Objective: To compare the effectiveness of Internet-based acceptance and commitment therapy (iACT) and bibliotherapy-based ACT (bACT) in reducing anxiety and depression among women with breast cancer. Methods: In this three-arm randomized controlled trial, 99 participants were recruited from three oncology centers in Iran and randomly assigned to iACT, bACT, or a control group (n=33 each). Interventions lasted 6 weeks with a 4-week follow-up. The Beck Anxiety Inventory and Beck Depression Inventory-Short Formwere used to assess outcomes at baseline, post-intervention, and follow-up. Data were analyzed using generalized estimating equations (GEE). Results: The iACT group showed significantly greater reduction in depression and anxiety than the control group at postintervention and follow-up (all p < 0.001, except for post-intervention anxiety in iACT group was p = 0.001). The bACT group showed delayed but significant improvement in anxiety at follow-up (p > 0.001), but not in depression. Between-group comparisons revealed superior outcomes for iACT over bACT in both anxiety and depression post-intervention, though the difference in anxiety diminished by follow-up. Conclusions: Internet-based ACT was more effective than bACT or standard care in reducing anxiety and depression in breast cancer patients. These results support the broader use of guided Internet-based ACT interventions in oncology settings. Trial Registration: Iranian Registry of Clinical Trials: IRCT20231209060301N1.
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