Purpose: This study aimed to compare the effects of a mobile health intervention based on social cognitive theory with standard care on maximal mouth opening, exercise compliance, and self-efficacy in patients receiving proton and heavy ion therapy for head and neck cancer.

Methods: This open-label, parallel-group, randomized, superiority trial involved a self-developed "Health Enjoy System" intervention. We assessed maximal mouth opening, exercise compliance, and self-efficacy at baseline (T₀), post-treatment (T₁), and at 1 month (T₂) and 3 months (T₃) after radiotherapy. Generalized estimating equations were used to analyze differences between the groups over time, with results reported as P values and 95% confidence intervals (CIs).

Results: The study included 44 participants. At T₃, the intervention group showed a 6 mm greater increase in maximal interincisal opening than the control group (mean difference = 6.0, 95% CI = 2.4 to 9.5, P = 0.001). There was also a significant difference in exercise compliance between the groups (mean difference = 31.7, 95% CI = 4.6 to 58.8, P = 0.022). However, no significant difference in self-efficacy was found between the groups.

Conclusion: This study demonstrated that an mHealth intervention incorporating behavior change theory could effectively enhance or maintain maximal mouth opening in patients undergoing proton and heavy ion therapy for head and neck cancer in China. This approach provides valuable support during and after treatment.