Utility of Protein Kinase C Beta II Immunohistochemistry in Differential Diagnosis of Ewing Sarcoma.

The diagnosis of Ewing sarcoma can be challenging, particularly when the tumor is present in an atypical location and resembles histologic mimics. The hallmark feature of Ewing sarcoma is chromosomal translocation, t(11;22)(q24;q12), involving EWSR1 and ETS gene family members. For decades, fluorescence in situ hybridization with a break-apart EWSR1 probe has been the diagnostic gold standard. However, EWSR1 rearrangements have been identified in other malignancies; thus, the detection of chimeric EWSR1 transcripts has become a preferable approach. Occasionally, insufficient tissue, severe RNA degradation, or economic constraints hamper molecular testing. This study evaluated Protein Kinase C Beta II (PKC β II) expression in >1000 tumors and assessed the utility of PKC β II immunohistochemistry in the differential diagnosis of Ewing sarcoma. Tumors harboring EWSR1::FLI1 (n=26), EWSR1::ERG, EWSR1::ETV4 (n=1), and FUS::ERG (n=6) fusions were evaluated, revealing strong diffuse immunoreactivity, although a patchy pattern was seen in 3 cases. Undifferentiated round cell sarcomas (n=46), including BCOR-, CIC-, NFATC2-, NUTM1-, and PATZ1 rearranged/fusion-sarcomas were negative. Two of the 130 synovial sarcomas, including 1 with a poorly differentiated morphology, showed diffuse, moderate-to-strong positivity. One of the 26 poorly differentiated carcinomas from the head and neck region, probably small cell lung carcinoma metastasis, showed strong PKC β II expression. Neuroblastomas (>50%) expressed PKC β II, although none showed a strong diffuse pattern. Diffuse moderate-to-strong immunoreactivity was observed in 2 sarcomatoid mesotheliomas and 2 metastatic melanomas. Diffuse but weak staining was observed in 73% (11/15) of the T-cell lymphoblastic lymphomas, including 10 CD99-positive cases. Similarly, weak predominantly patchy staining was seen in half (40/80) of other non-Hodgkin lymphomas and sporadically in embryonal rhabdomyosarcoma, Merkel cell carcinoma, small cell lung carcinoma, and Wilms tumor. Thus, diffuse and strong PKC β II immunoreactivity appears to be a reliable diagnostic marker for distinguishing classic Ewing sarcoma from histologic mimics.