Effect of Wire vs Magnetic Seed Localization on Lumpectomy Cavity Size

Abstract Purpose/Objectives To assess whether an association exists between surgical localization technique and size of lumpectomy cavity on radiation (RT) planning CT scan. Adjuvant RT with boost to lumpectomy cavity has been shown to improve local control compared to adjuvant whole breast radiation alone, but larger cavity sizes can lead to worse cosmetic outcomes following boost administration, which could lead providers to omit boost. Therefore, decreasing cavity size could increase guideline-concordant boost administration and minimize adverse cosmetic outcomes. Materials/Methods A retrospective review was conducted of all patients undergoing breast conserving surgery with either wire- or magnetic seed -guided lumpectomy followed by adjuvant RT at a single institution from 2018 to 2021. Data were collected from pre-surgical work-up, surgical pathology, and radiation planning. Women undergoing bracketed wire localization and patients treated by a surgeon who only performed wire localized procedures were excluded. The primary outcome was lumpectomy cavity size as measured on planning CT. We first conducted an overlapping weights propensity score analysis to account for imbalance between groups in age, BMI, breast size as measured on planning CT scan, pre-operative imaging tumor volume, neoadjuvant therapies, lumpectomy histology (DCIS alone vs DCIS + invasive vs invasive alone vs invasive with lobular features vs no residual), and multifocality. Multivariable analysis (MVA) of CT cavity volume included the above weighted variables as well as surgeon and radiation oncologist. Secondary analyses included MVA of total pathologic volume, bivariable analysis of boost delivery, bivariable analysis of electron vs photon boost, stratification by surgeon, and fixed effect model for year of surgery. Results Of 617 women who received lumpectomy during the study period, 387 were included in final analysis. Patients who underwent seed localization were less likely to have multifocal disease, less likely to have calcifications on mammogram, more likely to have ultrasound measurements for pre-op imaging, and had smaller tumor size on pre-op imaging. Four surgeons performed all cases, with rates of seed use per surgeon ranging from 27.7% to 70.7% but generally increasing throughout the study period. There was no difference between wire and seed localization in the need for additional margins based on intra-operative margin analysis (58.4 vs 62.7%, p = 0.5). There was no difference in positive margins (6.4 vs 5.4%, p = 0.81) or second surgeries (9.4 vs 8.1%, p = 0.79). Rates of close margins were the same for DCIS (23.4 vs 17.3%, p = 0.35) and invasive carcinoma (7.6 vs 6.8%, p = 0.97) between techniques. Initial uncorrected bivariable analysis shows wire localization has a non-significant trend toward increased CT cavity volume (4.56cc, p = 0.15) and a significant association with total pathology volume (21.7cc, p = 0.004). For the primary outcome, breast size, time from surgery to simulation, and surgeon were all significantly associated with CT cavity volume but there was no significant difference by localization technique (p=0.38). For pathology volume, there was a non-significant trend toward increased specimen volume with wire localization (p = 0.07), and significant associations with BMI, histology, and pre-op imaging volume. When stratified by surgeon, there was no surgeon for whom one localization technique led to significantly different CT or pathology volume over the other. There were no significant changes of the treatment effect over time (p = 0.79). There was no significant difference between wire and seed localization in indicated boost delivery (85% vs 79%, p = 0.14) or electron boost (42% vs 56%, p = 0.13). Conclusion There was no significant difference in CT cavity size between wire localization and magnetic seed localization, suggesting that the choice between these surgical techniques does not impede RT boost delivery. Citation Format: Michael Dykstra, Jessica Thompson, Jessica Aldous, Shannon Jiang, Tasha Hughes, James Hayman, Aleksandar Dragovic, Jennifer Shah, Alfred Chang, Corey W. Speers, Michael Sabel, Lesly Dossett, Matthew Schipper, Reshma Jagsi. Effect of Wire vs Magnetic Seed Localization on Lumpectomy Cavity Size [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-09-05.