Carcinomatous leptomeningitis from bladder cancer mimicking immune-related cerebellitis: A case report.

We describe the case of a 65-year-old man with a medical history of smoking, hypertension, dyslipidaemia, gastroesophageal reflux disease, and anxiety-depressive disorder. The patient initially presented with dysuria and haematuria. Contrast-enhanced computerized tomography scan revealed a muscle-invasive bladder tumour and rectal wall thickening. Biopsies confirmed high-grade urothelial carcinoma (stage pT2) and rectal adenocarcinoma. The patient underwent radiotherapy and chemotherapy with gemcitabine plus cisplatin, which was later switched to carboplatin due to renal impairment. Chemotherapy was discontinued approximately 12 months after symptom onset, and immunotherapy with Avelumab was initiated. Four months following the start of immunotherapy, a fluorodeoxyglucose positron emission tomography-computed tomography scan showed no evidence of metastases. Shortly thereafter, he developed neck pain and postural instability. Brain magnetic resonance imaging scan revealed cerebellar oedema and leptomeningeal enhancement. Although neuroradiologists suggested carcinomatous leptomeningitis, oncological evaluation considered it unlikely. Alternative hypotheses included paraneoplastic cerebellitis and immunotherapy-induced leptomeningitis. Despite corticosteroid therapy, the patient's condition worsened. Cerebrospinal fluid analysis revealed markedly elevated protein levels and the presence of epithelial cells, confirming leptomeningeal metastatic spread of urothelial origin. Leptomeningeal carcinomatosis is a rare manifestation of bladder cancer and is difficult to diagnose due to non-specific symptoms and potential overlap with immunotherapy-related side effects. Neuroimaging and CSF cytology remain essential for diagnosis. There is currently no standardized treatment for leptomeningeal carcinomatosis from urothelial carcinoma; management is palliative. In our case, the patient died shortly after diagnosis. This case highlights the importance of considering this diagnosis even in rare tumour types, particularly in patients receiving immunotherapy.