Lehman CD, et al. - Given the potential utility of advanced diagnostics, such as magnetic resonance imaging (MRI) and gene expression profiles, to guide targeted treatment in patients with ductal carcinoma in situ (DCIS), researchers investigated the proportion of patients who converted to mastectomy after MRI and the reasons for those conversions and measured patient adherence to radiotherapy guided by the 12-gene DCIS score. Among 339 women with pure ductal carcinoma in situ, they conducted a nonrandomized clinical trial of a prespecified primary outcome. Conversion to mastectomy was noted in 19% of patients eligible for wide local excision after magnetic resonance imaging; conversions were based on magnetic resonance imaging findings in 38% and on other reasons in 62%. In 96% of women, wide local excision was the final surgical procedure as well as the first procedure after magnetic resonance imaging, and adherence to radiotherapy use guided by a 12-gene assay exceeded 90%. These findings support tailoring breast magnetic resonance imaging and a 12-gene assay to primary surgical treatment and radiotherapy, respectively, and suggest their value in informing patient and physician decision-making to support more targeted therapy.