PROJECT SUMMARY/ABSTRACT Breast cancer outcomes are disproportionately poor in low- and middle-income countries (LMICs) compared to high-income countries. Low breast cancer survival rates in LMICs are primarily attributable to advanced stage presentation and limited diagnostic and treatment capacity. Although national clinical practice guidelines for breast cancer in Tanzania recommend that all breast cancer patients receive hormone receptor (HR) testing, current pathology capacity is unable to meet this need. Pathologic diagnosis, including HR testing, is critical to determining the presence of cancer, extent of the disease, and planning treatment. However, HR testing is available at only two public hospitals in Tanzania. As a result, patients encounter turnaround times of weeks to months, with only a subset of eligible women receiving HR testing and guideline-concordant treatment. To reduce this evidence-to-practice gap, we propose to systematically identify the most effective implementation strategies across all public referral hospitals with pathology services in Tanzania to guide development of a multi-faceted and adaptable intervention (set of implementation strategies). We hypothesize that this intervention will increase the proportion of women who receive HR testing and guideline-concordant endocrine therapy, resulting in improved cancer care, quality of life, and overall short term survival of patients. First, we will conduct a formative evaluation to determine the organizational readiness for change and to identify barriers and facilitators to routine HR testing using mixed methods. Then, we will determine the optimal implementation strategies for the barriers and facilitators identified in the formative evaluation, and develop a multi-component intervention with methods informed by organizational theory. Finally, we will iteratively pilot test the multi-component intervention and will modify the intervention components based on outcomes measures. These studies will be led by Dr. Dianna L. Ng, a junior faculty member in the Department of Pathology at Memorial Sloan Kettering Cancer Center (MSK) with an interest in developing and testing theory-driven, multi-level interventions to increase integration of evidence-based cancer diagnostics in low-resource settings. The research will be carried out under the multidisciplinary mentorship of Dr. T. Peter Kingham (surgical oncologist, global oncology research), Dr. Jamie Ostroff (clinical psychologist, implementation science), Dr. Edda Vuhahula, (pathologist, capacity building in East Africa), and Dr. Britt-Marie Ljung (pathologist, resource- stratified diagnosis). MSK offers an outstanding environment for cancer care delivery and health services research. To achieve her goal of becoming an independent researcher, Dr. Ng has developed a structured career development plan aimed at advancing her knowledge and advanced skills in implementation science in global health.