A 69-year-old woman was treated by local irradiation for a malignant lymphoma of the left parotid gland. Three years after the radiation therapy, magnetic resonance imaging revealed heterogeneously enhanced masses in the left temporal lobe and left cerebellum. Thallium-201 chloride single photon emission computed tomography (Tl-SPECT) revealed high uptake and [11C]methionine positron emission tomography (Met-PET) revealed moderate uptake in both masses. Stereotactic biopsy was performed. The histological diagnosis was radiation necrosis. She was treated with steroids. Neurosurgeons should be aware of the difficulty in differentiating tumor recurrence from radiation necrosis even with Tl-SPECT and Met-PET, and the importance of obtaining a histological diagnosis for radiation necrosis.