Diagnosing paraneoplastic autoimmune encephalitis using TSPO PET: a case with an incidental parotid tumor

This case highlights the utility of translocator protein (TSPO, 18 kDa) PET imaging for differentiating neuro-inflammatory lesions in autoimmune encephalitis from brain metastasis, with an incidental finding of significant TSPO expression in a parotid tumor. A 71-year-old male patient presented with neurological symptoms and was evaluated with 18 F-fluorodeoxyglucose ( 18 F-FDG) positron emission tomography/magnetic resonance (PET/MR) and 18 F‑N‑fluoroacetyl‑N‑(2,5‑dimethoxybenzyl)‑2‑phenoxyaniline ( 18 F-PBR06) positron emission tomography/computed tomography (PET/CT) for suspected autoimmune encephalitis. Both imaging modalities demonstrated marked uptake in the brain and right parotid gland. However, only 18 F-FDG PET revealed evidence of hypermetabolism in the lung lesion and lymph nodes. The diagnosis of anti-Hu-associated autoimmune encephalitis was confirmed on the basis of cerebrospinal fluid findings and imaging results. Following treatment with antiepileptic agents and immunotherapy (intravenous immunoglobulin and corticosteroids), the patient's symptoms improved. Subsequent diagnostic procedures included a lung biopsy, which confirmed the presence of small cell lung cancer (SCLC) and an enhanced brain MRI, which revealed no evidence of metastasis, thereby further supporting the diagnosis of encephalitis. Additionally, the right parotid tumor exhibited clinical characteristics consistent with Warthin tumor, as evidenced by its stable follow-up.