Abstract PS3-09-19: Correlation Between Breast Cancer Index (BCI) and RSClin Late in Assessing 5-10 Year Recurrence Risk in Early-Stage Hormone Receptor-Positive Breast Cancer

Abstract Background: Hormone receptor-positive (HR+), HER2-negative early-stage breast cancer constitutes approximately 70-80% of all breast cancers. However, despite favorable short-term outcomes with adjuvant endocrine therapy, a subset of patients remain at risk for late recurrence beyond 5 years, necessitating personalized decisions about extending endocrine therapy. Genomic assays have emerged as valuable tools for refining risk estimates and guiding clinical decision-making in this setting. Two such tools—Breast Cancer Index (BCI) and RSClin Late—are often used to assess the likelihood of distant recurrence from years 5 to 10, avoiding unnecessary endocrine therapy in patients with low risk and extending it in those with high risk. BCI combines two independent biomarkers, HOXB13:IL17BR (H:I) and the 5-gene molecular grade index (MGI), that assess estrogen-mediated signaling and tumor grade, respectively, to provide both prognostic and predictive information for late recurrence and extended endocrine therapy benefit. RSClin Late integrates the Oncotype DX 21-gene recurrence score with clinical and pathologic features (age at surgery, tumor size, tumor grade) to generate individualized 5- to 10-year risk estimates. Despite overlapping indications, the degree of concordance between these two tools remains unclear. Methods: We conducted a retrospective analysis of all patients within our practice with HR+, HER2-negative early-stage breast cancer without lymph node involvement who underwent BCI testing between January 1, 2024 to June 6, 2025. A total of 27 eligible patients were identified. The BCI Prognostic Results (risk of recurrence with 5 total years of adjuvant endocrine therapy) and RSClin Late scores were extracted from the electronic medical record. The Spearman rank correlation coefficient was calculated with a 95% confidence interval (CI) to assess the monotonic relationship between the two scoring systems. This nonparametric method was selected to account for potential outliers, non-linear relationships, and skewed distributions. Results: The Spearman correlation coefficient (ρ) between the BCI Prognostic Result and RSClin Late was 0.413 (95% CI 0.039 to 0.685), with a p-value of 0.032, indicating a moderate, statistically significant positive correlation between the two measures. These findings suggest that while the tests may grossly align in terms of risk stratification, clinically meaningful discordance may occur in individual cases. Discussion: While our study is limited by a small sample size and larger analyses are needed to better characterize the degree and clinical impact of discordance between these tools, the moderate but statistically significant correlation observed highlights that BCI and RSClin Late, although both designed to predict late recurrence, are complementary but not interchangeable. BCI is a purely genomic assay focused on tumor biology, while RSClin Late integrates genomic (Oncotype DX) and traditional clinical and pathologic data, leading to possible variation in risk assignment. Both assays have been independently validated to inform decisions regarding extended endocrine therapy, but their moderate correlation raises questions about how to interpret conflicting results. Conclusion: In this real-world cohort of patients with early-stage HR+ N0 breast cancer, BCI and RSClin Late scores demonstrated a moderate but statistically significant correlation. This supports a degree of concordance but also underscores that the tests may not be interchangeable. Incorporating both genomic and clinical-pathologic perspectives into shared decision-making discussions may help personalize extended endocrine therapy decisions, avoid unnecessary toxicity, and improve long-term outcomes. Citation Format: K. Rupani, S. Wang, M. Diniz, P. Klein. Correlation Between Breast Cancer Index (BCI) and RSClin Late in Assessing 5-10 Year Recurrence Risk in Early-Stage Hormone Receptor-Positive Breast Cancer 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 PS3-09-19.