What happened, in summary

A 57-year-old woman came to the neurology emergency department with 1 month of progressive limb ataxia that severely limited her mobility, along with a 7 kg weight loss over 2 months. Her history included hypertension, Graves disease, vitiligo, prior hysterectomy with bilateral salpingo-oophorectomy, and active smoking. Brain MRI showed no invasive lesion. CT of the chest, abdomen, and pelvis showed multiple solid lesions in the right lung with mediastinal and hilar lymphadenopathy. Endobronchial ultrasound-guided biopsy of a right hilar lymph node showed small-cell lung cancer: the cells were TTF-1-positive and synaptophysin-positive, with a Ki-67 index of about 85%. The diagnosis was limited-stage small-cell lung cancer. Her disabling gait and coordination problems were attributed to paraneoplastic cerebellar degeneration, an immune-mediated neurological syndrome associated with cancer. Serum testing was strongly positive for anti-CV2/CRMP5 and anti-SOX1 onconeural antibodies, while CSF showed lymphocytic pleocytosis and elevated protein without malignant cells. She began chemotherapy with carboplatin AUC 5 plus etoposide 100 mg/m² every 21 days. Immunotherapy was not approved for first-line small-cell lung cancer at that time. After the second chemotherapy cycle, her balance and gait began to improve, and she completed 6 cycles without major complications. Restaging CT showed complete response in the right-lung lesions. She then received consolidative thoracic radiation, 30 Gy total, and prophylactic cranial irradiation. Post-radiotherapy PET/CT showed no progression or recurrence. At 1 year, her performance status had improved to ECOG 1. At 6 years after diagnosis, she remained in remission with no relapse on CT scans, ECOG 0, and independent walking, despite persistent anti-CV2/CRMP5 antibody positivity.