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

Vaccine-associated lymphadenopathy (VAL) is a common finding on (18)F-FDG PET/CT examinations after coronavirus disease 2019 (COVID-19) vaccination. However, data regarding VAL on (18)F-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 (18)F-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 (18)F-rhPSMA-7.3 PET was 45%. Up to a period of 8 wk after the last COVID-19 vaccination, SUVratio was positive (2.05 ± 0.17). Thereafter, SUVratio dropped significantly (1.35 ± 0.09) and approached the value of unvaccinated group (1.1 ± 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 = 0.93, P < 0.0001) and lymph node size and duration after vaccine (r = −0.88, P < 0.0008) was found. Conclusion: Increased uptake of the PSMA ligand (18)F-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 (18)F-rhPSMA-7.3 PET/CT examinations.