Pathology and genetics of adipocytic tumors

Adipocytic tumors are common mesenchymal neoplasms with considerable morphologic and genetic heterogeneity. The fruitful integration of morphology and cytogenetics in the past 15 years has not only enhanced the diagnostic accuracy, but also refined the various pathological classifications and subtypes in these tumors. The current WHO classification includes eleven benign subtypes, one intermediate and five categories of malignant fatty neoplasms with incorporation of relevant genetic findings. Of the benign tumors, lipomas have been extensively analyzed by chromosome banding which has shown that their cytogenetic patterns are heterogeneous. Still aberrations involving 12q13-->q15, 6p23-->p21 and loss of material from 13q are common and consistent findings. Among the malignant tumors, the t(12;16)(q13;p11) resulting in the fusion of DDIT3 and FUS genes is the hallmark of myxoid and round cell liposarcoma and is used as a highly specific and sensitive marker of this entity. The tumor in the intermediate group, atypical lipomatous neoplasm/well-differentiated liposarcoma which poses morphologic challenges due to close histological similarity to benign lipomas shows characteristic supernumerary rings and giant rod chromosomes due to amplification of the 12q14-->q15 region often involving the MDM2 oncogene. This review will focus on the pathological features of the various adipocytic tumors and relevant genetic findings reported in the literature.