Abstract Introduction: Access to expert, evidence based clinical decision making is crucial in maximizing the outcome of women with breast cancer, but is a scarce resource, especially in developing countries. The Navya Expert System is a patented, software based clinical decision support system that exhaustively searches and assimilates relevant medical literature and guidelines to make specific therapeutic recommendations for individual patients based on their clinical data. This study is a retrospective validation of Navya Expert System's output against tumor board decisions of a multidisciplinary group of expert breast cancer clinicians working in a tertiary care oncology center in India. Methods: Women with non-metastatic breast cancer who had already completed their loco-regional and systemic therapy based on the recommendations of the tumor board were included in the study. The protocol specified clinical and pathology data of these women were retrospectively abstracted from their case charts and processed through the Navya Expert System. The output was classified into major (neo-adjuvant chemotherapy versus upfront surgery and need for adjuvant chemotherapy, endocrine therapy and radiation therapy, respectively) and minor (breast conservation versus mastectomy, taxane versus non-taxane adjuvant chemotherapy and need for nodal radiation therapy) therapeutic decisions. Decisions discordant between the tumor board and the Navya Expert System were adjudicated by an expert panel of breast cancer clinicians from the same institution. Navya Expert System decisions were classified as discordant with appropriate clinical practice if they were in disagreement with both the tumor board and expert panel. All other Navya Expert System decisions were classified as concordant. The primary outcome of the study was concordance between the Navya Expert System and the tumor board or expert panel for major and minor therapeutic decisions. Results: A total of 76 patients involving 224 major and 224 minor therapeutic decisions were included in the study. Navya Expert System's output was concordant with the tumor board or expert review in 224/224 major decisions (100%, 95% CI 99.6%-100%) and 221/224 minor decisions (98.6%, 95% CI 97.1%-100%). Navya Expert System's output was concordant with the tumor board alone in 210/224 (93.75%, 95% CI 90.6%-96.9%) major decisions and 160/224 (71.4%, 95% CI 65.5%-77.3%) minor decisions. Most common reasons for discordance were non-prescription of HER2 targeted therapy by the tumor board due to financial constraints and non-use of nodal radiation for 1-3 node positive patients. Of the 64/224 Navya Expert System decisions discordant with the tumor board, only 3 were finally deemed discordant after review by the expert panel. Conclusions: Navya Expert System treatment recommendations, only requiring the input of commonly available clinical data, are highly concordant with those of a tumor board comprised of breast cancer experts with high level expertise. If these results can be prospectively validated, Navya Expert System has the potential to increase global access to evidence based clinical decision making in breast cancer. Citation Format: Nita Nair, Sudeep Gupta, Naresh Ramarajan, Gitika Srivastava, Vani Parmar, Anusheel Munshi, Shraddha Vanmali, Vaibhav Vanmali, Rohini Hawaldar, Rajendra A Badwe. Validation of a software based clinical decision support system for breast cancer treatment in a tertiary care cancer center in India [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-16-01.