Parosteal lipoma of the rib: a benign condition that mimics malignancy.

Parosteal lipomas (PL) account for 0.3% of all lipomas. They are composed of adipose tissue adjacent to the bone cortex and elicit bony reactive changes. Location of this typically slow-growing indolent lesion in the ribs is unusual. Albeit a benign condition, PL imaging findings may be misinterpreted as a malignant lesion. We present a patient with a PL in which a surgical biopsy was performed. Distinctive features of PL in the computed tomographic (CT) scan and magnetic resonance image (MRI) are described. Familiarity with the characteristic radiographic PL signs assist in avoiding invasive diagnostic studies. Lipomas are benign tumors consisting of mature fat cells. They are the most common mesenchymal neoplasias and account for 50% of all soft tissue tumors. Their location varies from superficial to deep seated lesions. Musculoskeletal lipomatous lesions may be located in bones, soft tissues, and may also affect joint and tendon sheaths. PL is a rare benign deep fatty-tissue tumor that arises contiguous to the periosteum without originating from it. In 1836, Seering was the first in describing this lesion and named it "periosteal lipoma." Upon realization that there are no adipocytes in the periosteum, the term was changed to the more descriptive "parosteal lipoma" by Power in 1888. This is a case report of a patient with a chest wall mass that on CT scan and MRI had the typical PL appearance. This pattern allows one to identify the lesion and thus, to differentiate it from malignant conditions.