The New Molecularly Defined CNS Sarcomas: FET::CREB Fusion-Positive Intracranial Mesenchymal Tumour, CIC-Rearranged Sarcoma, and DICER1-Mutant Primary Intracranial Sarcoma

The 2021 World Health Organization (WHO) Classification of Tumours of the Central Nervous System introduced three molecularly defined mesenchymal entities: intracranial mesenchymal tumour, FET::CREB fusion-positive (ICMT), defined by fusion of a FET RNA-binding protein gene (EWSR1 or FUS) to a cAMP-response element-binding protein (CREB) family gene; CIC-rearranged sarcoma, defined by fusion of the capicua transcriptional repressor gene (CIC); and primary intracranial sarcoma, DICER1-mutant, defined by mutation of the microRNA-processing gene DICER1. Each is genetically defined yet morphologically non-specific, and each mimics a commoner neoplasm: meningioma, Ewing or embryonal tumour, and high-grade glioma, respectively. This review reads all three through a single mechanistic lens, tracing each defining lesion to its diagnostic phenotype. We compare the available diagnostic modalities and propose a tiered testing algorithm, offered as an expert synthesis rather than a validated pathway. DNA methylation profiling recognises CIC-rearranged and DICER1-mutant sarcoma as distinct classes but has no dedicated class for ICMT, which it may misassign to meningioma, making fusion testing indispensable. Alone among the three, DICER1-mutant sarcoma may arise on a germline background, so its diagnosis should prompt genetic counselling and an offer of germline testing. Molecular characterisation has largely resolved diagnostic ambiguity without yet delivering therapy: candidate targets remain preclinical or anecdotal, and management rests on resection and borrowed adjuvant regimens.