Primary subdural tumor mimicking subdural hematoma: illustrative case.

backgroundRare pediatric subdural tumors can mimic subdural hematomas and can be caused by lymphoma, sarcoma, and cancer metastases. OBSERVATIONS: A 3-year-old boy with a history of chronic subdural hematoma, who had undergone multiple surgeries, was transferred to the authors' hospital with recurrent vomiting and lethargy. CT revealed a partially organized bilateral mass with heterogeneous intensities. Although the lesion was primarily located in the subdural space, a portion appeared to invade the brain. Gadolinium (Gd)-enhanced MRI revealed an enhanced nodule. Surgical findings revealed that the lesion bled easily, adhered to the arachnoid membrane, and invaded the brain parenchyma. Immunohistochemical and genetic analyses were performed, and the patient was diagnosed with an unclassifiable, malignant, intracranial tumor with epithelioid features and the HIST1H3B K37 mutation. Classification based on the methylation profile was conducted, but no definitive methylation class was identified. The tumor showed rapid regrowth after resection. The patient subsequently underwent chemoradiotherapy. Despite multimodal treatment, the patient died of the primary disease within 1 year of surgery. LESSONS: Primary subdural tumors in children are extremely rare; however, they should be considered as a differential diagnosis when Gd contrast MRI shows an enhanced nodule protruding into the brain. https://thejns.org/doi/10.3171/CASE25849.