The proportion of women who have received NACT for BC has noticeably increased in the last decades [1]. One of the main criteria for indication of NACT in early BC beside tumor biology characteristics is an involvement of the axillary lymph nodes [2,3]. The detection of suspicious axillary lymph node(s) is routinely performed by axillary palpation and supplementary axillary ultrasound (AUS). Suspicious lymph nodes can also be detected in further imaging examinations such as CT Scan or MRI, even if AUS remains standard for the imaging assessment of the axilla. The correct assessment of the nodal status is crucial for surgical planning after NACT. Suspicious lymph nodes should be confirmed by ultrasound-guided core needle biopsy [4]. If patients with nodal positive BC subsequently received NACT, the standard in the past used to be that these patients needed axillary lymph node dissection (ALND) according to their nodal status prior to chemotherapy regardless of the number of axillary lymph nodes involved and regardless of the response to NACT [5]. It is known that patients receiving ALND have a higher rate of morbidity, mainly caused by lymphedema [6,7]. Today, the standard of care has changed to using targeted axillary dissection (TAD) and/or Sentinel Node Biopsy (SLNB) in patients who become node negative after NACT as based on clinical assessment and imaging diagnostics [[8], [9], [10]]. Patients who are ycN + after NACT or who had extensive initial lymph node involvement still need ALNE. Patients who are cN0 before NACT receive SLNB after NACT nowadays.

Nevertheless, NACT often allows a downstaging of the axilla. The rate of nodal pathologic complete response (ypN0) following NACT is between 35 and 75 % depending on the intrinsic subtype [11,12]. Consequently, the looming question is, whether patients with conversion of axillary lymph nodes from cN + to ycN0 still need surgical staging of the axilla after NACT. Thus, there is a need of a reliable non-invasive diagnostic tool for restaging of axillary nodal status after NACT. This study investigated the diagnostic performance of AUS for restaging of axillary nodal status to evaluate whether AUS can achieve sufficient accuracy with regard to the determination of axillary nodal status after NACT.