Development and Usability of the “FORTEe Get Strong” App to Promote Physical Activity and Health Awareness in Children and Adolescents With Cancer During Intensive Treatment Using an App-Based Approach: Mixed Methods Study

Background: As survival rates for children, adolescents, and young adults with cancer improve, managing treatment-related side effects is increasingly important. Enhancing physical activity levels has been shown to be effective in reducing some of these effects. Digital interventions, such as mobile apps, offer engaging tools to promote physical activity in young populations. Objective: This article introduces the "FORTEe Get Strong" app (Nurogames GmbH) and presents a formative evaluation of its acceptability among children, adolescents, and young adults with cancer. Methods: The "FORTEe Get Strong" app was developed within the multicenter FORTEe trial. Grounded in behavior change theories, the app uses gamification to deliver a child-friendly platform promoting physical activity and health-related knowledge. An embedded mixed methods design, with qualitative insights contextualizing quantitative findings, was applied. User experience was assessed using a self-developed questionnaire combining Likert scale items (1-5: "not at all satisfied" to "very satisfied," 1-3 for participants aged