Subdiaphragmatic Lymph Nodes Uptake on 18F-FDG PET/CT After COVID-19 Vaccination in the Thigh.

ABSTRACT: Since worldwide COVID-19 vaccination, 18F-FDG uptake in reactive axillary lymph nodes has been frequently observed in PET/CT studies. We describe a patient with breast cancer who underwent 18F-FDG PET/CT 7 days after receiving COVID-19 vaccination in the right thigh. 18F-FDG uptake was observed in nonenlarged right-sided inguinal, iliac, and para-aortic lymph nodes. As the thigh can be used as an alternate site for COVID-19 vaccine injection in case of lymphedema in both arms or for adequate axillary staging in patients with breast cancer, physicians should be aware of such 18F-FDG uptake pattern.

Since the launch of large-scale COVID-19 vaccination, 18F-FDG uptake in reactive axillary lymph nodes (LNs) is frequently observed in PET/CT studies.[1-7] A 74-year-old woman underwent 18F-FDG PET/CT for staging of left breast cancer 2 weeks after receiving the first dose of the Oxford-AstraZeneca COVID-19 vaccination in the right arm. 18F-FDG PET/CT showed high uptake in the primary tumor (A, black arrow) without signs of LN or distant metastasis and moderate LN uptake in the right axilla related to vaccination (A, red arrow). After 3 cycles of neoadjuvant chemotherapy and 7 days after repeat Pfizer-BioNTech COVID-19 vaccine in the right thigh, she underwent 18F-FDG-PET/CT for response evaluation. Complete metabolic response was observed for the primary tumor, but 18F-FDG uptake was observed in nonenlarged right-sided inguinal, iliac, and para-aortic LNs related to the vaccination in the thigh (B–E, red arrows). In patients who previously received COVID-19 vaccine in the deltoid muscle, PET/CT studies can show 18F-FDG–positive LNs in the ipsilateral axilla that should not be mistaken for cancer, highlighting the need for the nuclear medicine physician to check timing and side of prior vaccination. The thigh can be used as an alternate injection site, for example in case of lymphedema in both arms.[8] COVID-19 vaccination in the thigh might also be an option for adequate axillary staging in patients with breast cancer.[9,10] Physicians should be aware of the previously described 18F-FDG uptake pattern in the subdiaphragmatic LN chains that vaccination in the thigh may provoke.