Early High-Grade Transformation of IDH-Mutant Central Nervous System WHO Grade 2 Astrocytoma: A Case Report.

High-grade transformation of low-grade gliomas has long been a poor prognostic factor during therapy. In 2016, the World Health Organization (WHO) Classification of Tumors of the Central Nervous System (CNS) adopted isocitrate dehydrogenase (IDH) mutation status in the classification of diffuse astrocytomas. The 2021 classification denoted glioblastomas as IDH-wildtype and graded IDH-mutant astrocytomas as 2, 3, or 4. Gemistocytic morphology, a large proportion of residual tumor, the patient's age, and recurrence after radiotherapy were previously mentioned as risk factors for high-grade transformation of low-grade gliomas. We report a 34-year-old male patient initially diagnosed with IDH-mutant grade 2 astrocytoma according to the 2021 WHO classification of CNS tumors. As the first surgical resection achieved gross total resection on postoperative MRI, no adjuvant therapy was given and regular follow-up was planned. On 1-year follow-up MRI, two new enhancing nodular lesions appeared at the ipsilateral brain parenchyma abutting the surgical resection cavity. Salvage craniotomy achieved gross total resection, and the pathologic diagnosis was IDH-mutant WHO grade 4 astrocytoma. We describe this tumor in terms of the previous WHO classification to evaluate the risk of high-grade transformation and discuss possible risk factors leading to high-grade transformation of low-grade astrocytoma.