Undiff. Pleomorphic Sarcoma : Differential Diagnosis
Differential Diagnosis of Undifferentiated Pleomorphic Sarcoma (UPS): Pleomorphic Leiomyosarcoma: The distinction between UPS and pleomorphic leiomyosarcoma (LMS) can be quite challenging. About 10-15% of retroperitoneal LMS have pleomorphic, anaplastic cells with deeply eosinophilic cytoplasm. They may be admixed with more typical spindle cells with blunt-ended nuclei and perinuclear vacuoles. Some cases appear to arise from well-differentiated areas nearby. The interpretation of immunohistochemical markers of muscle differentiation must be done with great caution. One must keep in mind that the distribution and intensity of muscle markers may be diminished in pleomorphic areas of LMS in comparison to the classic areas. On the other hand, myofibroblasts in UPS often show focal or patchy staining for muscle markers. Smooth muscle-specific actin (monoclonal antibody 1A4) and pan-muscle actin (antibody HHF35) show strong diffuse cytoplasmic reactivity in most LMS. In contrast, myofibroblasts in UPS can show wispy, peripheral expression of actin (tram-track pattern). Desmin positivity is variable in LMS and is ranges from 50% to 100% in various studies. Strong diffuse desmin positivity generally favors muscle origin for the tumor. Other myogenic markers such as h-caldesmon and smooth muscle myosin heavy chain are less sensitive than actin and desmin in the diagnosis of LMS. They are even less commonly expressed in myofibroblastic cells of UPS.