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Why is it advantageous to mark clinically positive lymph nodes before starting neoadjuvant systemic therapy (NAST)?
Because the lymph node migrates
In case of clinical response, the previously positive node might not be detectable without a marker after NAST
For guidance of radiotherapy
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Neoadjuvant systemic therapy (NAST) has become more popular for treating breast cancer patients. The aim is to downstage or eradicate the disease and so enable breast-conserving surgery (instead of mastectomy) or a surgical approach in the first place. In case of a clinically node-positive (cN+) diagnosis, administration of NAST targets to eradicate the disease in the lymph nodes, thereby potentially enabling omission of axillary lymph node dissection (ALND) and reducing the risk of post-surgical morbidities such as lymphœdema. Pathologic complete response (pCR) is currently considered the most significant indicator of the success of NAST and is evaluated in the tumour as well as in the axillary lymph nodes.
To enable accurate assessment of the response in the breast and axilla, it is essential that both the positive lymph node(s) and the breast lesion are marked before NAST to be able to locate them correctly afterwards. Once detected and excised, the respective lymph nodes are analysed. ALND can only be avoided if no residual cancer burden can be detected in the lymph nodes. Accurate staging requires a highly sensitive and comprehensive analysis such as is possible with OSNA (One-Step Nucleic Acid Amplification). OSNA is a molecular assay which allows rapid analysis of the entire lymph node, thereby avoiding missing even tiny metastases.
Why is it difficult to locate previously positive lymph nodes after NAST with confidence?
Through NAST, nodal shrinkage may occur and in consequence, the marker device may shift into surrounding tissue.  Further to that, a lack of accuracy in detection is caused by poor or no visibility of the marker due to fibrosis induced by the systemic treatment. When ultrasound is performed intraoperatively, the quality of the localisation results is highly operator-dependent. Thus, difficulties to reliably locate the marker during surgery might occur.
A marker is needed that stays securely in place and can be identified during surgery with an easy detection method, ensuring safe and accurate localisation of the previously positive lymph node after NAST.
One solution for that is the Magseed® magnetic marker, which is CE-marked and FDA-cleared for long-term placement in soft-tissue. It can be easily and accurately located using the Sentimag® magnetometer, making it a highly intuitive, wire-free and non-radioactive system that can be used by any hospital.
 Hartmann et al. (2018): European Journal of Surgical Oncology. 44:1307–11.