Fibromatosis of the Breast

IMAGING FINDINGS: ​​

Standard Craniocaudal (CC) view of the right breast shows an irregular hyperdense mass with spiculated margins in the medial aspect of the breast. Architectural distortion is present in the lateral right breast due to prior lumpectomy. Corresponding ultrasound image demonstrates an irregular hypoechoic mass with posterior acoustic shadowing in the medial breast.
 
WHY IT MATTERS:

1. The majority of spiculated masses identified on mammography or ultrasound will yield malignancy at biopsy. However, there are a few more rare causes such as fibromatosis.  There is no way to make this diagnosis based on the imaging alone. 
2. Fibromatosis of the breast is also known as an extra-abdominal desmoid tumor of the breast.  It is benign but locally invasive so surgical excision is required. 
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FURTHER READING:
Mammary Fibromatosis (AJR 2009)