Absolute Lymphocyte Count After COVID-19 Vaccination Is Associated with Vaccine-Induced Hypermetabolic Lymph Nodes on (18)F-FDG PET/CT: A Focus in Breast Cancer Care

We aimed to predict the presence of vaccine-induced hypermetabolic lymph nodes (v-HLNs) on (18)F-FDG PET/CT after coronavirus disease 2019 (COVID-19) vaccination and determine their association with lymphocyte counts. Methods: In this retrospective single-center study, we included consecutive patients who underwent (18)F-FDG PET/CT imaging after messenger RNA– or viral vector–based COVID-19 vaccination between early March and late April 2021. Demographics, clinical parameters, and absolute lymphocyte count (ALC) were collected, and their association with the presence of v-HLNs in the draining territory was studied by logistic regression. Results: In total, 260 patients were eligible, including 209 (80%) women and 145 (56%) with breast cancer. The median age was 50 y (range, 23–96 y). The messenger RNA vaccine had been given to 233 (90%). Ninety (35%) patients had v-HLNs, with a median SUV(max) of 3.7 (range, 2.0–26.3), and 74 (44%) displayed lymphopenia, with a median ALC of 1.4 × 10(9)/L (range, 0.3–18.3 × 10(9)/L). An age of no more than 50 y (odds ratio [OR], 2.2; 95% CI, 1.0–4.5), the absence of lymphopenia (OR, 2.2; 95% CI, 1.1–4.3), and less than a 30-d interval from the last vaccine injection to the (18)F-FDG PET/CT (OR, 2.6; 95% CI, 1.3–5.6) were independent factors for v-HLNs on multivariate analysis. In breast cancer patients, the absence of lymphopenia was the only independent factor significantly associated with v-HLNs (OR, 2.9; 95% CI, 1.2–7.4). Conclusion: Patients with a normal ALC after COVID-19 vaccination were more likely to have v-HLNs on (18)F-FDG PET/CT, both of which might be associated with a stronger immune response to vaccination.