POORLY DIFFERENTIATED LIPOSARCOMA WITH SPINDLE CELL, ROUND CELL COMPONENTS AND FOCAL OSSEOUS AND CHONDROID METAPLASIA
Liposarcoma is one of the most common soft tissue sarcoma in adults. Liposarcomas are usually large and occur most frequently in the lower extremities, retroperitoneal, perirenal, mesenteric region and shoulder area.
Grossly, they are well circumscribed but not encapsulated. They are divided into five subtypes: well-differentiated, dedifferentiated, myxoid, round cell, and pleomorphic. Mixed forms occur as seen in this case. The common morphologic denominator of liposarcoma is the lipoblasts. The criteria used for identifying lipoblasts include a hyperchromatic indented or sharply scalloped nucleus, lipid-rich (neutral fat) droplets in the cytoplasm, and an appropriate histologic background.
Well-differentiated liposarcoma is the most common form with a peak incidence during sixth and seventh decades of life. Men and women are equally affected. Grossly, they are large multilobular lesions that range in color from deep yellow to ivory. They are further divided into three subtypes: lipoma-like, sclerosing, and inflammatory. Lipoma-like liposarcoma consists of mature fat with a variable number of lipoblasts. Sclerosing forms have dense fibrosis alternating with mature adipocytes. Inflammatory form consists of a dense lymphocytic or plasmacytic infiltrate superimposed on a lipoma-like or sclerosing form.
Dedifferentiated liposarcomas develop in approximately the same age group as well-differentiated liposarcomas, reaching a peak during the early seventh decade. The genders are affected equally. Dedifferentiation is considered a late complication in the natural history of well differentiated liposarcoma. Grossly, the lesions present as large multinodular masses ranging in color from yellow to yellow-tan admixed with firm tan-gray areas. Microscopically the nonlipogenic (dedifferentiated) component is typically abruptly separated from the juxtaposed well-differentiated component. The dedifferentiated areas can include large round cells, spindle cells, amianthoid fibers, and rhabdomyosarcomatous element.
Myxoid and round cell liposarcomas are considered as poorly differentiated and they account for one-half of liposarcomas. They occur in a younger age group with a peak incidence during the fifth decade. Myxoid liposarcomas grossly are multinodular, gelatinous masses and microscopically are consisted of proliferating lipoblasts in different stages of differentiation, a prominent anastomosing capillary network, and a mucoid matrix rich in hyaluronidase-sensitive acid mucopolysaccharides. Round cell liposarcomas grossly are opaque white nodules and microscopically are consisted of small cells with a distinctly acidophilic cytoplasm with presence of scattered lipoblasts. Pleomorphic liposarcoma is the least common and account for 10-15% of all liposarcomas. They have a disorderly growth pattern and an extreme degree of cellular pleomorphism, including bizarre giant cells.
Cytogenetically, 77% of myxoid liposarcomas are associated with a chromosomal translocation, t(12; 16) (q13; p11). This translocation appears specific for myxoid subtype, including tumors containing myxoid and round cell areas. One of the genes at this breakpoint turns out to be a transcription factor CHOP, which is involved in adipocytic differentiation. Trisomy 8 has been observed in 18% of the myxoid types as an additional nonrandom abnormality. Well-differentiated liposarcomas are characterized by telomeric association, ring chromosomes, and giant marker chromosomes which may contain homogeneously staining regions (hsr). The tumors are usually near-diploid with good chromosome morphology. Pleomorphic liposarcomas have hyperdiploid count, poor morphology, and multiple, complex non-recurrent aberrations.
Histologic grade is the most important prognostic factor. The grading parameters include degree of differentiation, necrosis and mitotic figures. Well-differentiated liposarcomas are non-metastasizing lesions with recurrence rate vary from 43% to 91% at different locations. Metastases was reported in about 50% of myxoid liposarcomas and 5 year survival rate is between 25% to 90% depended on grading. A 21% five-year survival rate was reported for pleomorphic liposarcomas.
Contributed by Hongbo Yu, MD, PhD, Antonia Sepulveda, MD, PhD and Uma Rao, MD