Breast cancer is the most frequent malignancy in women. Standard therapy for early-stage breast cancer is breast conserving surgery (BCS) followed by adjuvant whole breast irradiation (WBI), which usually requires a treatment time of 3-6 weeks. Several randomized controlled trials (RCTs) demonstrated that postoperative WBI after BCS significantly decreased the risk of local (in-breast) recurrence and improved breast cancer mortality as well as survival. However, one of the most common side-effects of radiotherapy is fatigue, which is reported in up to 80% of cancer patients during treatment. Especially in early-stage breast cancer patients it might be the only serious side-effect following adjuvant irradiation, as fatigue often significantly reduces quality of life due to resulting functional impairment and psychological distress. Fatigue additionally has a distinct socioeconomic impact: 75% of patients and 40% of caregivers are forced to change their employment status due to cancer-related fatigue. For reducing treatment-related toxicity, several RCTs consequently addressed the question whether adjuvant WBI could be omitted in early-stage, low risk breast cancer patients treated with endocrine therapy. However, all these trials detected up to seven-time increased local recurrence rates without WBI following BCS. Both, longer duration of radiotherapy and larger radiation field sizes are known to be associated with increase in treatment-related fatigue. Accelerated partial breast irradiation (APBI), delivered exclusively to the original tumor location and not to the surrounding breast tissue, might therefore be an alternative treatment option with fewer side-effects for early-stage, low risk breast cancer patients. Few previous trials have already reported comparable outcomes for highly selected low-risk breast cancer patients for APBI compared to conventional WBI. First results also point out that APBI compared to WBI might be associated with less severity and intensity of fatigue. One method for APBI is single-dose intraoperative radiotherapy (IORT) delivered directly to the tumor after resection. Data is still limited for APBI, hence current international and German guidelines suggest the use of APBI for low-risk early stage breast cancer patients but recommend the application of APBI preferably within a clinical trial.