“Hepatic Hot Quadrate Sign” on 18F-FDG PET-CT Mimicking Hepatic Involvement by Primary Mediastinal B-Cell Lymphoma

We report the case of a 25-year-old woman with refractory primary mediastinal B-cell lymphoma (PMBCL), presenting with a new FDG-avid hepatic lesion on surveillance 18 F-FDG PET-CT following treatment with a bispecific T-cell engager (BiTE). Subsequent imaging, including ultrasound and MRI, failed to confirm a mass, suggesting a hepatic pseudolesion secondary to collateral circulation from superior vena cava (SVC) obstruction. A repeat PET-CT with radiotracer administration via a leg vein confirmed the absence of hepatic involvement. This case underscores the importance of recognizing pseudolesions as a PET-CT pitfall in SVC obstruction, preventing unnecessary interventions.