Abstract PS3-10-10: Decision-making on adjuvant chemotherapy in early hr+ breast cancer: a prospective evaluation of oncotype dx® utility among swiss breast cancer experts

Abstract Background: The Oncotype DX ® test (21-gene Recurrence Score (RS) test) is widely used to guide adjuvant chemotherapy (CT) decisions in hormone receptor-positive (HR+), HER2-negative early breast cancer. However, variability in clinical interpretation persists in real-world settings. This study prospectively assessed how the Oncotype DX test influences CT decision-making among Swiss breast cancer experts. Methods: From a cohort of 325 patients with HR+/HER2− early breast cancer and available Recurrence Score ® results, 50 anonymized randomly selected cases were chosen. Ten board-certified Swiss medical oncologists, all specialized in breast cancer, independently evaluated each case and made CT recommendations before and after disclosure of the Recurrence Score results. Changes in treatment decisions were assessed using McNemar’s test and ANOVA. Results: The Oncotype DX test led to changes in treatment recommendations in 19.8% of cases. CT was recommended in 35.4% of pre-testing decisions, compared to 21% post-testing, representing a 14.4% net reduction. Changes in CDK4/6 inhibitor and osteoprotection recommendations were minimal (0.4% and 0.2%, respectively). The most pronounced impact was observed in cases with intermediate clinical risk, particularly tumors 2-4.5 cm and N0 status. Notably, 23 (46%) patients had tumors with node-positive disease. One-way ANOVA showed no significant difference in mean treatment recommendations before and after Oncotype DX testing (F(1,58)=0.29, p=0.59). However, McNemar’s test revealed a highly significant directional shift (p0.0001), primarily reflecting de-escalation of CT. Only one oncologist escalated treatment in a single case. Inter-observer variability decreased following Recurrence Score result disclosure. Conclusions: The Oncotype DX test significantly influenced adjuvant treatment decisions, primarily leading to chemotherapy de-escalation. It also increased decision consistency among Swiss breast cancer specialists, underscoring its value not only in guiding individual care but also in harmonizing expert judgment in routine practice. Citation Format: M. Vetter, S. Ebner, E. Kralidis, C. Kurzeder, A. Oseledchyk, E. Chiru, J. Landin, K. Schmutz-Kober. Decision-making on adjuvant chemotherapy in early hr+ breast cancer: a prospective evaluation of oncotype dx® utility among swiss breast cancer experts abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-10-10.