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Objective:
Unilateral axillary lymphadenopathy has been known to occur after COVID-19 vaccination. Such post-vaccination lymphadenopathy may mimic metastatic lymph nodes of breast cancer, and it is a challenging task to distinguish them. In our past experience, we had an impression that post-vaccination lymphadenopathy tends to occur at a deeper axillary level than metastatic lymph nodes of breast cancer. No previous reports are available in this matter. Therefore, we investigated if the localization of axillary lymphadenopathy on MRI can help distinguish between reactive and metastatic nodes.

Materials and Methods:
The study looked at preoperative MRI images of patients who underwent breast cancer surgery from June 2021 to October 2021. The metastatic lymphadenopathy group was made up of 45 patients, excluding patients with no lymph node metastasis or MRI images. The post-vaccination lymphadenopathy group ended with 37 patients, excluding patients with lymph node metastasis and patients with no MRI images. We defined lymphadenopathy as that with a cortical thickening or short axis greater than 5 mm. The axilla was divided into the ventral and dorsal parts on the axial plane using a horizontal line along the most anterior margin of the muscle group including the deltoid, latissimus dorsi or teres major muscles, in relative to the line along the lateral chest wall. The presence or absence of axillary lymphadenopathy for each area, the number of all visible lymph nodes, the long and short diameters of the lymph nodes, and the presence of fatty hilum and its configuration were recorded.

Results:
The number of visible axillary lymph nodes was significantly higher in the vaccination group (median [range], 15 nodes [3-36]) than in the metastasis group (6 nodes [1-18]) (P<0.001). As for the localization of enlarged lymph nodes, dorsal lymphadenopathy was observed in 13 patients (35.1%) in the vaccination group and only 2 patients (4.4%) in the metastasis group, a significant difference between the two groups (P<0.001). The sensitivity, specificity, positive predictive value, and negative predictive value of vaccination-related dorsal lymphadenopathy was 35.1%, 95.6%, 86.7%, and 64.2%, respectively.

Conclusion:
Dorsal lymphadenopathy occurs significantly in the vaccine group compared to the metastasis group. The presence of deep axillary lymphadenopathy may be an important point to distinguish between lymphadenopathy after COVID-19 vaccination and metastasis. The number of axillary lymph nodes may also be a clue.