Abstract
A 57-year-old man diagnosed with a metastatic bladder tumor was initiated on pembrolizumab treatment. (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography/computed tomography (PET/CT) imaging performed to evaluate treatment response showed numerical-dimensional and metabolic increase in the metastatic lesions. In the (18)F-FDG PET/CT imaging performed 8 weeks later due to suspicion of pseudoprogression, a significant regression of the lesions was observed, and the patient was diagnosed with pseudoprogression. Pseudoprogression should be kept in mind when (18)F-FDG PET/CT is performed after the use of immunotherapy, and evaluation with follow-up PET/CT is recommended to confirm that the patient has hyperprogression or pseudoprogression.