Effect of a group-based acceptance and commitment therapy (ACT) intervention on illness cognition in breast cancer patients

•This quasi-experiment examined a four-week group-based ACT for breast cancer patients in China.•Primary outcome variables were illness cognition, psychological flexibility, and quality of life.•These variables were measured at pre-, post, and 1- and 3-month follow-ups.•Effects of ACT on these variables were compared to those of a health education (HE).•The ACT group showed a greater rate of improvement than the HE group.