Axillary lymph node activation following Influenza vaccination on F-18 FDG PET/CT

Introduction: Influenza vaccination may cause transient lymph nodes activation in the draining axilla and cause false positive findings on F-18 FDG PET/CT imaging. This study investigated the influence of influenza vaccination on FDG PET/CT imaging in oncology patients.

Methods: Consecutive 485 adult patients with malignancy referred for whole body FDG PET/CT imaging between 10/1/2019 and 12/6/2019 were surveyed for recent influenza vaccination history, including date of vaccination and site of administration. Patients were categorized based on interval between vaccination and PET/CT imaging. PET/CT images were analyzed for number and uptake (SUVmax) of visualized axillary lymph nodes, location (right, left or none in deltoid muscle) and uptake (SUV max) of vaccination site.

Results: Of 485 patients, 179 (37%; mean ± SD age 66 ± 15 years, M:F:others = 90:88:1) responded they had received influenza vaccination recently. Of these, 27 patients (15%) had it within 1 week (group I), 30 (17%) within 2 weeks (group II), 19 (11%) within 3 weeks (group III), 73 (40%) within 4 weeks (group IV), and 30 (17%) > 4 weeks (group V) prior to PET/CT imaging. Axillary lymph nodes ipsilateral to the vaccination site were visualized in 48% (13/27 patients) in group I, 18% (5/28) in group II, 6% (1/18) in group III, 3% (2/73) in group IV and 10% (3/30) in group V, respectively, with no significant enlargement on CT. Three patients were excluded from the analysis due to confounding metastatic or immunotherapy-related axillary lymphadenopathy. Average number of positive ipsilateral nodes per patient was 9 in group I, 3 in group II, 5 in group III, 9 in group IV, and 2 in group V, respectively. SUVmax of lymph nodes were 3.0 ± 1.4 (mean ± SD) in group I, 2.2 ± 0.7 in group II, 3.5 in group III, 2.6 ± 0.3 in group IV, and 2.3 ± 0.4 in group V, respectively. Among patients with positive axillary nodes, vaccination site was seen at ipsilateral deltoid muscle in all except 1 in group I (92%; SUVmax 2.5 ± 1.8), while only 1 in group II (20%; SUVmax 1.2) and none in group III, IV and V. Some patients showed uptake at the vaccination site but not in axillary nodes: group I (50% (7/14); SUVmax 1.5 ± 0.6) and group II (8% (2/25); 1.6 ± 0.4). Conclusion: Axillary lymph nodes demonstrated increased FDG uptake in 50% of patients who received influenza vaccination within 1 week prior to imaging. The rate and number of lymph nodes visualization decreased with longer interval between vaccination and PET/CT imaging. Influenza vaccination can cause false positive uptake in axillary lymph nodes in oncology patients. It is useful to take recent vaccination history to avoid the assumption that they represent metastatic disease. Acknowledgements: Special thanks to our staff at the University of Iowa Hospitals and Clinics PET center for their ongoing excellent work and support.