What happened, in summary

This 34-year-old woman was admitted in April 2024 after a low-grade fever and 2 epileptic seizures within 10 hours. Examination showed neck stiffness and a right Babinski sign, but brain MRI and EEG were normal. CSF testing did not show infection. Specialized immune testing was positive for KLHL11 IgG antibodies in both serum and CSF, supporting KLHL11 encephalitis. Standard serum tumor markers and several other paraneoplastic markers were negative. During the same evaluation, breast ultrasound found a hypoechoic nodule in the left breast, mammography showed a calcified lesion, and FDG-PET/CT showed a hypermetabolic focus in the left upper outer breast. A small right-lung nodule was also seen on chest CT but was considered low risk at first. She started levetiracetam and efgartigimod for the neurologic syndrome. She declined glucocorticoids and plasmapheresis because of concern about side effects. After discharge, fever, dizziness, and gait instability developed, but these improved with weekly efgartigimod; after 4 weekly infusions, her ataxia score improved to 0. Fine-needle aspiration of the breast showed focal atypical ductal hyperplasia, so she underwent left partial mastectomy on May 29. Final pathology showed moderate-grade ductal carcinoma in situ that was ER-positive, PR-positive, HER2-positive, and Ki-67 15%. She then received IVIG to reduce relapse risk for autoimmune encephalitis, 20 fractions of left-breast radiotherapy, and tamoxifen. She remained asymptomatic on levetiracetam and tamoxifen, without recurrent fever, seizures, or ataxia. In January 2025, the right-lung nodule had enlarged, and thoracoscopic partial lung resection showed pulmonary adenocarcinoma in situ. This added a second early, locally treated cancer diagnosis.