Fluorodeoxyglucose (FDG)-Avid Pericardial Effusion With Reactive Mesothelial Proliferation Mimicking Malignancy.

Fluorodeoxyglucose (FDG)-avid pericardial effusion is frequently associated with malignancy, but inflammation and reactive mesothelial proliferation may produce similar imaging and histologic findings, creating diagnostic uncertainty. This overlap may lead to unnecessary invasive procedures if metabolic imaging is overemphasized. A 76-year-old man presented with progressive bilateral lower extremity edema and was found to have a large pericardial effusion. Imaging demonstrated FDG-avid pericardial and pleural thickening with hypermetabolic lymphadenopathy on PET-CT. Serial cytology, bronchoscopic biopsies, and immunohistochemistry did not demonstrate malignancy. Cardiac MRI showed pericardial inflammation without myocardial involvement. The patient was treated with pericardial drainage and corticosteroids, resulting in clinical improvement and resolution of the effusion. This case is unique because FDG-avid pleuropericardial abnormalities, hypermetabolic lymphadenopathy, and reactive mesothelial proliferation closely simulated malignant disease but ultimately represented a non-malignant inflammatory process. FDG uptake in pericardial effusion is not synonymous with malignancy, and PET-CT findings alone are insufficient for diagnosis. Integrating metabolic imaging with pathology and dedicated cardiac imaging can prevent misdiagnosis and avoid unnecessary invasive interventions, particularly in older or frail patients.