Primary tumour response on breast MRI as a predictor of axillary pathologic response in breast cancer patients treated with neoadjuvant chemotherapy

Objectives To investigate whether breast radiologic response on breast MRI can predict axillary pathologic response (ypN0 vs. ypN+) in breast cancer patients treated with neoadjuvant chemotherapy (NACT) in the total study population and in subgroups according to clinical nodal (cN)-status and oestrogen receptor (ER)-expression. Materials and methods This retrospective single-centre study consecutively included breast cancer patients who, between 2012 and 2022, had undergone baseline and post-NACT breast MRI. Measures of predictive ability, including sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic odds ratio (DOR) with 95% confidence intervals (CI), were calculated. Subgroup analyses were performed for cN-status and ER expression. Results Of 251 included patients, 48.6% were cN0 and 34.7% had ER-negative tumours. For axillary pathologic response prediction by breast MRI, the sensitivity was 84%, specificity 40.4%, PPV 48.3%, NPV 79.2%, and DOR 3.6 (95% CI: 1.83–7.00). The probability of achieving ypN0 was 60.2% in all patients, 82% in cN0 patients versus 39.5% in cN+ patients, and 78.2% in ER-negative tumours versus 50.6% in ER-positive tumours. High NPVs were observed in cN0 patients (92%) and ER-negative tumours (90.1%) and were lower in cN+ patients (62.2%) and ER-positive tumours (72.2%). Conclusions The probability of ypN+ given breast complete response on breast MRI was below 10% in cN0 patients and ER-negative tumours. Therefore, in these subgroups, breast complete response on breast MRI may be clinically useful to exclude ypN+. In cN+ patients and ER-positive tumours, due to lower NPVs, breast complete response on breast MRI may not be clinically useful to exclude ypN+. Key Points Question Accurate non-invasive methods to differentiate between patients with and without axillary pathologic complete response after neoadjuvant chemotherapy prior to axillary surgery are currently lacking . Findings Breast complete response on MRI after neoadjuvant chemotherapy indicates a low probability (< 10%) of axillary residual disease in clinically node-negative patients and oestrogen receptor-negative tumours . Clinical relevance Breast complete response on MRI may be a non-invasive, clinically useful tool to exclude patients with axillary residual disease after neoadjuvant chemotherapy prior to axillary surgery and guide patient selection for potential de-escalation of axillary treatment .