Atypical and malignant neoplasms showing lipomatous differentiation. A study of 111 cases.

One-hundred-eleven cases of histopathologically atypical or malignant lipomatous lesions in the somatic soft tissue and retroperitoneum were studied. These consisted of 48 differentiated fatty neoplasms of the somatic soft tissues (DFT-S), 21 fatty neoplasms of the retroperitoneum (DFT-R), 33 myxoid liposarcomas from various sites and nine pleomorphic liposarcomas. DFT-S were defined as lipomatous lesions composed of mature fat and containing atypical stromal cells or lipoblasts. In the somatic soft tissues, this group included lesions that would be classified using published criteria as "atypical lipoma", "pleomorphic lipoma", "well-differentiated lipoma-like liposarcoma", and "sclerosing liposarcoma". All of the DFT-R met previously published criteria for "well differentiated liposarcoma" or "sclerosing liposarcoma". We found no consistent histologic differences between the DFT-S and DRT-R. No pure "round cell" liposarcomas were encountered although many myxoid liposarcomas had "round cell" areas. Follow-up data were available in 80 cases (72%) with a mean follow-up period of over 7 years. Among the DFT-S there were no uncontrollable recurrences, distant metastases, or tumor-related deaths. The depth of the neoplasm correlated with the tendency for local recurrence; no neoplasms primary in the subcutis recurred; 29% of the tumors recurred when they originated in the deep soft tissues or within the muscle. None of the recurrent tumors demonstrated "dedifferentiation." DFT-R had a recurrence rate of 67% and, although there were no distant metastases, nine patients (43%) died of tumor. Five retroperitoneal tumors dedifferentiated but did not metastasize. In light of this experience, we believe that the term "atypical lipoma" is warranted for the DFT-S and "well differentiated liposarcoma" is an appropriate label for the DFT-R. The overall mean survival for the 52 cases of liposarcoma (excluding DFT-S) was 13.6 years. The mean survival in "well differentiated liposarcoma" (11.25 years) was between that for myxoid liposarcoma (16.25 years) and that for pleomorphic liposarcoma (7 years). Six patients (29%) with myxoid liposarcoma developed local recurrences and 6 patients (29%) developed distant metastases and died. Metastasis was always associated with a round cell (or pleomorphic) component with increased numbers of mitotic figures in either the primary tumor or a local recurrence.