T2-weighted image. The preoperative diagnosis was desmoid-type fibromatosis, and an open biopsy was performed to rule out a malignant tumor. Frozen section examinations during surgery were conducted for confirmation or exclusion of cell atypia and a negative margin. Surgically removed specimens showed infiltrative proliferation of spindle cells associated with dense connective bundles. The spindle cells had low mitotic activity and no atypia, and were positive for alpha smooth muscle actin and nuclear beta-catenin in immunochemistry. A definitive diagnosis of breast desmoid-type fibromatosis was established based on these findings. She is now 4 years post-surgery and remains disease-free. On mammography, fibromatosis appears as an ill-defined, dense mass.