Fat necrosis is a common entity that may pose a challenge to clinicians and breast imagers. It is a  benign non-suppurative inflammatory process of adipose tissue condition with a wide variety of presentations on mammography, ultrasound, and MRI. It is important to diagnose fat necrosis because it can often mimic carcinoma of the breast. Breast imagers should become familiar with the different manifestations of fat necrosis to avoid unnecessary biopsies.

CASE HISTORY:

A 52-year-old female patient presented with complaint of a painful lump in the right breast. The patient had a past operative history of bilateral reduction mammoplasty, 10 year before. On clinical examination, there was a hard lump in the retro areolar region of the right breast.

Mammogram with ultrasound correlation done.

DIAGNOSIS:

Calcified lesion in the retroareolar location of right breast -fat necrosis.

Nodular opacity in the upper outer quadrant of left breast –followed up with biopsy shows fibrocystic change with focal histiocytic reaction.

DISCUSSION:

The incidence of fat necrosis of the breast is estimated to be 0.6% in the breast, representing 2.75% of all breast lesions.

Most common etiologies are trauma, Post radiotherapy, anticoagulation, cyst aspiration, biopsy, lumpectomy, reduction mammoplasty, implant removal, breast reconstruction with tissue transfer, duct ectasia, and breast infection. Other rare causes for fat necrosis include polyarteritisnodosa, Weber-Christian disease, and granulomatous angiopanniculitis

Clinical presentation range from an incidental benign finding to a lump,single or multiple smooth round nodules to clinically worrisome fixed, irregular masses with overlying skin retraction Following injury to breast tissue, hemorrhage in the fat leads to induration and firmness, which demarcates and may result in a cavity caused by cystic degeneration. Other clinical features include ecchymosis, erythema, inflammation, pain, skin retraction or thickening, nipple retraction, lymphadenopathy. Fat necrosis is commonly seen in the superficial breast tissues and subareolar regions in obese women with pendulous breast.

Histopathologic Findings :

Macroscopically, early lesions appear as hemorrhagic foci or areas of indurated fat. It become bright yellow (saponification), chalky white (calcification), or yellow-gray (fibrosis). Some lesions may develop a central cavity because of liquefactive necrosis. Microscopically, early lesions show hemorrhage, anucleated adipocytes lipid-laden histiocytes, and multinucleated giant cells. Older lesions develop fibrosis with a few foamy histiocytes and multinucleated giant cells. Dystrophic calcification may occur in older lesions. Histogenesis of foamy histiocytes may be confirmed by positive CD68 and negative pan cytokeratin immunostains.

Imaging Findings:

On mammography, may appear normal initially. Common findings of fat necrosis are oil cysts ,coarse calcifications, microcalcifications or spiculated masses. Lipid cysts are pathognomonic of benign fat necrosis. Later on a well circumscribed lipid cyst with a thin water density capsule develops and with time, calcified salt of fat precipitates along the capsule of the oil cyst, known as liponecrosis macro/micro cysticacalcificans. The clustered, pleomorphic microcalcifications may be indistinguishable from those of malignancy. Fibrosis may lead to replacement of the radiolucent necrotic fat, resulting in the appearance of a focal asymmetric density, a focal dense mass, or an irregular spiculated mass on mammography.

On sonography, the appearance of fat necrosis ranges from a solid hypoechoic mass with posterior acoustic shadowing to complex intracystic masses that evolve over time. Cystic lesions appear complex with mural nodules or internal echogenic bands. Solid masses have circumscribed or ill-defined margins and are often associated with distortion of the breast parenchyma. A specific sonographic indicator of fat necrosis is a mass with echogenic internal bands that shift in orientation with changes in patient position.

CT appearance is based on the main components found in fat necrosis: liquefied fat, fibrosis, and inflammation. Liquefied fat would present on CT as low attenuation coefficients, fibrosis would present as soft tissue coefficients similar to fibroglandular tissue or linear densities resembling fibrous bands, and inflammation would present with enhancement after contrast injection .Calcifications typically are not evident until later in the evolution of fat necrosis when they become large in size.

MRI also has a wide spectrum of findings for fat necrosis and the appearance is the result of the amount of the inflammatory reaction, the amount of liquefied fat, and the degree of fibrosis. Lipid cyst appers as round or oval mass with hypointense T1-weighted signal on fat saturation images. Fat necrosis is usually isointense to fat elsewhere in the breast and shows low signal intensity on T1-weighted MRI, which may be due to its hemorrhagic and inflammatory content . Fat necrosis may show focal or diffuse and homogeneous or heterogeneous enhancement after the administration of IV paramagnetic contrast material. The amount of enhancement is correlated with the intensity of the inflammatory process .The “black hole” sign has been described as another characteristic on MRI to help diagnose fat necrosis, marked central hypointensity of the lesion on short tau inversion recovery (STIR) images when compared with surrounding fat.

Fat necrosis show FDG uptake on PET.

CONCLUSION:

Fat necrosis of the breast is a challenging diagnosis due to the various appearances on mammography, ultrasound and MRI. The appearance of fat necrosis is the result of the amount of the inflammatory reaction, liquefied fat, and the degree of fibrosis.

Dr. Shabna Jasmin K
Radiologist