oil cyst

Oil cysts in breast imaging refer to benign breast lesions where an area of focal fat necrosis becomes walled off by fibrous tissue.

Epidemiology

Occurs across all age and ethnic groups with a female predilection. Usually associated with blunt trauma, if present in males.

Clinical presentation

  • usually asymptomatic
  • tender or non-tender palpable lump
  • bruising, if recent trauma

Pathology

Fat debris from ruptured lipocytes tends to conglomerate to form a macroscopic pool of oil surrounded by lipid-laden macrophages or foam cells - known as an oil cyst. The wall can then calcify. Most often this occurs secondary to trauma or surgery; however, this is not always necessary.

Etiology

The etiology of oil cysts is felt to be most often an end-form of liquefaction fat necrosis of the breast resulting from trauma or surgery. However, some oil cysts arise independently, without a prior history of trauma or surgery.

Macroscopic appearance

Specimens are usually obtained from surgical excision or aspiration. Grossly, appears as viscus, oily fluid/material.

Microscopic appearance

Fat-filled macrophages and foreign body giant cells are usually present. There may be hemorrhage into fat. A fibrous capsule forms around the lipid contents. Following fatty acid saponification, there may be calcific precipitation.

Associations

Multiple intradermal oil cysts can be part of steatocystoma multiplex.

Radiographic features

Mammography

Typically seen as a radiolucent rounded mass of fat density +/- wall calcification, which if present typically appears as eggshell calcification. Lesions are usually well-circumscribed with a thin capsule. Rarely, fat-fluid levels may be present.

Ultrasound

On ultrasound, most oil cysts are hypo-echoic with smooth walls and show neither posterior acoustic enhancement or shadowing. Echogenicity varies. Fat-fluid levels are better characterized sonographically. When present, rim calcifications will demonstrate posterior acoustic shadowing. Complex features include thick walls, mural nodularity, and internal echoic bands. A very small proportion can represent an intracystic mass.

CT

Circumscribed, lucent, non-enhancing lesion +/- calcification.

MRI

T1WI: circumscribed, hyperintense lesion (isointense to fat)

Treatment and prognosis

Treatment is usually not required. Aspiration/biopsy is not recommended due to the inflammatory nature of oil contents.

Differential diagnosis