Impact of the Breast Cancer Index on extended endocrine therapy recommendations in patients from the BCI Registry study.

527 Background: The Breast Cancer Index (BCI) test is an established genomic assay that provides individualized risks of overall and late distant recurrence and predicts the benefit from extended endocrine therapy (EET) in patients with early-stage, HR+ breast cancer. Previous findings from the large, prospective BCI Registry have shown that physicians changed their EET recommendation in over 40% of patients following BCI testing. This analysis investigates how physicians integrate prognostic and predictive BCI results in real care settings to decide whether or not to recommend extension of endocrine therapy. Methods: The BCI Registry study (NCT04875351) is evaluating the long-term clinical outcome, decision-impact and medication adherence in about 3000 patients. Pre- and post-BCI physician treatment recommendations were compared using McNemar’s test. Clinicopathologic features of cases were correlated with BCI (H/I) predictive results and post BCI-EET recommendations using Fisher’s exact test. Differences in mean BCI prognostic risk were assessed using analysis of variance (ANOVA). Results: Pre- and post-BCI testing physician questionnaires were completed for 2850 patients. The percentage of patients recommended for EET decreased from 54.6% before BCI testing to 41.2% after BCI testing, while those not recommended for EET increased from 44.9% to 58.0% (p<.0001). Among patients not recommended for EET pre-BCI, physicians cited low risk of recurrence (47.4%), osteoporosis risk (16.7%), side effects (8.6%), or multiple reasons (24.4%). Among BCI (H/I)-High patients (N=1063), recommendations for EET increased from 60.4% pre-BCI to 90.6% post-BCI, while EET non-recommendations decreased from 39.1% to 8.0%. Conversely, in BCI (H/I)-Low patients (N=1787), EET recommendations decreased from 51.1% to 11.8% and EET non-recommendations increased from 48.3% to 87.8%. Post-BCI, BCI (H/I)-High patients who were not recommended EET (8.0%) had more favorable clinicopathological features, including more T1 tumors (83.5% vs. 68.1%, p=.009), more G1 tumors (44.7% vs. 14.4%, p<.0001) and lower mean BCI prognostic risk compared to those for whom EET was recommended (4.4% vs 9.2%, p<.0001). Conversely, BCI (H/I)-Low patients who were recommended EET (11.8%) had a larger proportion of T2/T3 tumors (40.7% vs. 21.7%, p<.0001), more G2/G3 tumors (79.2% vs. 62.8%, p<.0001), more N+(1-3) cases (43.6% vs. 15.7%, p<.0001) and higher mean BCI prognostic risk (8.1% vs 4.7%, p<.0001). Conclusions: BCI results meaningfully inform physician recommendations for EET. Post-BCI, physician EET recommendations increased to 91% in BCI (H/I)-High patients and decreased to 12% in BCI (H/I)-Low patients, highlighting the important treatment guidance provided by BCI. Integration of both predictive and prognostic BCI results help further personalize patient care.