The purpose of this randomized intervention study is to investigate the effects and biological mechanisms of a supervised 12-week progressive resistance training on fatigue and immunological and inflammatory biomarkers in breast cancer patients during adjuvant radiotherapy. To determine the effect of the exercise itself beyond potential psychosocial effects due to attention by trainers or the group support, patients in the control group have a comparable training schedule (i.e. 60 min, twice a week, for 12 weeks) but with relaxation training (Jacobsen method).