Fluorescence<i>In Situ</i>Hybridization for<i>MDM2</i>Amplification as a Routine Ancillary Diagnostic Tool for Suspected Well-Differentiated and Dedifferentiated Liposarcomas: Experience at a Tertiary Center

Abstract

Background. The assessment ofMDM2gene amplification by fluorescencein situhybridization (FISH) has become a routine ancillary tool for diagnosing atypical lipomatous tumor (ALT)/well-differentiated liposarcoma and dedifferentiated liposarcoma (WDL/DDL) in specialist sarcoma units. We describe our experience of its utility at our tertiary institute.Methods. All routine histology samples in whichMDM2amplification was assessed with FISH over a 2-year period were included, and FISH results were correlated with clinical and histologic findings.Results. 365 samples from 347 patients had FISH forMDM2gene amplification. 170 were positive (i.e., showedMDM2gene amplification), 192 were negative, and 3 were technically unsatisfactory. There were 122 histologically benign cases showing a histology:FISH concordance rate of 92.6%, 142 WDL/DDL (concordance 96.5%), and 34 cases histologically equivocal for WDL (concordance 50%). Of 64 spindle cell/pleomorphic neoplasms (in which DDL was a differential diagnosis), 21.9% showedMDM2amplification. Of the cases with discrepant histology and FISH, all but 3 had diagnoses amended following FISH results. For discrepancies of benign histology but positive FISH, lesions were on average larger, more frequently in “classical” (intra-abdominal or inguinal) sites for WDL/DDL and more frequently core biopsies. Discrepancies of malignant histology but negative FISH were smaller, less frequently in “classical” sites but again more frequently core biopsies.Conclusions. FISH has a high correlation rate with histology for cases with firm histologic diagnoses of lipoma or WDL/DDL. It is a useful ancillary diagnostic tool in histologically equivocal cases, particularly in WDL lacking significant histologic atypia or DDL without corresponding WDL component, especially in larger tumors, those from intra-abdominal or inguinal sites or core biopsies. There is a significant group of well-differentiated adipocytic neoplasms which are difficult to diagnose on morphology alone, in which FISH forMDM2amplification is diagnostically contributory.</jats:p