A novel sclerosing atypical lipomatous tumor/well-differentiated liposarcoma in a 7-year-old girl: report of a case with molecular confirmation.

Atypical lipomatous tumor (ALT)/well-differentiated liposarcoma (WDL)/dedifferentiated liposarcoma (DDL) is a common type of liposarcoma in late adulthood. However, pediatric ALT/WDL/DDL is extremely rare, and only 3 cases have been described in children younger than 10 years of age. Notably, none of these cases harbored MDM2 gene amplification. Here, we reported a sclerosing ALT/WDL in a 7-year-old Chinese girl. Histologically, in most areas, the neoplastic cells were embedded within the collagenous background, and typical lipogenic areas were inconspicuous throughout the sclerotic areas. In addition, scattered small foci of atypical osseous/chondrous elements were identified. Notably, a small typical lipoma-like ALT/WDL area was detected in the periphery of the mass. Immunohistochemically, all the neoplastic components demonstrated positivity for MDM2, CDK4, and p16. Fluorescence in situ hybridization revealed MDM2 gene amplification in all the tumor components. To the best of our knowledge, this is the first example of MDM2-amplified ALT/WDL in this age group.