Abstract PS1-46: Magnetic seeds to aid targeted axillary dissection

Introduction: Indications for accurate removal of a previously identified, involved or highly suspicious axillary lymph node include: 1) after neoadjuvant chemotherapy (NACT) to ensure pathological assessment of the correct index node (targeted axillary dissection, TAD) 2) for women with an abnormal/indeterminate node on imaging who may be able to avoid axillary lymph node clearance (ALNC) because they meet Z0011 criteria or may be eligible for POSNOC trial inclusion 3) guided removal of a specific axillary lymph node (eg Rotter’s node recurrence, diagnostic biopsy, inclusion of specific node in ALNC). Dual localisation is either not viable or has an unacceptably high false negative rate in these scenarios and marking of the index node is advised. Although conventional markers are often used prior to NACT, finding that marked node at surgery can pose a challenge. Wire localisation has variable outcomes and scheduling constraints. Magnetic seed localisation, with MagseedTM, allows for easy placement and excision of targeted nodes. Seeds can be accurately placed, do not migrate and can now be placed many months prior to surgery, at the start of NACT. The aim of this study was to report our early experience. Methods: Patients were identified by searching an imaging database for women who had undergone ultrasound (US)-guided MagseedTM insertion into an abnormal axillary lymph node between August 2018 and July 2020. Data on intended use and surgical outcomes were collected retrospectively from electronic patient records (EPR). Patients were categorised into 3 groups as described above. Results: 37 patients were identified, 17 in group 1, 13 in group 2 and 7 in group 3. Only 1 patient had complications with MagseedTM insertion, requiring a second attempt to successfully mark the required node. In 1 patient the MagseedTM was found on the surgical drape, thought to have been lying adjacent to the node intended for excision, this patient was therefore excluded from further evaluation in the results, as were 3 patients with missing data. One patient has yet to come to surgery, all other MagseedsTM have been successfully retrieved. Group 3 indications were disparate, and this group will not be described further. Group 1 - in 10 of 15 patients post NACT the MagseedTM-marked node was found to be the sentinel node by routine mapping techniques (Technetium 99 +/- Patent V blue dye injection), identified as either hot or hot and blue. Of the 15 patients, 6 remained node positive on intra-operative assessment and underwent ALNC whilst 9 (60%) were spared ALNC. 2 patients avoided false negative results as the Magseed™-marked node contained residual disease yet was neither hot nor blue. Group 2 - Ten patients underwent sentinel lymph node biopsy using routine mapping techniques and MagseedTM-guided excision of a specifically targeted node. In 7 the MagseedTM-marked node was found to be a sentinel node (hot or hot and blue). Only 3 patients had sufficient disease to warrant completion ALNC. Conclusion: Insertion of a magnetic seed into a biopsy-proven positive node prior to planned TAD or into an abnormal node identified at diagnosis facilitates specific node retrieval for pathological assessment, with minimal associated complications, and has wide versatility. Magnetic seed localisation reduces the false negative rate and provides confidence in the correct assessment of the axilla thereby sparing patients unnecessary ALNC if they have had a complete pathological response to NACT or have limited axillary disease. The ability to insert the seed prior to the surgical date allows for flexibility with scheduling in comparison to wire localisation. Citation Format: Kathryn E Harborough, V Sinnett, K Downey, K Krupa, J E Rusby, P A Barry. Magnetic seeds to aid targeted axillary dissection [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 PS1-46.