Beyond N Staging in Breast Cancer: Importance of MRI and Ultrasound-based Imaging

Simple Summary Breast cancer is the most frequent cancer affecting women and metastatic breast cancer is still the leading cause of death from all cancers in women, accounting for about 3.6% of all deaths in women. The N-stage represents the main prognostic factor affecting the rate of recurrence and the therapeutic management so that a correct staging of the axillary lymph node status is fundamental. Since clinical examination of the axillary cavity is associated with a high false negative rate, reaching values of 45%, the role of imaging becomes crucial to obtain an accurate assessment of loco-regional lymph nodes at the time of diagnosis. In this setting, Ultrasound and Magnetic Resonance Imaging (MRI) represent two important diagnostic tools. In particular, MRI represents an accurate and reproducible technique, which allows an accurate staging of the "N-stage". The correct N-staging in breast cancer is crucial to tailor treatment and stratify the prognosis. N-staging is based on the number and the localization of suspicious regional nodes on physical examination and/or imaging. Since clinical examination of the axillary cavity is associated with a high false negative rate, imaging modalities play a central role. In the presence of a T1 or T2 tumor and 0-2 suspicious nodes, on imaging at the axillary level I or II, a patient should undergo sentinel lymph node biopsy (SLNB), whereas in the presence of three or more suspicious nodes at the axillary level I or II confirmed by biopsy, they should undergo axillary lymph node dissection (ALND) or neoadjuvant chemotherapy according to a multidisciplinary approach, as well as in the case of internal mammary, supraclavicular, or level III axillary involved lymph nodes. In this scenario, radiological assessment of lymph nodes at the time of diagnosis must be accurate. False positives may preclude a sentinel lymph node in an otherwise eligible woman; in contrast, false negatives may lead to an unnecessary SLNB and the need for a second surgical procedure. In this review, we aim to describe the anatomy of the axilla and breast regional lymph node, and their diagnostic features to discriminate between normal and pathological nodes at Ultrasound (US) and Magnetic Resonance Imaging (MRI). Moreover, the technical aspects, the advantage and limitations of MRI versus US, and the possible future perspectives are also analyzed, through the analysis of the recent literature.