Cocreation of a Digital Psychological Intervention to Improve Mental Health Among Survivors of Breast Cancer (SerenApp): Protocol for a Usability and Pilot Randomized Controlled Trial

Background Survivors of breast cancer frequently experience anxiety, stress, and depression during the follow-up and survivorship phases, when access to psychosocial support is often limited. Although psychological interventions have demonstrated effectiveness, their implementation in routine clinical practice remains challenging. Digital mental health interventions offer a promising alternative. However, many existing solutions show limited personalization, engagement, and long-term adherence, partly due to insufficient involvement of end users in their design. Objective This study aims to develop and pilot-test a cocreated digital psychological intervention (SerenApp) aimed at improving psychological well-being among women during breast cancer follow-up and to evaluate its feasibility, usability, and preliminary effectiveness. Methods This mixed methods study adopts a participatory cocreation approach structured into 3 phases. Phase 1 focuses on identifying unmet psychosocial needs and co-designing the app’s content and functionalities through workshops involving patients and health care professionals, applying design thinking principles and the Cocre-Ar-E (cocreate, assemble, and evaluate) framework. Phase 2 involves the technical development of the intervention using an agile scrum framework methodology, followed by usability and technical validation (alpha testing). Phase 3 consists of a pilot randomized controlled trial (n=90; 45 participants in each of the intervention and control groups) conducted in Andalusia and the Canary Islands, Spain. Outcomes include acceptability, usability, adherence, psychological well-being, and cost-effectiveness. Validated instruments include the Cancer Patients’ Stress Coping Questionnaire (CAEPO), Patient Health Questionnaire-9 (PHQ-9), Hospital Anxiety and Depression Scale (HADS), EQ-5D-5L, System Usability Scale (SUS), Client Satisfaction Questionnaire-8 (CSQ-8), Client Service Receipt Inventory (CSRI), and Mobile App Rating Scale (MARS). Quantitative analyses will include ANOVA, multilevel regression models, and cost-effectiveness analyses (incremental cost-effectiveness ratio and incremental cost-utility ratio), whereas qualitative data will be analyzed using reflexive thematic analysis supported by NVivo software. Results Funding was awarded by the Instituto de Salud Carlos III (PI24/01193) in 2024. The SerenApp project began in January 2025 with an exploratory phase and a comprehensive literature review. The cocreation phase was conducted between September 2025 and May 2026, involving 24 breast cancer survivors and 15 multidisciplinary health care professionals. Development and alpha testing are ongoing, with recruitment expected to begin in 2027. Data analysis has not yet started, and the main study findings are expected to be published in 2028. Conclusions This protocol describes the development and pilot evaluation of SerenApp. By integrating the perspectives of survivors, health care professionals, researchers, and technology developers, the study seeks to develop an intervention that is relevant, acceptable, and feasible within real-world health care settings. The findings generated through the cocreation process, usability evaluation, and pilot randomized controlled trial are expected to contribute to the evidence base on participatory approaches in digital mental health and inform the future refinement, implementation, and large-scale evaluation of SerenApp. Trial Registration ClinicalTrials.gov NCT07169539; https://clinicaltrials.gov/study/NCT07169539 International Registered Report Identifier (IRRID) DERR1-10.2196/87151