Chronic expanding hematoma following Gamma Knife irradiation for primary central nervous system lymphoma: illustrative case.

backgroundChronic expanding hematoma (CEH) is a rare disease characterized by slow expansion, especially reported after Gamma Knife radiosurgery (GKRS) for cerebral arteriovenous malformations. No cases have been reported following treatment for primary central nervous system lymphoma (PCNSL). Vascular endothelial growth factor and vascular endothelial growth factor receptor-1 (VEGFR-1) have been reported to be responsible for CEH induction. OBSERVATIONS: A 56-year-old woman underwent partial tumor removal for PCNSL in the left temporo-occipital lobe at another hospital. One and a half months later, a new lesion was observed, and she underwent GKRS, including the residual lesion. She was treated with multiple chemotherapy regimens including R-MPV (rituximab, methotrexate, procarbazine, vincristine) but relapsed repeatedly and was administered tirabrutinib. Four years after GKRS, the gadolinium-enhanced lesion slowly grew. Tumor recurrence or radiation necrosis was suspected, and surgical removal was performed. The tissue was composed of nonmalignant brain tissue and fibrinized hematoma, which demonstrated strong expression of VEGFR-1 on immunostaining, and the pathological diagnosis was CEH. LESSONS: In this patient, CEH could have resulted from VEGFR-1 expression due to GKRS. For a patient with slow lesion growth following GKRS for PCNSL, surgical removal should be considered, taking into account the possibility of CEH along with recurrence and radiation necrosis. https://thejns.org/doi/10.3171/CASE258.