Evaluation of PRDM10 Gene Rearrangement by Immunohistochemical and Molecular Methods in Pleomorphic Soft Tissue Tumors

Abstract Undifferentiated pleomorphic sarcomas are aggressive soft tissue tumors that do not exhibit any identifiable histological differentiation. Undifferentiated pleomorphic sarcomas (UPS) occupies a significant place among soft tissue tumors, although less commonly seen benign mimickers like low-grade myxofibrosarcomas (low-grade MFS), superficial CD34-positive fibroblastic tumors (SC34FT), myofibroblastic sarcomas, myxoinflammatory fibroblastic sarcomas (MIFS) and pleomorphic hyalinizing angiectatic tumors (PHAT) should not be forgotten in the differential diagnosis. In recent years, PRDM10 fusion transcript has been detected in soft tissue tumors with pleomorphic morphology. In our study, we aimed to detect PRDM10 gene rearrangement in soft tissue tumors with pleomorphic morphology using immunohistochemical and molecular methods and to evaluate them with clinicopathological findings. Pleomorphic soft tissue tumors with low mitotic and necrotic scores; which originally diagnosed as UPS, MFS, myofibroblastic sarcoma, and PHAT (totally 33 cases), were selected. In our study, five cases showed immunohistochemical positivity with PRDM10; two cases showed both nuclear and cytoplasmic staining while three showed only cytoplasmic staining and four cases had break-apart signals with FISH. We found that two cases initially diagnosed as low-grade UPS which showed break-apart signals in FISH, may actually be SC34FT considering the clinical, morphological, immunohistochemical and molecular findings. The lack of PRDM10 staining in one of these two cases suggests a low sensitivity of the PRDM10 immunomarker. In conclusion, soft tissue tumors that has PRDM10 gene rearrangement, superficial location, low mitotic activity, no necrosis and CD34 positivity exhibit distinct clinical and prognostic features, suggesting potential overlap with the tumor spectrum of SC34FT.