Atypical lipoma.

In a previous clinico-pathologic and prognostic study of a Swedish national series of liposarcoma the designation atypical lipoma was suggested for a group of well-differentiated non-metastasizing liposarcomas characterized by only moderate nuclear atypia. In the present report, 21 new cases of atypical lipoma are described. The tumors were mainly composed of univacuolated fat cells without cellular or nuclear atypia, but also showed univacuolated fat cells with enlarged, moderately polymorphic, dark nuclei. In two of the tumors a few multivacuolated fat cells with scalloped nuclei were found. Small multinucleated cells with overlapping, peripherally arranged nuclei, reminiscent of so-called floret-like cells as in pleomorphic lipoma, could occasionally be seen. Areas of generally delicate linear or patchy fibrosis with atypical nuclei were a common finding. Two atypical lipomas were studied ultrastructurally. The ultrastructural findings suggest that the nuclear polymorphism is of a neoplastic nature rather than of a regressive changed. Nine atypical lipomas were located intramuscularly, two intermuscularly and ten subcutaneously. Five of the subcutaneous tumors and three of the inter-and intramuscular tumors recurred one or more times. Most of them, particularly the subcutaneous ones, had primarily been diagnosed as ordinary lipomas, but re-examination of the histological sections after one or more recurrence revealed overlooked nuclear atypia. Metastasis or dedifferentiation at recurrence was not seen in any case.