The World Health Organization classified lipoma and lipomatous lesions as follows in the year 2002 based on histopathological appearance of the lesions.
BENIGN LIPOMATOUS TUMORS
LIPOMA: A lipoma is a benign tumor composed of mature white adipocytes and is the most common soft tissue mesenchymal neoplasm in adults, usually a circumscribed, non-encapsulated aggregate with clear cytoplasm, absence of vascularity and atypia of metaplasia.
LIPOMATOSIS: (Madelung disease, Launois-Bensaude syndrome): Lipomatosis is a diffuse overgrowth of mature adipose tissue. It occurs in a variety of clinical settings and can affect different anatomic regions of the body. Diffuse lipomatosis usually occurs in individuals under 2 years of age but it may also arise in adults.
ANGIOLIPOMA: A subcutaneous nodule consisting of mature fat cells, intermingled with small and thin-walled vessels, a number if which contain fibrin thrombi. Angiolipomas most frequently present as multiple subcutaneous small nodules, usually tender or painful. There is no correlation between the intensity/occurrence of pain and the degree of vascularity.
LIPOBLASTOMA/ LIPOBLASTOMATOSIS: Lipoblastoma is a type of subcutaneous benign fatty tumor. Types include benign lipoblastomatosis, a tumor also knows as an embryonic lipoma, which usually occurs in children under three years old. This is a tumor of brown fat cells. The extremities are most commonly involved, but locations in the mediastinum, retroperitenoeum, trunk, head and neck, and various organs (lung, hear, parotic gland) have been described.
LIPOMATOSIS OF NERVES: Lipomatosis of nerve is characterized by infiltration of the epineurium by adipose and fibrous tissue. The tissue grows between and around nerve bundles thereby causing enlargement of the affected nerve. Patients present with a gradually enlarging mass in the affected area that may be asymptomatic or associated with motor 0r sensory deficits. Patients with macrodactyly have symmetrical or asymmetrical enlargement of the affected finger(s) with enlargement of the involved bones. Imaging studies show fusiform enlargement of the nerve with fatty infiltration.
HIBERNOMA: Hibernoma is a rare benign adipose tumor composed at least in part of brown fat cells with granular, multivacuolated cytoplasm. This brown gat component is admixed in variable proportion with white adipose tissue. Residual brown fat, mostly seen around cervical and axillary lymph nodes, should not be classified as hibernoma, Hibernoma is a relatively slow growing tumor of the subcutaneous tissue. At least 10% of cases are intramuscular. Hibernomas are usually painless. MRI reveals non-fat septations in hibernoma, not found in lipoma. On CT scan, hibernoma has a tissue attenuation intermediate between fat and skeletal muscle and is enhanced with contrast.
MYOLIPOMA: Myolipoma of soft tissue is a benign tumor exhibiting features of mature smooth muscle and mature adipose tissue. The majority of cases are deeply located and involve the abdominal cavity retroperitoneum, and inguinal areas. The trunk wall and extremities may also be involved; such cases are subcutaneous and may grow deeply to involve the superficial muscular fascia.