BackgroundDifficulty in preoperatively assessing the risk for occult invasion or surgery that precludes future accurate axillary mapping in patients with ductal cancer in situ (DCIS) account for overutilization of SLND.MethodsProspective, multicenter, cohort study, including women with any DCIS planned for mastectomy or DCIS grade 2 and > 20 mm, any DCIS grade 3, any mass-forming DCIS and any planned surgery. Patients received an interstitial SPIO injection during breast surgery, but no upfront SLND was performed. If invasion was identified on final pathology, delayed SLND (d-SLND) was performed separately with the coadministration of isotope ± blue dye (BD). Study outcomes were proportion of upfront SLNDs that were avoided, detection rates during d-SLND, and impact on healthcare costs.ResultsIn total, 78.7% of study participants (N= 254, mean age 60 years, mean DCIS size 37.8 mm) avoided upfront SLND. On d-SLND (median 28 days, range 9–46), SPIO outperformed Tc99with (98.2% vs. 63.6%,p< 0.001) or without BD (92.7% vs. 50.9%,p< 0.001) and had higher nodal detection rate (86.9% vs. 32.3%,p< 0.001) and with BD (93.9% vs. 41.4%,p< 0.001). Only 27.9% of all SLNs retrieved were concordant for Tc99and SPIO. Type of breast procedure (WLE vs. oncoplastic BCT vs. mastectomy) affected these outcomes and accounted for the low performance of Tc99(p< 0.001). d-SLND resulted in a 28.1% total cost containment for women with pure DCIS on final pathology (4190 vs. 5828 USD,p< 0.001).ConclusionsMarking the SLN with SPIO may avoid overtreatment and allow for accurate d-SLND in patients with DCIS.Supplementary InformationThe online version contains supplementary material available at 10.1245/s10434-022-13064-0.