Abstract GS1-10: Radioactive Iodine Seed placement in the Axilla with Sentinel lymph node biopsy after neoadjuvant chemotherapy in breast cancer: Results of the prospective multicenter RISAS trial

Rationale and research objective: As a result of neoadjuvant chemotherapy (NAC), approximately 1/3 of clinically node positive (cN+) breast cancer patients convert to a pathologic complete response (pCR) of the axilla. Targeted Axillary Dissection appears to be most accurate to evaluate axillary treatment-response, but evidence to support this is limited. The RISAS trial was set up to determine the accuracy of sentinel lymph node biopsy (SLNB) combined with excision of the pretreatment marked positive lymph node for axillary staging after NAC in cN+ patients. Methods: This prospective, multicenter trial included cT1-4N1,2,3b patients treated with NAC. Prior to NAC, an iodine seed was placed in the pathologically confirmed positive lymph node. After NAC, all patients underwent SLNB together with excision of the marked lymph node (RISAS-procedure) followed by axillary lymph node dissection (ALND). The identification rate and the accuracy (i.e. false negative rate (FNR) and negative predictive value (NPV)) were calculated for the RISAS procedure. Assuming an FNR of 2%, a prevalence of a positive ALND of 64% and a 10% drop-out rate, a sample size of 248 patients was needed to determine non-inferiority. The NULL hypothesis of inferiority would be rejected at a significance level of 5% if the upper bound of the two-sided 90% Clopper-Pearson confidence interval of the observed FNR would be below the non-inferiority margin of 6.25%. Secondary objectives included the accuracy of the SLNB and excision of the marked lymph node separately. Results: A total of 252 pathologically proven cN+ patients provided informed consent of whom 227 underwent the RISAS procedure/Targeted Axillary Dissection (SLNB combined with excision of the marked positive lymph node). The identification rate of the RISAS procedure is 98% (223/227). Preliminary analysis shows a false negative rate of approximately 5% and a negative predictive value of approximately 91%. Furthermore, by combining SLNB with excision of the marked lymph node, both the identification rate and the accuracy are improved compared to either the SLNB alone or the excision of the marked lymph node alone. Conclusion: This prospective multicenter validation trial shows that the RISAS procedure/Targeted Axillary Dissection is most suitable in terms of identification rate and accuracy to replace ALND for axillary staging after NAC in cN+ patients. The trial was funded by the Dutch Cancer Society (KWF, grant number 2015-8023). Citation Format: Janine Simons, Thiemo JA v Nijnatten, Linetta B Koppert, Carmen C van der Pol, Paul J v Diest, Agnes Jager, David v Klaveren, Boen LR Kam, Marc BI Lobbes, Maaike de Boer, Cees Verhoef, Paul RA Sars, Harold J Heijmans, Els RM van Haaren, Wouter J Vles, Caroline ME Contant, Marian BE Menke, Leonie HM Smit, Wendy Kelder, Marike Boskamp, Ingeborg Mares-Engelberts, Ernest JT Luiten, Marjolein L Smidt. Radioactive Iodine Seed placement in the Axilla with Sentinel lymph node biopsy after neoadjuvant chemotherapy in breast cancer: Results of the prospective multicenter RISAS trial [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr GS1-10.