Long-term follow up of pure myxoid liposarcomas with special reference to local recurrence and progression to round cell lesions

Myxoid and round cell liposarcomas cytogenetically share the t(12;16)(q13;p 11) translocation, and represent a continuous morphological spectrum includ ing pure myxoid liposarcoma (PML), mixed (myxoid/round cell) liposarcoma (M XL) and pure round cell liposarcoma (RCL). It has been documented that a cr itical amount of RC lesion in low-grade, MXL is associated with poor progno sis. The present longterm follow-up study examined the factors that influen ce the progression of PML to RCL and several other prognostic factors and c ompared the results with those of liposarcomas initially diagnosed as MXL. Using 34 primary PML and their local recurrences after curative resection, and two initially diagnosed MXL, the appearance of RC lesion in PML was exa mined, and the prognostic significance of the local recurrence rate (times/ month), tumor size, infiltrative proliferation (defined as entrapping of st riated muscle fibers in the tumor), tumor necrosis and mitotic figures. The follow-up period ranged from 19 to 378 months (mean, 173.4 months). Based on Cox's proportional hazards regression model, a recurrence rate greater t han or equal to 0.01 in 24 PML with recurrences (P = 0.037) and the presenc e of infiltrative proliferation in 34 PML (P = 0.043) were significant inde pendent factors associated with poor prognosis. Round cell lesions were det ected in the last recurrences of two surviving PML These results indicate t hat through their recurrence, PML might potentially become malignant, indep endent of the proportion of RC lesions in the tumor. The appearance of RC l esions in subsequent recurrences may indicate a progression from PML to MXL .