Fibromatosis arising from the pectoralis major muscle mimicking breast cancer

Fibromatoses are soft tissue tumors composed of fibroblasts which commonly appear in the muscular aponeurosis of the abdomen. Mammary fibromatoses occur in only 0.2% of breast neoplasms and have been reported in association with prior breast augmentation and Gardner's syndrome. Multiple imaging modalities have been used to characterize the appearance of breast fibromatosis; however, it remains a tissue diagnosis given the variability both within and across modalities. We present the case of a 25-year-old female with a history of palpable breast mass who was evaluated with ultrasound, diagnostic mammography, MRI, and CT. Ultrasound-guided biopsy revealed fibromatosis, and MRI ultimately revealed that the mass was arising from the pectoralis major muscle and extensively involved the chest wall.