A team at Stanford University in Stanford, CA, evaluated F-18 FDG-PET/MRI scans of six children and young adults before and after COVID-19 vaccination. The scans demonstrated thymus activation in addition to local lymph node reactions, the group found.

"Understanding postvaccination imaging findings in patients with cancer is important because these results may be confounded with tumor relapse or metastasis," wrote first author Gaurav Luthria, MD, PhD, and colleagues.

In patients with cancer, an increase in the size and metabolic activity of lymph nodes can indicate tumor recurrence or progression and play an important role in guiding treatment decisions, the author explained.

Several studies have demonstrated that vaccine-related lymphadenopathy may confound disease assessment in oncology patients, the researchers added. In this study, they hypothesized that F-18 FDG-PET/MRI may reveal activation in the thymus in pediatric oncology patients in addition to lymph node reactions.

The group assessed scans of six children with extrathoracic cancer before and after COVID-19 vaccination. They analyzed both PET and MRI biomarkers, such as uptake of F-18 FDG radiotracer (maximum standard uptake values, or SUVmax), mean apparent diffusion coefficient on MRI, and size of the thymus and axillary lymph nodes.

There was a significant increase in the metabolic activity in the ipsilateral axillary lymph nodes across all patients after COVID-19 vaccination, according to the findings. The mean SUVmax increased by a factor of three, from 0.87 before vaccination to 2.61 after vaccination (p = 0.03).