Abstract
Immune checkpoint inhibitors （ICI） are standard treatments for recurrent and metastatic head and neck cancers. They cause immune-related adverse events （irAE）, which differ from conventional chemotherapy side effects, and their management is important. We report a case of maxillary gingival squamous cell carcinoma that developed irAE encephalitis after initial pembrolizumab administration. An 84-year-old female with maxillary gingival squamous cell carcinoma （cT4aN1Mo） received radiotherapy （70Gy） and achieved a complete response; however, local recurrence occurred 6 months after radiation therapy. Pembrolizumab therapy was initiated as the patient refused surgery. Fourteen days after the first dose, she developed weakness and fatigue and was urgently admitted to the hospital. We suspected neurological irAE related to ICI and consulted a neurologist; myositis, myasthenia gravis, and Guillain–Barré syndrome were ruled out. On the 11th day after hospitalization, the patient developed an impaired consciousness. Cerebrospinal fluid tests were negative for bacteria and viruses but showed increased mononuclear cell and protein levels. Magnetic resonance imaging of the head revealed no abnormal findings that could explain the impaired consciousness. The patient was diagnosed with irAE encephalitis and was started on steroid pulse therapy. After steroid administration, the impaired consciousness due to encephalitis resolved, but the patient was unable to walk independently; she was transferred for rehabilitation and later recovered mobility. The tumor significantly shrank after one pembrolizumab dose and had disappeared at discharge. Although local recurrence occurred 1 year and 9 months after pembrolizumab administration, the patient had no subjective symptoms, such as pain, and received best supportive care before dying 5 months later.
