We report a case of subacute cerebellar infarction mimicking metastasis on prostate-specific membrane antigen (PSMA) PET/CT. A 77-year-old man with prostate cancer treated with androgen deprivation therapy and radiotherapy with rising prostate-specific antigen was referred for Ga-PSMA PET/CT. Apart from PSMA-expressing tumor of the left prostate, PSMA PET/CT demonstrated radiotracer uptake in the right cerebellar hemisphere, corresponding to a site of subacute infarction demonstrated on an MRI of the brain performed 35 days previously. As cerebellar infarcts are considerably less common than cerebral infarcts, they may not be anticipated as a potential cause for false-positive radiotracer uptake on PSMA PET.

Introduction: This study aims to evaluate discrepant findings between positron emission tomography/magnetic resonance imaging (PET/MRI) and positron emission tomography/computed tomography (PET/CT) in a cohort of oncological patients and to undertake a phantom study to assess the potential for extended PET acquisitions to lead to false-positive findings on PET/MRI. Methods: Discrepant findings from a series of 106 patients undergoing sameday 18 F-fluorodeoxyglucose (FDG)-PET/CT and PET/MRI were reviewed. Phantom studies explored the potential for PET acquisition time to contribute to discrepancy.Results: There were 14 discrepant cases, 5 (35.7%) of which related to PET/ MRI acquisitions that had been extended to 10 min. Three of these five cases proved to be falsely positive. Phantom studies showed greater contrast recovery and signal to noise ratio for 10-min PET/MRI acquisitions compared to 2min acquisitions using PET/CT. There were no discrepancies when PET/CT showed disseminated disease (P = 0.036). Conclusions: Extended PET/MRI acquisitions used to accommodate multiple MRI sequences may be associated with false-positive findings compared to PET/CT. PET/MRI is more likely to have incremental value when the prior probability for disseminated disease is low.