Abstract Background: Women with breast cancer account for the largest cohort (23%) of over 18 million cancer survivors in the United States. Of the nearly 4 million breast cancer survivors, many, including individuals with metastatic breast cancer, will require lifelong treatment for chronic conditions. As part of a multi-site study including diverse tumor types, breast cancer survivors are being recruited to explore the feasibility and outcomes of a digital health coaching (DHC) program following the completion of primary therapy to support self-management of symptoms and general wellness. Trial design: A randomized-wait list control design will evaluate outcomes related to engagement in a DHC program following completion of primary therapy. Participants will be enrolled in a 6-month DHC program delivering evidence-based content via weekly calls, text, and email, in addition to usual support from their respective healthcare teams. The intervention group will receive coaching upon enrollment. The control group will receive coaching after a 6-month monitoring period. Twelve months of longitudinal clinical, patient-reported outcomes, activity tracking (via Fitbit), and microbiome data will be collected from both groups. Eligibility criteria: The breast cancer cohort will consist of individuals with both localized and metastatic disease within 1 year of completion of primary therapy, defined as treatment of curative intent, first-line or later, from which the individual is advancing to active surveillance or follow-up, with or without maintenance therapy. All study participants must be >18 years old, able to read and speak English, have access to mobile technology, and agree to wear and have data collected from an activity tracker. Specific aims: 1) Assess the feasibility and acceptability of a DHC program and its effect on participant health self-efficacy; 2) characterize associations between participant symptoms, physical/psychosocial well-being, and health self-management and gut microbiota changes; and 3) explore patient-generated health data outcomes among participants (patient reported and wearable biometrics outcomes). Statistical methods: Feasibility and acceptability will be assessed through descriptive statistics. Feasibility for this study is defined as a retention rate ≥70%, and acceptability is defined as ≤20% scoring “not at all helpful.” Secondary aims will be assessed using linear mixed models. Cross-cohort analysis of physical function, measured by the PROMIS Physical Function 10a, will be conducted. Assuming a power of 0.8 and significance level of .05, we will be able to detect a 2.51 difference in PROMIS score means between the control and intervention group (SD 10) with a sample of 500 total participants. Sub-group analysis will be conducted with the anticipated 250 participants in the breast cancer cohort with a focus on feasibility, acceptability, and descriptive and exploratory endpoints for this population as a whole and comparing early stage and metastatic participants. Target and present accruals: 500 individuals with diverse tumor types will be enrolled across 5 collaborating sites, of which 4 ill include individuals with breast cancer. An estimated 250 breast cancer participants (approximately 50% of the total study population), including 150 with de novo or progressive metastatic breast cancer, are planned to be recruited. To date, 1 Midwestern site has enrolled the first 7 breast cancer survivors. Two additional sites plan to enroll in July 2022, with the remaining 2 sites to begin enrollment by the end of 2022. Citation Format: Robin M. Lally, Rachael L. Schmidt, Gisele Tlusty, Elizabeth K. Arthur, Laura K. Flora, Jessica L. Krok-Schoen, Jessica T. Jones, Meagan Whisenant, Abbey Kaler, Debu Tripathy, Leorey Saligan, Marilyn Hammer, Austin Barr, Lindsey Jackson, Jennifer Loftis, Kelly J. Brassil. Breast Cancer Cohort of the Comprehensive Outcomes for After Cancer Health (COACH) Study: Study Protocol [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT2-06-01.