Lipogenic component shows atypical fat with lipoblasts.
Non-lipogenic sarcoma appears variable, can be high-grade (e.g. pleomorphic sarcoma, myxofibrosarcoma) or low grade (e.g. fibroblastic) spindle cells.
A subset will show heterologous differentiation (rhabdomyosacoma, leiomyosarcoma, osteo- or chondrosarcoma, angiosarcoma).
Like well-differentiated LPS shows amplified sequence of Chr 12q14-15.
Immunohistochemistry: S100+, MDM2+ and CDK4+; keratin-.
Poor prognosis, local recurrence in up to 40% and metastisis in 20% of cases.
Retroperitoneal LPS may extend into the kidney and simulates a primary renal malignancy, particularly:
RCC with sarcomatoid change: look for well differentiated (non-transformed) areas of RCC; keratin+, MDM-, CDK4-.
Muscle predominant angiomyolipoma: mature fat and with abnormal thick hyalinized vessels; HMB45+.
Examine "normal fat" around the tumor/kidney, which may represent well-differentiated LPS of the dedifferentiated LPS.
Leiomyosarcoma: spindle cells with cigar-shaped nuclei; actin and desmin+; MDM and CDK4-.