Dedifferentiated liposarcoma (DDL), occurring in up to 10% of well differentiated liposarcoma cases, has similar histologic features to that of undifferentiated high-grade pleomorphic sarcoma (UHGPS); the former develops in a background of atypical lipomatous tumors/well differentiated liposarcoma, whereas the latter shows no specific line of differentiation. The retroperitoneum and thigh represent the most common anatomic locations for both the sarcomas. Despite their morphologic similarity, the issue of whether these 2 sarcomas share overlapping immunohistochemical and molecular features has not been well studied. We examined the expression of the lipogenic tumor-related markers peroxisome proliferator-activated receptor gamma (PPAR-gamma), CDK4, and MDM2 in 15 cases of DDL and 45 cases of retroperitoneal/thigh UHGPS. Patients with DDL ranged from 31 to 82 years (mean 63 y) with a male:female ratio of 5:3. Patients with UHGPS ranged from 14 to 80 years (mean 52 y) with a male:female ratio of 3:2. All 15 DDLs expressed CDK4 and MDM2 (100%), and 8 of 15 cases expressed PPAR-gamma (53%). Twenty-three of 45 (51%) UHGPS expressed at least 1 of these 3 markers. We also studied MDM2 and CDK4 gene amplification by fluorescence in situ hybridization in 28 immunohistochemically positive cases, including 5 DDLs and 23 UHGPSs. All 5 cases of DDL showed MDM2 and/or CDK4 amplification (100%), whereas 6 of 45 UHGPSs showed MDM2 and/or CDK4 amplification (13%). Our results demonstrate that (1) the lipogenic tumor markers CDK4 and MDM2 can be used as surrogate immunohistochemical markers for the diagnosis of malignant lipomatous tumors with high sensitivity; (2) approximately 26% of retroperitoneal/thigh UHGPS cases that were positive for PPAR-gamma, CDK4, or MDM2 by immunohistochemistry showed characteristic CDK4 and MDM2 gene amplification, suggesting that a subset of UHGPS cases represent DDL despite lacking histologic evidence of lipoblasts.
