Generalized Lymph Node Activation after Influenza Vaccination on 18F FDG-PET/CT Imaging, an Important Pitfall in PET Interpretation

We report on a 59-year-old female patient with an infected vascular graft investigated with 18F FDG-PET/CT. The first of two studies showed FDG activity in the left deltoid and ipsilateral axillary lymph nodes explained by influenza vaccination the day prior. The second 18F FDG-PET/CT showed multiple FDG-avid lymph nodes on both sides of the diaphragm without tracer accumulation at the vaccination site. Three months later the CT was negative for lymphadenopathy within the chest or abdominal region. Although influenza vaccination is a potential source of false positive results in FDG PET studies, generalised lymph node activation post vaccination is a rare finding with only one prior published report in individuals infected with HIV-1. This case emphasizes the necessity of taking a history of vaccination prior to a FDG PET study, and consideration of a vaccine-related immune response even without evidence of tracer activity at the vaccination site when generalised FDG-avid lymphadenopathy is encountered.