Atypical lipoma, atypical intramuscular lipoma, and well differentiated retroperitoneal liposarcoma: a reappraisal of 30 cases formerly classified as well differentiated liposarcoma.

Thirty cases formerly diagnosed as Grade I lipogenic liposarcoma (well differentiated liposarcoma) were reviewed. The basic histologic pattern in all was that of adult fat modified by the presence of cells with enlarged, hyperchromatic nuclei; in most cases there was also a component of myxoid and/or fibrous tissue. The length of follow-up ranged from two to 30 years. Nine of the tumors were located in the subcutaneous layer. None of these recurred after excision, not even those which were simply "shelled out," and none metastasized. The term "atypical lipoma" is proposed for this group. Thirteen were located within or between muscles of the limbs, limb girdles, and head and neck. Nine of these recurred at least one, but there were no metastases and no deaths due to tumor. These were designated "atypical intramuscular lipoma." The remaining eight originated in the retroperitoneum. Although none of these patients developed metastases, five suffered inoperable recurrence and three died as a result of the neoplasm. It is suggested that the term "well differentiated retroperitoneal liposarcoma" be retained for cases of this type.