Stage IV small cell lung cancer: complete tumor response with recovery from paraneoplastic limbic encephalitis

What happened, in summary

A 67-year-old man with hypertension, ischemic heart disease, chronic stage IIIA renal insufficiency, and heavy active smoking was hospitalized in November 2024 with severe symptomatic hyponatremia caused by syndrome of inappropriate antidiuretic hormone secretion. Whole-body CT and FDG PET found a left lung hilar tumor with metastases to the adrenal glands, diaphragmatic lymph nodes, lungs, and peritoneum. Bronchoscopy showed tumor involvement of the tracheal and bronchial wall. Histology confirmed small cell lung cancer, with tumor cells positive for TTF-1 and chromogranin-A and a high Ki-67 of 90%. In December 2024, after his first cycle of carboplatin, etoposide, and atezolizumab, he developed worsening short-term memory loss, apraxia, a right mesial temporal lobe seizure, tremor, aggression, confusion, and worsening amnesia. Brain MRI showed bilateral hippocampal changes consistent with limbic encephalitis. CSF and serum were positive for GABA-B receptor IgG, CSF IL-6 was elevated, and no tumor cells were found in CSF. Atezolizumab was stopped, and carboplatin plus etoposide was continued alone for a total of 6 cycles. Seizure activity resolved with lacosamide. Neurologic treatment also included high-dose corticosteroids, IVIG, plasmapheresis stopped after 2 sessions, rituximab, and then tocilizumab every 4 weeks. PET after 4 chemotherapy cycles showed complete tumor response. He later had full neurologic recovery, remained clinically well, and April 2025 brain MRI showed improvement of limbic encephalitis changes with progression toward hippocampal atrophy. The neurologic syndrome was treated alongside the cancer, not as a separate cancer spread to the brain; CSF testing found no tumor cells, and the pattern supported an antibody-mediated paraneoplastic process.