24th May 2025

Fat Necrosis

Fat necrosis is a benign breast condition that commonly occurs after direct trauma or surgery to the breast. Whilst it can affect women of any age, middle-aged women seem to be most at risk. Due to its variable character on both examination and imaging, fat necrosis can often mimic malignancy and therefore, may warrant further investigation.

Fat necrosis is characterised by the saponification of fatty tissue, often in response to localised inflammation as a result of direct trauma. The most common causes include breast biopsy, surgery (lumpectomy, reduction, reconstruction, enlargement), lipomodelling and trauma related to seat belts. However, up to 50% of patients may have no history of prior surgery or trauma.

Clinical examination usually reveals a painful or painless, firm, round lump, with a predilection for the subareolar and periareolar regions. The lump may also be associated with changes to the overlying skin, such as erythema, bruising, dimpling or a retracted nipple. Given that the onset can be considerably delayed to the causative event (>10 years), fat necrosis can have a variable appearance on imaging and where possible, comparison with previous imaging is essential.

In the acute phase, ultrasound will show increased echogenicity around the lesion. In the days to months after, the lesion typically develops into an ill-defined complex cystic lesion surrounded by oedematous fat. After several years, fat necrosis may appear as a spiculated mass with calcified walls and posterior shadowing. Mammography, on the other hand, is usually characterised by an ill-defined, irregular, spiculated mass, with or without associated calcification. With time, the lesion will develop into an ‘oil cyst’, characterised by a curvilinear, calcified wall and homogenous centre. 

Due to the potential for diagnostic uncertainty, it may be necessary to biopsy these lesions in order to establish a definitive diagnosis. Histology reveals necrotic fat cells surrounded by macrophages, multi-nucleate giant cells and variable inflammatory cells. More established lesions may show varying degrees of fibrosis and calcifications, as part of a scar formation.

Fat necrosis does not carry an increased risk of breast cancer and therefore, the majority of patients do not require treatment. Surgery is usually avoided, due to the association with developing further fat necrosis. For that reason, intervention is limited to those with symptomatic fat necrosis, persistent/enlarging lesions or for aesthetic purposes.