2-[¹⁸F]fluoro-2-deoxy-d-glucose (FDG) is known to accumulate in benign conditions such as infection and inflammation as well as in malignancy. Vaccination may cause transient inflammation of lymph nodes, which may induce false-positive findings on FDG-positron emission tomography (PET) imaging. This study investigated the influence of influenza vaccination on FDG-PET/CT imaging in normal subjects.

Between November 2008 and March 2009, a total of 172 examinees underwent FDG-PET/CT during an annual cancer-screening program at our hospital, 83 of whom had a history of recent non-adjuvanted seasonal influenza vaccination. They were asked the date and injection site of the vaccination. Examinees were divided into 2 groups based on the interval after vaccination using a cutoff value of 7 days (1 week). Two double board-certified nuclear medicine physicians and radiologists visually interpreted the FDG-PET/CT images with reference to PET/CT fusion and CT images and checked the location and the number of abnormal accumulations by consensus reading.

Intervals between vaccination and FDG-PET were less than 7 days in 5 examinees, and 7 days or more in 78 examinees. Unexpected accumulations were visualized in 4 examinees in the axilla and medial upper arm, and all of them belonged to the group who underwent vaccination less than 7 days previously. In the second group there was no abnormal FDG accumulation.

Recent influenza vaccination before FDG-PET/CT examination may cause ipsilateral axillary lymph node accumulations, especially within several days after vaccination. Questionnaires about vaccination can help to avoid false interpretation of FDG avid axillary lymph nodes.