Axillary
lymphadenopathy caused by the high immunogenicity of
messenger RNA (
mRNA)
COVID-19 vaccines presents radiologists with new diagnostic dilemmas in differentiating
vaccine-related benign reactive
lymphadenopathy from that due to malignant causes. Understanding axillary anatomy and lymphatic drainage is key to radiologic evaluation of the axilla. US plays a critical role in evaluation and classification of axillary lymph nodes on the basis of their cortical and hilar morphology, which allows prediction of metastatic disease. Guidelines for evaluation and management of axillary
lymphadenopathy continue to evolve as radiologists gain more experience with axillary
lymphadenopathy related to
COVID-19 vaccines. General guidelines recommend documenting vaccination dates and laterality and administering all
vaccine doses contralateral to the site of primary
malignancy whenever applicable. Guidelines also recommend against postponing imaging for urgent clinical indications or for treatment planning in patients with newly diagnosed
breast cancer. Although
conservative management approaches to axillary
lymphadenopathy initially recommended universal short-interval imaging follow-up, updates to those approaches as well as risk-stratified approaches recommend interpreting
lymphadenopathy in the context of both vaccination timing and the patient's overall risk of metastatic disease. Patients with active
breast cancer in the pretreatment or peritreatment phase should be evaluated with standard imaging protocols regardless of vaccination status. Tissue sampling and multidisciplinary discussion remain useful in management of complex cases, including increasing
lymphadenopathy at follow-up imaging, MRI evaluation of extent of disease, response to
neoadjuvant treatment, and potentially confounding cases. An invited commentary by Weinstein is available online. ©RSNA, 2022.