Temporary Reactive Response of Axillary Lymph Nodes to COVID-19 Vaccination on ¹⁸F-rhPSMA-7.3 PET/CT in Patients with Prostate Cancer

Vaccine-associated lymphadenopathy (VAL) is a common finding on ¹⁸F-FDG PET/CT examinations after coronavirus disease 2019 (COVID-19) vaccination. However, data regarding VAL on ¹⁸F-rhPSMA-7.3-ligand PET are currently lacking. This study assesses the prevalence, temporal response to vaccination, and characteristics of VAL. Methods: Two hundred thirty-three consecutive vaccinated and 41 unvaccinated patients with confirmed prostate cancer who underwent ¹⁸F-rhPSMA-7.3 PET/CT were retrospectively analyzed. Size and uptake of the axillary lymph nodes were measured. Ratios of SUV_max of ipsilateral to contralateral axillary lymph node (SUVratio) were determined. The characteristics of SUVratio in respect to the duration of PSMA avidity in the axillary lymph node after COVID-19 vaccination was analyzed. Results: The prevalence of VAL on ¹⁸F-rhPSMA-7.3 PET was 45%. Up to a period of 8 wk after the last COVID-19 vaccination, SUVratio was positive (2.05 6 0.17). Thereafter, SUVratio dropped significantly (1.35 6 0.09) and approached the value of unvaccinated group (1.1 6 0.2). SUVratio of metastatic axillary lymph nodes was very high (.11) and can be easily detected visually or semiquantitatively. In 3 patients, we observed suspected development and consecutively confirmed involving metastasis of axillary lymph node with SUVratio between 4.0 to 6.6. Correlation between SUVratio and lymph node size (r 5 0.93, P, 0.0001) and lymph node size and duration after vaccine (r 5 20.88, P, 0.0008) was found. Conclusion: Increased uptake of the PSMA ligand ¹⁸F-rhPSMA-7.3 by axillary lymph nodes is common after COVID-19 vaccination and can persist for 8 wk. This finding should be considered in the interpretation of ¹⁸F-rhPSMA-7.3 PET/CT examinations.