Rasmussen encephalitis without atrophy and increased ¹¹C-methionine PET uptake mimicking glioma: A case report

Diagnosing Rasmussen encephalitis presents a significant challenge when cortical atrophy is absent on MRI. We present a case of Rasmussen encephalitis in a 13-year-old boy with lack of atrophy and imaging features mimicking glioma, including increased ¹¹C-methionine (MET)-PET uptake, which led to diagnostic uncertainty. The patient had been previously diagnosed with chronic autoimmune encephalitis and epilepsy for about 6 years prior. Initial MRI revealed an FLAIR hyperintensity in the left frontal lobe and basal ganglia region without cortical atrophy, which remained stable for several years. However, 1 year after the last follow-up, the lesion demonstrated progressive enlargement. MRS demonstrated an increased choline/NAA ratio, increased CBF on ASL, and increased accumulation on MET-PET primarily in the gray matter adjacent to the FLAIR hyperintense lesion. Because these imaging characteristics could not rule out the possibility of glioma, we decided to perform open biopsy via a small craniotomy to establish definitive diagnosis while minimizing the burden on the patient. The pathological examination revealed cortical thickening, increased astrogliosis, and CD3+ and CD8+ T-cells infiltration. A final diagnosis of Rasmussen encephalitis was established, with follow-up MRI demonstrating a gradual reduction in the lesion signal intensity. The current case highlights the difficulties of distinguishing Rasmussen encephalitis from glioma in the absence of cortical atrophy, as they share similar MRI features and MET-PET uptake patterns. In such cases, a minimally invasive biopsy is recommended to establish a definitive diagnosis and reduce patient burden.