Ordinary lipoma and its variants contain mature white fat, sometimes with fibroblastic or other mesenchymal elements. The term lipomatosis covers a heterogeneous group of entities representing regional diffuse growth of mature fat that does not have known morphologically distinctive features, except for lipomatosis of the nerve, which is discussed in more detail later.

Some lesions designated as lipomas, such as synovial lipoma with villous pattern (lipoma arborescens) and lipoma of the hernia sac, are more likely reactive hyperplasias than true neoplasms. Furthermore, lumbosacral lipoma in children is associated with neural tube closure defects and could be a developmental anomaly.

Atypical lipoma is discussed in Chapter 15 with liposarcomas, because it is essentially synonymous with well-differentiated liposarcoma.

Several special lipoma types are considered specific clinicopathologic entities, as listed previously. All of these, except angiolipoma and spindle cell lipoma, are quite rare. Hibernoma is a tumor resembling brown fat, and chondroid lipoma has some resemblance to it. Lipoblastoma is a clinically and genetically distinctive childhood lipomatous tumor. Fibrohistiocytic lipoma is a peculiar benign lipomatous tumor in the leg, having a mesenchymal fibrohistiocytic-like element.

Finally, fat atrophy, fat necrosis, sclerosing lipogranuloma silicone granuloma, and lymphedema in obese patients are discussed as examples of non-neoplastic lesions that can simulate lipomatous tumors. Angiomyolipoma is included among perivascular epithelioid cell tumors (PEComas), a group of tumors with HMB45-positive and smooth muscle actin–positive components (see Chapter 19).