Radiotherapy With a 12-Gene Expression Assay for Ductal Carcinoma In Situ: A Randomized Clinical Trial.

Importance: Breast ductal carcinoma in situ (DCIS) requires personalized treatment given its variable natural history. This study reports the first prospective oncologic outcomes of radiotherapy decisions as guided by 12-gene molecular assay, the DCIS score (DS). Objective: To assess surgical outcomes following preoperative breast magnetic resonance imaging (MRI) in women with DCIS and estimate 5-year and 10-year ipsilateral breast event (IBE) rates in participants given DS-based postoperative radiotherapy recommendations after local excision (WLE). Design, Setting, and Participants: Women with screen-detected DCIS who were eligible for WLE were enrolled to a single-arm, multicenter trial conducted at 75 institutions within the Eastern Cooperative Oncology Group-American College of Radiology Imaging Network (March 2015 to April 2016) and received a preoperative breast MRI-guided surgical treatment. Those who underwent successful WLE were advised to omit radiotherapy for low DS (<39) and receive radiotherapy for intermediate/high DS (≥39). Those achieving WLE as final treatment and successful DS assay (n = 171) were included in a prespecified analysis. Participants were followed up every 6 months for DCIS or invasive IBE, and 5-year IBE rates were analyzed from July to November 2023. Intervention: DS-based postoperative radiotherapy recommendations. Main Outcomes and Measures: Five-year IBE rates, with 95% CIs. Results: Among the 339 participants, the mean (SD) age was 59.1 (10.1) years. A total of 171 women (50.4%) received WLE for pure DCIS with free surgical margins and had DS data available. A total of 159 (93.0%) adhered to DS-based radiotherapy recommendations; 7 of 82 patients (8.5%) with a low DS underwent radiotherapy, and 5 of 89 patients (5.6%) with an intermediate/high DS declined radiotherapy. Over median (range) follow-up of 5 (0.5-5.0) years, 8 of 171 women experienced IBEs (4.8%; 95% CI, 2.4%-9.4%). IBE rates were similar for participants with a low DS(5.1%; 95% CI, 1.9%-12.9%) and participants with an intermediate/high DS (4.5%; 95% CI, 1.7%-11.7%). Among the 159 women who had adhered to DS-based radiotherapy recommendations, IBE rates were similar for participants with a low DS (5.5%; 95% CI, 2.1%-14.1%) and intermediate/high DS (4.8%; 95% CI, 1.8%-12.3%). Conclusions and Relevance: The findings of this prespecified analysis of a clinical trial suggest that DS-guided radiotherapy post-WLE for DCIS shows markedly lower 5-year IBE rates (approximately 5%) for intermediate/high DS than previously reported data following WLE alone. Despite a limited sample size, these data potentially provide support for radiotherapy use in patients with intermediate/high DS, and omission when DS is low, although confirmatory studies are needed. Trial Registration: ClinicalTrials.gov Identifier: NCT02352883.