A true-cut biopsy of the lesion was taken. Histopathologic examination showed the presence of multiple lobules of adipose tissue with a myxoid matrix, in which proliferating thin vascular channels showing branching and cordlike arrangement were seen (Figure 6). There was no mitosis, cellular atypia, or necrosis. The differential diagnosis of such a lesion was a lipoma, a liposarcoma, or a dermoid cyst. This histopathologic picture was consistent with the diagnosis of lipoblastoma.
First described by Jaffe,2 a lipoblastoma is a developmental anomaly characterized by continued proliferation of lipoblasts in the postnatal period.3 While adipose-derived tumors account for only 6% of soft tissue tumors in children, lipoblastomas account for 30% of them. There is no preference for any racial group, nor is there any specific pattern of inheritance.4
Lipoblastomas occur as 2 clinicopathologic types. The more common type is the well-circumscribed type that is located in the superficial soft tissues. The second type is of the diffuse infiltrative variety, which is deep and poorly circumscribed, with a tendency to grow in tissue spaces and musculature. The latter is uncommon and tends to recur after surgical resection. This variety of lipoblastoma has been termed lipoblastomatosis. Lipoblastomas typically arise from the limbs or trunk.4 The head and neck, retroperitoneum, inguinal canal, peritoneal cavity, mediastinum, and lung are the other reported sites of lipoblastomas.4,5 Associations with this tumor have been reported in the past, such as hemangiomas and juvenile aponeurotic fibromas.6
Grossly, these tumors range from 3 to 5 cm in diameter but may be much larger. The clinical presentation is a gradually increasing, painless soft tissue mass. The mass is usually asymptomatic but may cause dysfunction because of mass effect.4 Lipoblastomas in the neck may cause respiratory insufficiency.7 The growth rate of the tumor may be rapid, and a history of noticeable swelling may be of only a few weeks' duration.7
Both lipoblastoma and lipoblastomatosis have a benign course, though they both have a tendency to recur in spite of complete excision. The recurrence rate ranges from 10% to 25%8 and is more common with the diffuse variety (lipoblastomatosis).8 Lipoblastomas that occur on the back have a high recurrence rate (50%) and are associated with intraspinal extension.9 There have been no reports of metastases, making the designation "blastoma" confusing, since this term is usually reserved for malignant tumors. Spontaneous resolution of lipoblastomas has never been reported.
The gross specimen of lipoblastoma is usually a soft, lobulated mass with a pale yellow appearance on a cut surface. The characteristic microscopic findings include a lobular arrangement of fat cells separated by fibrous connective tissue septa or trabeculae. These septae are rich in capillaries and venules, which may be dilated and arranged in a plexiform manner. The fat cells show varying degrees of differentiation. Mitotic figures are observed, but there are no atypical forms seen. Differentiation with liposarcoma on microscopy is often difficult. The absence of hyperchromatic nuclei and of atypical forms favors the diagnosis of lipoblastoma.4
On plain radiographs, lipoblastomas appear as a soft tissue density. On CT, this tumor characteristically contains regions of fat density, separated by septae of soft tissue density, and does not enhance following contrast administration.7 Solitary cases of lipoblastomas that cause underlying bone enlargement have been reported. The extent of the lesion can be accurately assessed using CT.7 Lipoblastomas show intratumoral fat stranding on CT, but differentiation with liposarcoma is not possible using this modality.6 Lipoblastomas show high intensity signals both on T1and T2-weighted images. On fat suppression, they still show areas of increased intensity that distinguish these lesions from lipomas. However, lipoblastomas may have varied signal intensity on T1-weighted images. This atypical MRI manifestation on T1-weighted images was ascribed to the presence of an excessive amount of immature fat and myxoid tissue, intratumoral infarction, extensive mucoid, and cystic degeneration within the tumor.10
The differential diagnosis of a neck mass presenting in infancy includes a cystic hygroma, branchial cleft cyst, thyroglossal duct cyst, hemangioma, vascular malformation, and an abscess.7 Dermoid cyst and liposarcoma contain the characteristic fat attenuation on CT similar to that of a lipoblastoma, but these tumors rarely present before the second or the third decade.7 Lipoblastomas may also resemble myxoid liposarcoma. The tumor karyotype is very helpful in differentiating myxoid liposarcoma from a lipoblastoma.11 In the differential diagnosis, the age of the patient is of greatest importance, as myxoid liposarcomas are exceedingly rare in children younger than 10 years of age.
Complete surgical excision is the treatment of choice.6 Recurrent lesions are best imaged with MRI to assess the extent of the tumor and to plan the reconstruction, if necessary.12 The major concern with lipoblastoma in children is to completely excise the tumor, to avoid leaving residual tumor, and to prevent recurrences. Complete surgical excision with at least 2 years of follow-up is the preferred therapy.5
CONCLUSION
Lipoblastomatosis is a rare, fat-containing, benign tumor that should be considered as a differential diagnosis of recurrent soft tissue neck masses in infants and children. Although histopathologic examination is required for confirmation of the diagnosis, CT and MR are useful in narrowing the differential diagnosis and in evaluating the extent of the tumor for surgical planning.
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