Abstract PO5-20-01: SENTINOT2- Use of superparamagnetic iron oxide tracer to avoid unnecessary sentinel lymph node biopsies

Background/Purpose: The use of superparamagnetic nanoparticles of iron oxide tracers (Magtrace) and electromagnetometers (SentiMag) for the detection of sentinel lymph nodes (SLN) during breast cancer surgery has been demonstrated to be noninferior to traditional radioisotope (RI) and blue dye detection, with additional safety benefits. Previous work has shown that transcutaneous detection of SLN with Magtrace/Sentimag is possible for over 30 days. Because of the safety, efficacy, and long detection window of Magtrace, we aim to investigate the use of Magtrace to prevent unnecessary SLN biopsies (SLNB) in breast cancer surgery. SLNB are commonly performed during breast conserving surgeries (BCS) or mastectomy for patients with a preoperative diagnosis of ductal carcinoma in-situ (DCIS). Because metastasis is not expected in patients diagnosed with DCIS, SLNB may be unnecessary and potentially harmful. However, in 15-25% of cases, unexpected invasive carcinoma is found during the post-surgical histopathological analysis of the resected breast tissue. For these cases, if SLNB are forgone during the initial surgery, SLNB during a second (delayed) procedure is the usual standard of care to evaluate SLN for metastasis. However, potential changes to lymphatic drainage following the initial resection may affect tracer localization to SLN. Consequently, we aim to compare SLN detection rates during delayed SLNB with Magtrace administered prior to initial BCS or mastectomy and subsequent RI tracer administered prior to delayed SLNB. Methods: SENTINOT2 is an ongoing international trial with BCM as the sole US site. For eligible patients with a preoperative diagnosis of DCIS, Magtrace will be administered prior to BCS or mastectomy. If invasive carcinoma is found from the post-surgical histopathological analysis, patients will receive delayed SLNB within 4 weeks of their initial surgery. Prior to delayed SLNB, patients will be randomized into two groups differing in the order of modality used for SLN detection (Magtrace or RI). Subjects with the following conditions will be excluded from the study: hypersensitivity to Magtrace, iron overload disease, pregnancy, and lactation. The total expected accrual for this study is 538 subjects globally and 50 subjects at BCM. Results: Currently, 19 patients have been enrolled in SENTINOT2. The subject population is 11% Asian, 26% Black/African American, and 47% Caucasian with non-Hispanic ethnicity. 5% of subjects identified as Hispanic Caucasian and 11% of subjects declined to report their race or ethnicity. 79% of patients had mastectomies, while 21% had BCS. After post-surgical histopathological analysis, 21% (4/19) patients were determined to have invasive carcinoma and received delayed SLNB. 50% (2/4) of these patients were randomized to have Magtrace as their first SLN detection modality (Mag-RI) while the other 50% (2/4) had RI as their first SLN detection modality (RI-Mag). Table 1 shows comparable SLN detection rates between Magtrace and RI for patients who received delayed SLNB. Only 1/4 patients showed SLN metastasis. For this patient, the positive SLN was successfully detected by both modalities. Conclusion: Magtrace has shown potential in delayed SLN detection and the use of Magtrace has prevented 79% (15/19) of enrolled patients from receiving unnecessary SLNB. Table 1. Number of Lymph Nodes (LN) Detected by Magtrace/Radioisotope, Biopsied and Positive for Metastasis Citation Format: Mary Bajomo, Ivan Marin, Jessica Montalvan, Margarita Riojas-Barrett, Logan Healy, Elizabeth Bonefas, Stacey Carter, Alastair Thompson. SENTINOT2- Use of superparamagnetic iron oxide tracer to avoid unnecessary sentinel lymph node biopsies [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-20-01.