DEDIFFERENTIATED LIPOSARCOMA - A CLINICOPATHOLOGICAL ANALYSIS OF 155 CASES WITH A PROPOSAL FOR AN EXPANDED DEFINITION OF DEDIFFERENTIATION

We studied 155 cases of dedifferentiated liposarcoma to define its cli nicopathologic features and behavior, in particular how the extent and grade of dedifferentiation affected outcome. Tumors occurred in late adult life (median, 61.5 years; range, 21-92 years), most commonly in the retroperitoneum (106 cases), extremities and trunk (32 cases), and scrotum/spermatic cord (13 cases). The majority of dedifferentiated l iposarcomas presented as de novo lesions, whereas the remainder develo ped as a late complication of a preexisting well-differentiated liposa rcoma after an average interval of 7.7 years. At the time of presentat ion, most of the dedifferentiated liposarcomas displayed extensive are as of high-grade dedifferentiation resembling malignant fibrous histio cytoma or high-grade fibrosarcoma, whereas a minority contained only a reas of low-grade dedifferentiation resembling fibromatosis or well-di fferentiated fibrosarcoma, Divergent myosarcomatous or osteosarcomatou s differentiation was observed focally in six cases. The behavior of d edifferentiated liposarcomas was that of a high-grade sarcoma with a l ocal recurrence rate of 41%, a metastatic rate of 17%, and disease-rel ated mortality of 28%. The most important prognostic factor was locati on in that retroperitoneal tumors had significantly worse survival tha n those in other sites. Tumors were divided into those having less tha n or those with more than 25% dedifferentiation, and dedifferentiated zones were classified into low grade or high grade. Neither low-grade dedifferentiation nor a low percentage of dedifferentiation was associ ated with an improved outcome for the tumors examined in this study; h owever, in no cases was the absolute size of the dedifferentiated focu s <2 cm. Therefore, this study did not determine a minimum, or thresho ld, amount of dedifferentiation below which outcome was more favorable . The behavior of liposarcomas in which the dedifferentiated component was entirely low grade was more similar to that of traditional dediff erentiated liposarcoma than to that of well-dedifferentiated liposarco ma. Our study supports the expansion of the definition of dedifferenti ated liposarcoma to include tumors with low-grade dedifferentiation an d also suggests that low-grade dedifferentiation represents a precurso r lesion of high-grade dedifferentiation.