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

Rationale: We aimed to predict the presence of vaccine-induced hypermetabolic lymph nodes (v-HLN) on 18F-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 [18F]-FDG PET/CT imaging after mRNA- or viral vector-based COVID-19 vaccination between early March and late April 2021. Demographic, clinical parameters and absolute lymphocyte count (ALC) were collected and their association with the presence of v-HLN in the draining territory was studied by logistic regression. Results: Two hundred and sixty patients were eligible, including 209 (80%) women and 145 (56%) with breast cancer. The median age was 50 years (range, 23-96). Two hundred thirty-three patients (90%) received the mRNA vaccine. Ninety (35%) patients had v-HLN with a median SUVmax of 3.7 [range, 2.0-26.3] and 74 (44%) displayed lymphopenia with a median ALC of 1.4 G/L [range, 0.3-18.3]. Age ≤ 50 years (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 the delay from the last vaccine injection to the date of [18F]-FDG PET/CT, if < 30 days (OR 2.6, 95%CI 1.3-5.6), were independent factors for v-HLN in multivariate analysis. In breast cancer patients, the absence of lymphopenia was the only independent factor significantly associated with v-HLN (OR 2.9, 95%CI 1.2-7.4). Conclusion: Patients with normal values of ALC after COVID-19 vaccine were more likely to have v-HLN on [18F]-FDG PET/CT, which might both be associated to a stronger immune response to vaccination.