CD30 expression in an emerging group of mesenchymal spindle cell neoplasms with ALK fusion detected by flow cytometry and immunohistochemistry

Abstract An emerging group of spindle cell neoplasms harboring fusions involving NTRK or non‐ NTRK kinase genes often share characteristic S100 and/or CD34 expression; however, the diagnostic utility of immunohistochemical stains is not well established in this family owing to their lack of specificity. Recently, CD30 expression in spindle cell neoplasms with kinase gene fusions, such as NTRK , BRAF , RAF1 , and RET , has been increasingly identified. We herein report a 10‐year‐old girl with high‐grade spindle cell sarcoma of the neck. Prior to histopathological evaluation, flow cytometry (FCM) analysis and touch smear cytology of the tumor tissue revealed CD34+ and dimCD30+ spindle cell populations. Histopathologically, the case was characterized by monomorphic spindle‐shaped cytomorphology with CD30, S100, and CD34 positivity and harbored close similarities with spindle cell neoplasms with NTRK or non‐ NTRK gene fusions. Subsequently, a comprehensive next‐generation sequencing sarcoma panel identified a rare PLEKHH2::ALK fusion, and a diagnosis of ALK ‐rearranged spindle cell neoplasm was made. The patient showed significant tumor response to single‐agent treatment with alectinib, an ALK‐tyrosine kinase inhibitor. This case supports that CD30 is expressed in an ALK ‐rearranged mesenchymal neoplasm. The benefit of the early detection of CD30 expression by FCM for a prompt diagnosis and treatment is highlighted in the context of an aggressive clinical course. This case represents a learning experience regarding the need to the check the status of CD30 expression in these tumors and suggests the potential clinical benefits of CD30‐targeted therapy.