OBJECTIVE:The MARI procedure marking the axillary lymph node with radioactive iodine (I) seeds is a new minimal invasive method to assess the pathological response of nodal metastases after neoadjuvant systemic treatment (NST) in patients with breast cancer. This method allows axilla-conserving surgery in patients responding well to NST. METHODS:Prior to NST, proven tumor-positive axillary lymph nodes were marked with a I seed. This marked lymph node is the so-called MARI-node. After NST, the MARI node was selectively removed using a γ-detection probe. A complementary axillary lymph node dissection was performed in all patients to assess whether pathological response in the MARI node was indicative for the pathological response in the additional lymph nodes. RESULTS:A tumor-positive axillary lymph node was marked with a I seed in 100 patients. The MARI node was successfully identified in 97 of these 100 patients (identification rate 97%). Two patients did not undergo subsequent axillary lymph node dissection, leaving 95 patients for further analysis. The MARI node contained residual tumor cells in 65 of these 95 patients. In the other 30 patients, the MARI node was free of tumor, but additional positive lymph nodes were found in 5 patients. Thus, the MARI procedure correctly identified 65 of 70 patients with residual axillary tumor activity (false negative rate 5/70 = 7%). CONCLUSIONS:This study shows that marking and selectively removing metastatic lymph nodes after neoadjuvant systemic treatment has a high identification rate and a low false negative rate. The tumor response in the marked lymph node may be used to tailor further axillary treatment after NST.