= rare rapidly growing benign tumor in children <3 years of age (= postnatal proliferation) composed of mesenchymal cells ranging from prelipoblasts (spindle cells) to mature adipocytes

Origin: derived from fetal adipose tissue

Frequency: 30% of adipocytic tumors in children

Path: encapsulated immature adipose tissue (embryonal fat) separated by septa into multiple lobules

Histo: uni- and multivacuolated lipoblasts interspersed between spindle / stellate mesenchymal cells + suspended in myxoid stroma

Molecular features: translocation involving 8q11–13 with rearrangement of Plag1 gene

Median age: 12–18 months of age (range, newborn to 16 years); <3 years of age (in >90%); M÷F = 2÷1

Types:

1. lipoblastoma proper = circumscribed encapsulated form in superficial adipose tissue (⅔)
2. lipoblastomatosis = diffuse nonencapsulated infiltrative form in deep adipose tissue (⅓)

Location: subcutaneous tissue of extremities (70%), head and neck, trunk, mediastinum, mesentery, perineum, retroperitoneum

Size: 3–5 cm

• gradually increasing painless soft-tissue mass
• no enhancement for tumors composed purely of lipocytes
• marked heterogeneous enhancement for tumors composed predominantly of lipoblasts

US:

• homogeneous finely textured echogenic (relative to liver / adjacent muscle) mass
• may contain occasionally cystic / focal hypoechoic ares

CT:

• tumor of fat density separated by fibrous septa
• intratumoral fat stranding

MR (preferred preoperative evaluation):

• high (mature adipocytes) / intermediate SI (immature adipocytes) on T1WI + T2WI with a predominance of lipocytes
• heterogeneous texture with T1-hypointense + T2-hyperintense areas ← excessive amount of immature fat (lipoblasts) / myxocollagenous stromal tissue / intratumoral infarction / extensive mucoid + cystic degeneration
• CHARACTERISTIC areas of high signal intensity on fat-suppressed images ← predominance of myxocollagen (NEVER in lipoma!)

Prognosis: may evolve into mature lipoma; capable of invading surrounding tissue (= lipoblastomatosis); NO metastatic potential

Rx: wide surgical resection; recurrence in 10–25% for lipoblastomatosis

DDx:

1. Teratoma (calcifications)
2. Lipoma (very rare in children)
3. Well-differentiated liposarcoma (no distinguishing imaging features, patient age <3 years allows reliable differentiation)
4. Dermoid cyst