Background & Objectives: Cancer is a disease characterized by the uncontrolled growth of abnormal cells. Among all types of cancer, lung cancer has the highest incidence and mortality rates compared with other cancers. This type of cancer typically presents no symptoms in its early stages, which often leads patients to seek medical attention only after the disease has progressed. Cancer not only poses chronic physical risks but also presents significant psychological challenges, increasing the need for psychological support among these patients. Many patients experience concerns about survival, which can negatively impact their mental health and emotional regulation, often resulting in anxiety and depression. The relationship between changes in cancer patients' mindsets and their health–promoting behaviors has gained global importance. Given that cancer has numerous causes and treatment methods, psychological and behavioral factors are as crucial as physical factors. Alongside drug treatments, psychological interventions can effectively address mental health issues. Two key psychological constructs related to the mental health of cancer patients are emotional regulation and tolerance of ambiguity. This study evaluates the effectiveness of acceptance and commitment therapy on emotional regulation and tolerance of ambiguity in patients with lung cancer.

Methods: This quasi–experimental research employed a pretest–posttest design with a control group. The statistical population included all patients with lung cancer who were referred to the Association for the Support of Patients with Lung Cancer in Tehran City, Iran, in 2023. According to the records, 132 men had cancer. Among them, 30 people were selected purposively and randomly assigned to the experimental (15 people) and control (15 people) groups. The inclusion criteria for participation in the study were as follows: diagnosed with lung cancer, completed chemotherapy, provided consent to participate, aged 20 to 50 years, and no mental disorders. The exclusion criteria for participation in the study were as follows: withdrawal from therapy, more than 2 absences from therapy sessions, or concurrent involvement in psychotherapy. The experimental group received treatment based on acceptance and commitment in 8 weekly 90–minute sessions, and the control group did not undergo any psychological treatment during this period. Two participants in the control group withdrew from the study, and two participants in the experimental group were absent for more than two therapy sessions. Ultimately, 13 participants remained in each group. The data were collected using the Ambiguity Tolerance Scale–I (MSTAT–I) (McLean, 1993) and the Emotion Regulation Questionnaire (ERG) (Gross & John, 2003) at two time points, before and after the therapeutic intervention, in two groups. After the research concluded, the treatment program was also implemented for the control group to uphold ethical standards. Data analysis consisted of two parts: descriptive and inferential statistics. Descriptive statistics included frequency, percentage, mean, and standard deviation, while inferential statistics involved univariate analysis of covariance. The analysis was conducted using SPSS version 27, with a significance level of 0.05.

Results: After controlling for pretest scores on ambiguity tolerance, suppression, and reappraisal, the difference between the experimental and control groups at posttest was statistically significant (p < 0.001). Furthermore, the effect size analysis indicated that the intervention accounted for 48.1%, 49.1%, and 70.8% of the variance in ambiguity tolerance, suppression, and reappraisal scores, respectively.

Conclusion: Based on the findings of the present study, acceptance and commitment therapy is effective in increasing ambiguity tolerance and emotion regulation among patients with lung cancer. Therefore, this therapeutic approach may be used in healthcare settings to improve the psychological status of these patients.