The Effect of Wire vs Magnetic Seed Localization on Lumpectomy Cavity Size

PURPOSE:To assess whether an association exists between surgical localization technique and lumpectomy cavity size on radiotherapy planning CT scan. METHODS AND MATERIALS:A single-institution retrospective review was conducted of women undergoing breast conserving surgery with wire- or magnetic seed- guided lumpectomy followed by adjuvant radiotherapy from 2018 to 2021. Patients of a surgeon only performing one localization technique or undergoing bracketed localization were excluded. The primary outcome was lumpectomy cavity size on simulation CT. Confounding due to imbalance in patient and tumor factors was addressed with overlap weights derived from a propensity score analysis and used in a weighted multivariable analysis (MVA). Secondary outcomes included positive margins, total pathologic volume, boost delivery, and boost modality. RESULTS:Of 617 women who received lumpectomy during the study period, 387 were included in final analysis. Tumors of patients undergoing seed localization were more likely unifocal, assessable by ultrasound, and smaller. Seed use rates ranged from 27.7% to 70.7% per surgeon. There was no difference in positive margins (6.4 vs 5.4%, p = 0.79) or second surgeries (9.4 vs 8.1%, p = 0.79) between groups. Close margin rates were similar for DCIS (p = 0.35) and invasive carcinoma (p = 0.97). In unadjusted bivariable analyses, wire localization was associated with larger total pathology volume (p = 0.004) but localization technique showed no association with CT cavity volume (p = 0.15). After adjusting for potentially confounding variables listed in the methods, MVA failed to show an association between localization technique and either CT cavity (p=0.35), or total path volume (p = 0.08). There was no difference in indicated-boost delivery (p= 0.15) or electron boost (p= 0.14) by localization technique. CONCLUSIONS:There was no significant difference in CT cavity size by localization technique, suggesting choice between surgical techniques does not impede radiotherapy boost delivery.