ISRA May 2022

Prevalence of Increased FDG PET/CT Axillary Lymph Node Uptake Beyond 6 Weeks after mRNA COVID-19 Vaccination

Yael Eshet 1,2 Michal Eifer 1,2 Noam Tau 1,2 Yousef Alhoubani 1 Nayroz Kanana 1,2 Liran Domachevsky 1,2
1Department of Diagnostic Imaging, Chaim Sheba Medical Center, Israel
2Sackler School of Medicine, Tel Aviv University, Israel

Introduction: As avid axillary lymphadenopathy can be seen on FDG PET/CT in association with recent COVID 19 mRNA vaccination, Recommendations were published, advising scheduling imaging either before or at least 6 weeks after the final vaccination dose to eliminate false-positive results for cancer, based on data extracted from influenza vaccine, showing normalization of FDG uptake within 40 days.

We aimed to assess the prevalence of FDG PET/CT–avid axillary lymph nodes beyond 6 weeks after the second dose of mRNA-based BNT162b2 COVID-19 vaccine.

Materials and Methods: Consecutive adult patients referred for FDG PET/CT at our institution, who underwent the examination at least 42 days after the second dose of Pfizer-BioNTech (BNT162b2) COVID-19 vaccine, were evaluated for the presence of FDG-avid axillary lymphadenopathy ipsilateral to the injection site.

Results: Among 169 eligible subjects (median age, 65 years 6±14; 49% women), total of 29% (49 of 169) had positive axillary uptake 7–10 weeks after second vaccination, divided to 42%, 31%, 25%, and 19% on 7th, 8th, 9th, and 10th weeks, respectively.

Discussion: This study shows that avid axillary lymph node uptake at FDG PET/CT can be detected in more than a quarter of our patient population even beyond 6 weeks after the second dose of the mRNA-based COVID-19 vaccination. Compared with a previous study showing normalization of FDG uptake within 40 days of receiving an inactivated H1N1 influenza vaccine, we found uptake persistence even at 70 days. Physicians should be aware of this potential pitfall.