Atypical Cortical Presentation of Anti-Ri Paraneoplastic Encephalitis Mimicking Tumor

Purpose Anti-Ri paraneoplastic neurological syndrome (PNS) typically manifests as brainstem or cerebellar dysfunction and is most commonly associated with breast cancer. Methods and Results We report the case of a 54-year-old woman who presented with transient executive dysfunction, confusion, and headache. Brain MRI revealed a tumor-like lesion in the right frontal cortex. Stereotactic brain biopsy showed inflammatory changes without evidence of malignancy. Serum testing was strongly positive for anti-Ri antibodies, and systemic workup identified right breast cancer with axillary lymph node metastasis. The patient received breast cancer treatment alongside high-dose steroid pulse therapy and four weekly doses of rituximab, resulting in marked resolution of the cortical lesion and neurological symptoms. Conclusion To our knowledge, this is the first reported case of anti-Ri PNS involving the frontal cortex. While current PNS-Care diagnostic criteria enabled a definitive diagnosis, further investigation is warranted to elucidate the underlying pathomechanism of cortical involvement in anti-Ri PNS.