With hundreds of millions of coronavirus disease 2019 (COVID-19) mRNA-based vaccine doses planned to be delivered worldwide in the upcoming months, it is important to recognize positron emission tomography with computed tomography (PET/CT) findings in recently vaccinated immunocompetent or immunocompromised patients. We aimed to assess PET/CT uptake in the deltoid muscle and axillary lymph nodes of patients that received a COVID-19 mRNA-based vaccine, and to evaluate its association with patients’ age and immune status. All consecutive adult subjects undergoing PET/CT scans with any radiotracer at our center during the first month of a national COVID-19 vaccination rollout (between 23 December 2020 and January 27, 2021) were included. Data regarding clinical status, laterality and time interval from recent COVID-19 mRNA vaccination was prospectively collected and retrospectively analyzed, and correlated with deltoid muscle and axillary lymph nodes uptake. Of 426 eligible, recently vaccinated, subjects (median age, 67±12 years; 49% female), 377 (88%) underwent PET/CT with F-18-fluorodeoxyglucose (FDG) and positive axillary lymph node uptake was seen in 45% of them. Multivariate logistic regression analysis revealed a strong inverse association between positive FDG uptake in ipsilateral lymph nodes and patients’ age (Odds Ratio [OR]=0.57, 95% CI, 0.45-0.72; p<.001), immunosuppressive treatment (OR=0.37, 95% CI, 0.20-0.64; = 0.003) and presence of hematological disease (OR=0.44, 95% CI, 0.24-0.8; = 0.021). No such association was found for deltoid muscle uptake. The number of days from the last vaccination and the number of vaccination doses were also significantly associated with increased odds of positive lymph nodes uptake. Following mRNA-based COVID-19 vaccination, a high proportion of patients showed ipsilateral lymph node axillary uptake, which was more common in immunocompetent patients. This information will help recognize PET/CT pitfalls and may hint about the patient's immune response to the vaccine.Copyright © 2021 by the Society of Nuclear Medicine and Molecular Imaging, Inc.
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