Primary mismatch repair-deficient IDH-mutant astrocytoma in child: Unusual entity

Introduction: "Primary Mismatch Repair-Deficient IDH-mutant Astrocytoma" (PMMRDIA) is a newly identified high-grade glioma with a poor prognosis. It has not been officially recognized as a distinct entity in the 5th edition of the WHO Classification of Central Nervous System Tumors 2021, making its diagnosis challenging. Case presentation: To highlight this entity, we present the case of a 7-year-old boy with a family history of cerebral tumor among the father's relatives and first-degree parental consanguinity. He experienced increased intracranial hypertension and inguinal cafe<acute accent>-au-lait macules. Brain imaging revealed a cystic and solid mass with irregular enhancement in the left parietal lobe, indicating a possible Pilocytic astrocytoma. The final diagnosis of PMMRDIA was confirmed through histological, immunohistochemical, and molecular analysis. Clinical discussion: This tumor is mainly observed in children and is characterized by microsatellite instability (MSI) and high tumor mutational burden (TMB). MSI is known to occur in recurrent IDH-mutant gliomas as a resistance mechanism towards alkylating chemotherapy. Conclusion: In PMMRDIA, MSI can be a good predictive biomarker for responding to immunotherapy, which could improve the outcome of these children.