The Mind programme for cancer patients was developed by this project PI through the integration of ACT, mindfulness and CFT components specifically adapted to the needs of a cancer population. This intervention aims at improving well-being, preventing subsequent distress, and promoting adaptation to the disease and posttreatment period. A recent pilot study presented preliminary findings on this intervention, suggesting high acceptability and efficacy in improving self-reported psychological health in breast cancer patients undergoing Radiotherapy treatment. Nevertheless, this study's small sample size, methodology (inactive control group), and exclusive reliance on self-reported data limit the interpretation and generalization of results, creating an avenue for the optimization and further testing of the programme through more robust and reliable methods. The aim of this project is therefore to optimize the Mind programme taking into consideration the results from its pilot study and to conduct a Randomized Controlled Trial on the efficacy of the intervention in improving not only mental health outcomes but also biological markers, as well as on its cost-effectiveness, in women with breast cancer. The superiority of the Mind programme will be compared to a support group intervention through the analysis of changes in cancer-specific quality of life, depressive symptoms and anxiety severity, psychological experiences, and immunological and epigenetics markers related to mental health and breast cancer prognosis. All participants will receive the intervention that shows better results.