Vaccine-associated hypermetabolic lymphadenopathy on 18F-FDG PET/CT: Experience from a single center in Mexico

Introduction: Vaccination against COVID-19 started in Mexico in late 2020. Soon we identified vaccine-associated hypermetabolic lymphadenopathy (VAHL) on ¹⁸F-FDG PET-CT scans in axillary or supraclavicular lymph nodes ipsilateral to the vaccination site. Incidence may vary according to vaccination rates among different countries or regions. Differentiation between malignant and benign nature lymphadenopathy could be a diagnostic challenge in certain tumors. The purpose of this study is to determine the overall incidence of VAHL after vaccination in a center from a Latin American nation and to assess its relevance in oncological PET-CT interpretation.

Methods: From February to July 2021, a total of 1483 consecutive patients who underwent ¹⁸F-FDG PET/CT in our center were interviewed regarding the type of vaccine, vaccination site, and date of vaccination. A total of 127 vaccinated patients have been included: 62 patients received mRNA vaccines, 49 patients viral vector vaccines, and 16 patients live attenuated vaccines; subunit vaccines were not presented in our population. Cumulative incidence analysis was calculated. The population was categorized according to the type of COVID-19 vaccine, first and second dose, and by post-vaccination days as follows ≤7, 8-14, 15-21, 22-31. In all PET/CT studies, the location, intensity of uptake, and node size were recorded.

Results: We found a cumulative incidence of 8.56% of vaccine-associated hypermetabolic lymphadenopathy. A statistical significance differenced was found in SUVmax according to post-vaccination days for mRNA (p: <0.0001), viral vector (p: <0.0001), and live attenuated vaccines (p: < 0.0002), lymph node short-axis ranges from 6-11 mm (mean 9 mm) and axillary lymph nodes (67%) were more frequently observed than axillar and supraclavicular nodes together (33%). The first doses were associated with higher and longer SUVmax uptake (p: 0.0024). No statistical differences were documented between SUVmax uptake regarding the type of COVID vaccine.

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Conclusions: VAHL is frequent after the COVID-19 vaccine, regardless of the type of vaccine administered. In our population, we documented 8.56% among all PET/CT studies. We recommend an incidence analysis of VAHL by nation or center since vaccination rates may vary per country or region. According to our data, the fourth week of vaccination status was associated with the lowest SUVmax uptake. The timing ¹⁸F-FDG PET-CT scan should be considered on diagnosis and the time intervals from vaccination. This practice could minimize false and equivocal reports in oncological patients.

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