Does “Low-Grade” Dedifferentiated Liposarcoma Exist? The Role of Mitotic Index in Separating Dedifferentiated Liposarcoma From Cellular Well-differentiated Liposarcoma

BACKGROUND: Subjective, varying criteria identify "low-grade" dedifferentiation in well-differentiated/dedifferentiated liposarcoma (WD/DDLPS). The value of mitotic rate (MR) in defining DDLPS is not confirmed. We studied all patients with the resection of their primary or first recurrence retroperitoneal WD/DDLPS at our institution to determine the value of MR in diagnosing DDLPS and if MR associates with patient survival. DESIGN: Ninety-eight patients with retroperitoneal WD/DDLPS operated at our institution from January 1, 1989 to December 31, 2013 were included. Cases were defined as acellular (AC) WDLPS, LS0-4 (tumors with non-lipogenic areas and MR 0-4/10HPFs) or LS5+(non-lipogenic areas, MR≥5/10 HPFs) and graded using the French system. Kaplan-Meier survival estimates with log-rank test and multivariate Cox (mCox) analyses were performed. RESULTS: Follow-up was available on all patients (median 9.3 y, range 0.02-23.16 y). Kaplan-Meier demonstrated a significant ( P =0.004) difference in disease-specific survival (DSS) among the 3 groups. mCox demonstrated no difference in DSS between the AC and LS0-4 groups (HR 1.51; 95% CI 0.57-3.99, P =0.412) but significantly lower DSS in the LS5+group compared with the AC group (HR 2.68; 95% CI 1.07-6.71, P =0.035). The difference in DSS was not significant between grade 2 and 3 tumors ( P =0.094). DSS between MR 5-19/10 HPFs and MR20+/10 HPFs subgroups was significant ( P =0.007) but by mCox did not reach significance (HR 2.47; 95% CI 0.96-6.35, P =0.060). CONCLUSION: This study confirms that MR distinguishes DDLPS from WDLPS with non-lipogenic areas, also known as cellular WDLPS. For consistency in diagnosis and research, only WD/DDLPS with≥5 mitoses/10 HPFs should be considered DDLPS.