Lipomatous tumors.

Lipomatous tumors are a common group of mesenchymal lesions. Over the years the major changes in the classification of lipomatous tumors have included the addition of several new variants of lipoma, the use of the term atypical lipoma for well differentiated liposarcoma of the subcutaneous tissue, and recognition of the entity dedifferentiated liposarcoma. Lipomas, the most common lipomatous tumor, account for nearly one-half of all benign lesions. In their typical form they seldom present diagnostic problems for the pathologist. However, lipomas occurring in deep locations (e.g., intramuscular lipoma, perineural lipoma) or those having unusual features (e.g., chondroid lipoma, lipoma with hibernoma, cellular angiolipoma, spindle cell/pleomorphic lipoma) may be confused with liposarcoma. Recent cytogenetic studies have reaffirmed the separate nature of many of the variants of lipoma. Solitary lipomas commonly have rearrangements of chromosome 12, a finding not encountered in multiple lipomas or in spindle cell/ pleomorphic lipoma. Liposarcoma is the most common adult soft tissue sarcoma. It seldom arises from subcutaneous tissues or from preexisting lipomas, and is seldom found in children. The hallmark of liposarcoma is the immature fat cell or lipoblast. Diagnostic lipoblasts have an eccentric, hyperchromatic nucleus, which is indented or scalloped by the presence of one or more fat vacuoles. It is important that these cells occur in the appropriate histologic background because similar cells can be seen in a variety of nonlipomatous lesions (e.g., silicone reaction). Failure to apply strict criteria in identifying such cells and in noting the milieu in which they occur can lead to overdiagnosis of liposarcoma. Liposarcomas are divided into several subtypes: well differentiated, myxoid, round cell, pleomorphic, and dedifferentiated. Liposarcomas can be conceptualized as occurring in two broad groups, myxoid/round cell liposarcoma and well differentiated/dedifferentiated liposarcoma. Myxoid/round cell liposarcomas occur in middle-aged adults primarily as an extremity lesion. Tumors range from pure myxoid (low grade) to pure round cell (high-grade lesions) with some cases having transitional features. Behavior can be related to the amount of round cell areas. A consistent chromosome abnormality t(12;16) characterizes this spectrum of lesions. Well differentiated/dedifferentiated liposarcomas, in contrast, occur in late adult life as extremity or retroperitoneal lesions. They consist of mature fat interlaced with atypical hyperchromatic cells and rare lipoblasts. These lesions recur commonly, but they do not metastasize. Their behavior is strongly influenced by location, with retroperitoneal lesions having the worse prognosis. As a long-term complication of the disease, these lesions may progress histologically (dedifferentiate), a phenomenon that confers upon them metastatic potential. Dedifferentiation is largely a time-dependent phenomenon that occurs in sites in which there is a high likelihood for clinical persistence of disease (e.g., the retroperitoneum). Dedifferentiated liposarcomas occur in an age group similar to well differentiated liposarcoma, but are found far more commonly in the retroperitoneum. Most occur as de novo lesions, with only a small percentage occurring as a late complication of well differentiated liposarcoma, as described above. They consist of well differentiated areas associated with nonlipogenic sarcoma usually resembling high-grade malignant fibrous histiocytoma or fibrosarcoma. However, the range of histologic features occurring in the dedifferentiated areas is more varied than previously appreciated. Low-grade areas resembling fibromatosis or low-grade fibrosarcoma may be seen as the sole expression of dedifferentiation or may co-exist with high-grade sarcoma. (ABSTRACT TRUNCATED)