[DICER1-mutant primary intracranial sarcoma: analysis of five cases].

<b>Objective:</b> To investigate the clinicopathological characteristics and differential diagnosis of DICER1-mutant primary intracranial sarcoma. <b>Methods:</b> Five cases of DICER1-mutant primary intracranial sarcoma at Sanbo Brain Hospital, Capital Medical University, Beijing, China during May 2013 to November 2024 were collected. The clinical and imaging data were retrieved. Histological evaluation, immunohistochemical staining and next generation sequencing were performed. Additionally, a literature review was conducted. <b>Results:</b> All five DICER1-mutant primary intracranial sarcomas were located in the supratentorial region, with one case involving the basal ganglia. There were two males and three females. The median age at diagnosis was 25 (14.0, 30.5) years. Morphologically, they were characterized by high-grade spindle cell sarcoma, with brisk mitotic activity and cytoplasmic eosinophilic globules. Myxoid degeneration, necrosis, and invasion into surrounding brain tissue were observed in some cases. The tumor cells showed diffuse staining of vimentin and variable expression of myogenic marker (desmin), with or without focal MyoD1 and/or Myogenin expression. Four tumors exhibited diffuse, strong expression of TLE1 and p53, while only three tumors showed loss of ATRX (nuclear) expression. Two cases showed mosaic loss of H3K27me3 expression in neoplastic cells. The Ki-67 proliferation index was high (40%-80%). Various neuronal markers, such as synaptophysin, NF, SOX2 and MAP2, were expressed in all tumor samples. Genetically, all tumors samples harbored biallelic abnormalities of DICER1. One was a hotspot missense mutation in the RNase Ⅲb domain within exon 25 on one allele (p.E1813 or p.D1810), while the other allele had mutations including a germline mutation in one case, a somatic mutation in two cases, and a copy number deletion in two cases. In addition, these sarcomas showed alterations in TP53 (4/5), ATRX (3/5), and the genes of the mitogen-activated protein kinase pathway (3/5). Finally, all five cases were diagnosed as DICER1-mutant primary intracranial sarcoma. All patients underwent craniotomy that led to complete tumor resection. Three patients received adjuvant radiotherapy and chemotherapy, with progression-free survival time of 28, 48, and 50 months, respectively. Patient 2 succumbed to the tumor after 3 months post-surgery due to rapid progression and tumor dissemination. Patient 5 was lost to follow-up 3 months after the surgery. <b>Conclusions:</b> DICER1-mutant primary intracranial sarcoma is a newly defined tumor entity in the fifth edition of the World Health Organization Classification of Central Nervous System Tumors, and commonly occurs in children and young adults. High-grade malignant spindle cells are their typical morphological feature. Eosinophilic cytoplasmic globules and myogenic differentiation can help establish the diagnosis. This study suggests that DICER1-mutant primary intracranial sarcomas exhibit immunophenotypic neuronal differentiation. Rendering the diagnosis of DICER1-mutant primary intracranial sarcoma largely relies on detecting DICER1 pathogenic alterations or DNA methylation profiling.