Lipoblastoma/Lipoblastomatosis
Synonym: Embryonic lipoma
Key features
Benign neoplasm of immature fat cells that arises primarily on the extremities of children
The circumscribed form is termed lipoblastoma, and the diffuse form is termed lipoblastomatosis
Histopathologically, indistinguishable from myxoid liposarcoma
Introduction
This is a rare variant of lipoma, and it represents a benign tumor of immature fat cells that occurs almost exclusively in children. When circumscribed, this neoplasm is termed lipoblastoma, and when diffuse, it is known as lipoblastomatosis.
History
The term lipoblastoma was coined by Vellios and colleagues in 1958 when they reported a case of lipoblastomatosis.
Epidemiology
Lipoblastoma and lipoblastomatosis are tumors of infancy. Occasionally, these neoplasms are described in older children, or even teenagers, but the majority occur before the age of 3 years. Congenital tumors have been observed. Boys are affected twice as often as girls.
Pathogenesis
Lipoblastomas are neoplasms of immature (embryonic) white fat, and they probably represent residual areas of immature fat. The latter is suggested by sequential biopsies showing maturation to mature fat and by the almost exclusive occurrence in infancy and childhood. Lipoblastomas commonly display structural alterations, i.e. translocations, inversions, insertions or rings, of 8q11–q13 which lead to rearrangement of PLAG1, a finding that argues for a clonal neoplasm.
Clinical features
Most tumors are circumscribed and limited to the subcutis, and hence represent lipoblastomas. They occur primarily on the extremities and less often in the head and neck region or on the trunk. These super-ficial lesions clinically simulate ordinary lipomas, presenting as slowgrowing, soft, mobile, painless nodules, measuring 3–5 cm in diameter.
Lipoblastomatosis affects the deeper soft tissue and often diffusely infiltrates adjacent skeletal muscle. This process can occur in the mediastinum, mesentery, and retroperitoneum. Rare tumors weigh up to 1 kg. Depending on the location, the tumor may result in symptoms of local obstruction or compression.
Pathology
Grossly, lipoblastomas are paler than ordinary lipomas, and they have a myxoid quality. Histologically, it is a lobulated neoplasm. The stroma is often myxoid, contains a plexiform vascular network, and is divided into lobules by connective tissue septa. Deep-seated tumors often lack the lobulation, and they may grow in a diffuse pattern. The tumors are composed of fat cells spanning the range of maturation from immature stellate and spindled mesenchymal cells, to lipoblasts with a single cytoplasmic vacuole that indents a peripherally located nucleus, to small and mature adipocytes. Lipoblastomas are paucicellular, cytologically bland, and lack mitotic activity. Serial biopsies of a lipoblastoma may show gradual maturation to a mature fatty tumor. The adipocytes in lipoblastomas stain positively for S100, CD56, and CD34. Molecular demonstration of PLAG1 rearrangements may be helpful in confirming the diagnosis.
Differential diagnosis
Histopathologically, lipoblastoma and myxoid liposarcoma may be indistinguishable. Clinical parameters that aid in the differential diagnosis are the age of the patient and the location of the tumor. Myxoid liposarcomas almost always develop in individuals older than 20 years of age, and they are located in the deep soft tissue, rather than the subcutis. Nevertheless, in a child, an immature fat tumor that contains areas of marked cellularity, loss of lobulation, and mitotic figures should raise concern for a myxoid liposarcoma.
Treatment
Lipoblastoma, if treated with complete excision, rarely recurs. Unusual cases of spontaneous resolution without treatment exist. In a series of 14 patients with lipoblastomas/lipoblastomatosis, 22% recurred after excision. Wide excision is the therapy of choice for the diffuse form of the disease.