FDG PET/CT Scan after Vaccination in Times of Pandemic.

We found the images published by Eifer & Eshet (1) of a 72-year-old woman with breast cancer treated with surgery and complementary treatment of great interest. Ten days before the follow-up FDG-PET / CT scan, she was vaccinated against SARS-CoV-2 in her right shoulder; the study showed new abnormal uptake images in the right deltoid and two axillary lymph nodes on the same side, of average size.

These abnormal uptakes in the FDG-PET / CT scan after vaccination have been described since 2006, initially by Williams et al. (2) in a 59-year-old man with large B-cell lymphoma in remission, which showed an increase in uptake in a left axillary lymph node; the patient had recently been immunized, the time elapsed or the type of vaccine was not specified. Similar findings have been described in patients vaccinated against influenza (3), including against H1N1 (4). This false positive is frequent and was reported in 29.3% of patients with different malignant tumors who were recently vaccinated against influenza (4). Also, abnormal uptake of the spleen due to influenza vaccine was recently reported (5).

Abnormal lymph node uptake is likely consistent with kinetics and lymphocyte proliferation after vaccination (3).

The keys to interpreting increased uptake of the axillary lymph nodes as a false positive are as follows: increased activity of 18F-fluorodeoxyglucose (FDG) in the ipsilateral deltoid muscle, the affected nodes are normal in size, and the antecedent of recent vaccination (14 days earlier) (3,4). The standard uptake value (Standardized Uptake Value) of the axillary nodes reported near the vaccination site is 1.3 to 3.4 (3).

Global vaccination against SARS-CoV 2 will make the appearance of this false positive frequent and the medical community must take this finding into account (1).