e11504 Background: Numerous studies have demonstrated the cost utility of gene expression-based assessment of recurrence risk in breast cancer. Cost savings rely primarily on decreased use of adjuvant chemotherapy in patients predicted to be low-risk. Breast Cancer Index (BCI) is a gene expression-based test that significantly predicts overall risk of recurrence, late (≥5y) recurrence and likelihood of benefit from extended (≥5y) endocrine therapy in patients with ER+, LN- breast cancer. This study evaluated the potential cost utility of BCI from a US third-party payer perspective. Methods: A fact-based economic model was developed which projected the cost and effectiveness of BCI in a hypothetical population of patients with ER+, LN- breast cancer compared to standard clinicopathologic diagnostic modalities. Patients flowed through the model based on patterns of care and BCI data. Costs associated with adjuvant chemotherapy, toxicity, follow-up, endocrine therapy, and recurrence were modeled over 10 yrs. Model inputs were based primarily on published literature, and supplemented by interviews with disease experts and payers. Sensitivity analyses were performed around key inputs to estimate effects on the model. Results: Use of BCI is projected to be cost saving in this patient population, with a net cost savings of $4,005 per patient tested after accounting for BCI cost. Gross cost savings were projected to be achieved through targeted use of adjuvant chemotherapy ($5,785), reduced recurrence in patients receiving extended endocrine therapy based on BCI ($2,350), and reduced recurrence in previously non-compliant patients ($370). Sensitivity analyses demonstrated that results were most sensitive to chemotherapy utilization in low- and intermediate-risk patients, cost of adjuvant chemotherapy, cost of recurrence, and percentage of patients classified as low risk. Conclusions: BCI is projected to be cost saving in an ER+, LN-, breast cancer patient population. Cost savings are achieved through projected impact on adjuvant chemotherapy use, extended endocrine therapy use, and endocrine therapy compliance. These findings require validation in additional cohorts, including studies of real-world clinical practice.