# Case report: Atypical lipomatous tumor of the thigh in a four-year-old girl

## TL;DR

A 4-year-old girl was diagnosed with an atypical lipomatous tumor in her thigh, which was initially mistaken for a more common pediatric tumor.

## Contribution

This case highlights the importance of considering ALT in children with atypical adipocytic tumors and using MDM2 and CDK4 immunohistochemistry for accurate diagnosis.

## Key findings

- The tumor showed nuclear positivity for MDM2 and CDK4 and negativity for PLAG1.
- Fluorescence in situ hybridization revealed MDM2 gene amplification.
- No recurrence or metastasis was observed one year after surgery.

## Abstract

Atypical lipomatous tumors (ALTs) are locally aggressive adipocytic malignancies that frequently occur in middle-aged adults. We report the rare case of an ALT of the thigh that occurred in a 4-year-old girl. Since the tumor was initially diagnosed as a lipoblastoma by incisional biopsy, marginal resection was performed. Histopathological findings of the surgical specimen revealed the proliferation of mature and variously sized adipocytes, as well as ectopic ossification; these features differ from the typical findings of lipoblastoma. Immunohistochemical findings showed nuclear positivity for a murine double minute 2 (MDM2) and cyclin-dependent kinase 4 (CDK4) and negativity for pleomorphic adenoma gene 1 (PLAG1). Fluorescence in situ hybridization showed abnormal amplification of the MDM2 gene. The patient was thus finally diagnosed as having an ALT. No signs of local recurrence or metastasis were noted 1 year postoperatively. This case is instructive in the differential diagnosis of primary adipocytic tumors. Lipoblastomas are the most common adipocytic tumors in children, but if a tumor is located in the deep tissue or imaging findings are not typical, the possibility of ALT should be considered and immunohistochemistry for MDM2 and CDK4 should be added.