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 <8 y) and open-ended questions. Quantitative data were analyzed descriptively and inferentially, including chi-square tests for differences in app usage by age, sex, and diagnosis, and Kruskal-Wallis tests to compare app feature ratings between age groups. Free text responses underwent qualitative content analysis. Results: The app was published in May 2023; it integrates gamified features to teach health knowledge. Exercise videos address endurance, strength, flexibility, coordination, and gait. Feedback on user experience was provided by 53 patients aged 5-19 years (mean 11.4, SD 3.8 years, 95% CI 10.4-12.5; 27/53, 51% male). App use did not differ significantly by age groups (χ²3=5.1; P=.16; Cramer V=0.135), sex (χ²1=3.4; P=.06; φ=0.110), or diagnosis (P=.54; Cramer V=0.168). The app was generally well-received, with a mean Likert scale score of 4.14 (SD 1.04, 95% CI 3.84-4.43). For individual components, mean scores ranged from 3.52 (SD 1.44, 95% CI 3.09-3.95) for exercise content to 4.37 (SD 0.82, 95% CI 4.14-4.60) for design. Kruskal-Wallis tests revealed no significant differences among age groups. Qualitative analysis identified key categories regarding overall app evaluation, specific features, target group perception, usability, and suggestions for improvement. While participants highlighted the app's design and gamification, concerns arose regarding age-appropriateness, lack of supervision, and adaptability of exercises. Conclusions: This study provides novel insights into user experience of children, adolescents, and young adults with cancer with a gamified exercise and health education app. Unlike previous studies focusing on survivorship, this evaluation offers a comprehensive understanding of how digital tools can support patients in maintaining physical activity during intensive treatment. Apps with interactive, gamified elements may complement clinical care by providing low-threshold access to exercise and health education. The findings advance the field by identifying key facilitators and barriers to engagement. Future research should assess adherence, behavioral outcomes, and effectiveness in larger samples to inform clinical implementation.