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Fat necrosis of the breast is a benign nonsuppurative inflammatory lesion affecting the adipose tissue. It accounts for < 3% of all breast lesions and is usually the result of trauma (including blunt injury, surgery, or radiation) to the breast. Fat necrosis of the breast manifests as an ill-defined nontender breast mass; local skin changes may also be present. Age-appropriate breast imaging is indicated in all patients with a palpable breast mass. On mammography, fat necrosis may appear as an oil cyst; on ultrasound, it may appear as a breast mass of variable echogenicity. Clinical and imaging features can resemble those of breast cancer. If there is any suspicion of malignancy or if imaging is inconclusive, image-guided biopsy is indicated. Treatment is not usually required, but surgical excision should be considered if malignancy cannot be excluded, or imaging and pathology findings are discordant.
Imaging findings 
- Breast ultrasound: variable; may be solid or cystic and anechoic or hyperechoic 
- Fluid-filled oil cyst
- Coarse rim calcifications
- Spiculated mass
MRI breast with and without contrast (not routinely ordered) 
- Round or oval lesions with hypointense T1 signals when fat suppression is performed
- Irregular or spiculated mass
- Indication: clinical suspicion of malignancy 
- Findings