18F-Fluorodeoxyglucose-Positron Emission Tomography/Computed Tomography (FDG PET/CT) in the Characterization of Massive Cardiac Infiltration by Peripheral T-cell Lymphoma, Not Otherwise Specified (PTCL-NOS).

Peripheral T-cell lymphoma, not otherwise specified, is an aggressive and heterogeneous neoplasm with high metabolic activity and potential for extranodal infiltration. Cardiac involvement is exceptional and may present with nonspecific symptoms or abnormalities of the conduction system. We present the case of a 41-year-old patient with constitutional symptoms and complete atrioventricular block, in whom cardiac magnetic resonance imaging identified an infiltrative mass involving the right atrium and adjacent myocardial structures. 18F-Fluorodeoxyglucose-Positron Emission Tomography/Computed Tomography (FDG PET/CT) demonstrated an extensive hypermetabolic mediastinal lesion, with superior vena cava compression and cardiac infiltration, allowing more accurate delineation of the metabolic tumor burden and guidance of biopsy toward the most viable tissue. Histopathologic and immunohistochemical analysis confirmed peripheral T-cell non-Hodgkin lymphoma, not otherwise specified. This case emphasizes the utility of multimodality imaging and, particularly, 18F-FDG PET/CT for characterizing tumor extent, differentiating viable tissue from inflammatory or necrotic changes, and optimizing the diagnostic strategy in aggressive lymphomas with cardiac involvement.