The Carcinoma Mimic: A Case of Breast Fibromatosis.

Fibromatosis of the breast is a rare and locally aggressive disease that mimics carcinoma on imaging. We present a case of a 17-year-old female who presented with a painless, palpable 8 cm lump in her left breast. Breast ultrasound highlighted features concerning for malignancy - a large, irregular, hypoechoic mass with posterior acoustic shadowing. However, the absence of a hyperechoic halo, presence of posterior acoustic attenuation, and low vascularity pointed towards the correct diagnosis. In addition, on magnetic resonance imaging (MRI), breast fibromatoses correspond to hyperintense signals on T2, which, though a nonspecific finding, may be seen with myxoid tissue and hypointense signals on T1. Following contrast administration, there was progressive enhancement as opposed to the typical washout kinetics of breast carcinoma. Diagnosis of breast fibromatosis was confirmed on core needle biopsy. Although the differential for an irregular hypoechoic breast mass should be broad, it can be narrowed with history and radiographic findings. This case report highlights the unique ultrasound and MRI findings seen in breast fibromatoses to spread awareness of active surveillance and improve early detection.