Abstract BACKGROUND Neurotoxicity is one of the serious and life-threatening complications of CAR-T treatment in patients with diffuse large B-cell lymphoma. Our objective is to identify the findings in brain PET/CT, their evolution over time, and their clinical correlation. We also aim to compare the sensitivity of PET/CT with magnetic resonance imaging (MRI). MATERIAL AND METHODS This prospective study includes 33 patients with diffuse large B-cell lymphoma treated with CAR-T therapy. Whole-body and brain [18F]FDG PET/CT scans were performed pre-treatment and 30 days post-therapy. In cases of suspected immune effector cell-associated neurotoxicity syndrome (ICANS), urgent structural imaging (CT or MRI) and [18F]FDG PET/CT brain scans were conducted, among other tests. RESULTS The mean age was 62±12 years, with a male-to-female ratio of 19:14. 70% of patients (23/33) developed grade 1, 2, or 3 cytokine release syndrome (CRS). 27% (9/33), all with prior CRS, showed symptoms of ICANS (3/9 G1, 3/9 G2 and 3/9 G3 ICANS). The most common symptoms were tremor, disorientation, and bradypsychia, starting on average on the 7th day post-infusion (range 1 to 10). Urgent [18F]FDG PET/CT brain scans were performed on 7 of the 9 ICANS patients, showing diffuse cortical hypometabolism compatible with encephalopathy in 5 (5/7) and no significant findings in 2/7. At the 30-day post-treatment evaluation, complete normalization was observed in 2/5 patients, improvement with persistent mild global cortical hypometabolism in 2/5, and one patient (1/5) died before the 30 days (with G3 ICANS and severe cortical and subcortical hypometabolism on PET imaging). Urgent brain MRI was performed on the 7 patients with ICANS and urgent [18F]FDG PET brain scans, and it was normal in 5/7 and pathological in 2/7 (both with grade 3 ICANS). CONCLUSION [18F]FDG PET/CT brain scan is useful for the diagnosis and follow-up of patients with CAR-T related neurotoxicity (ICANS) and shows greater sensitivity than MRI. Future studies should determine the relationship between acute phase PET findings and long-term prognosis.