Purpose: Although there is evidence that breast cancer patients benefit from exercising during treatment, exercising during radiotherapy and especially the effects on upper-limb dysfunctions have been infrequently assessed. Therefore, we primarily aimed to confirm our interventions’ feasibility and secondarily aimed to affect upper-limb dysfunctions and fatigue. Methods: Twenty-two breast cancer patients scheduled for radiotherapy were allocated to an intervention (IG) or a passive control group (CG) as they preferred. IG exercised 3×/week during 6 weeks of radiotherapy: cycling endurance, handheld vibration, and balance training. We documented adverse events and training compliance (feasibility) and assessed the range of shoulder motion (ROM), isometric hand grip strength, vibration sense on the first metacarpophalangeal joint of the affected upper limb, and fatigue. Results: We observed no adverse events and a training compliance of 98 %. IG’s ROM improved significantly (abduction: 11°; 95% confidence interval [CI] 5 to 20; external rotation: 5°, 95% CI 0 to 10), as did the hand grip strength (1.6 kg, 95% CI −0.6 to 3.1), while CG’s ROM did not change. CG’s vibration sense worsened (−1.0 points, 95% CI −1.5 to −0.5), while IG’s remained stable. Changes in general fatigue levels between IG (−2.0 points, 95% CI −3.0 to −1.0) and CG (0.5 points, 95% CI −1.0 to 4.5) revealed significant differences ( P = .008) Conclusions: Our intervention proved to be feasible and provides novel findings: it reduced fatigue levels and interestingly, handheld vibration exercises improved upper-limb function due to shoulder ROM, hand grip strength, and vibration sense.