Intraoperative placement of catheters in the tumor bed during breast-conserving surgery (BCS) enables postponed targeted boost irradiation in high risk breast cancer patients. Twenty-three patients with high risk breast cancer underwent neoadjuvant chemotherapy and multifractionated perioperative brachytherapy as a boost to the tumor bed using three-dimensional (3D) CT-based planning. Plastic catheters for brachytherapy were implanted during surgery and targeted irradiation was delivered in the course of 2-3 weeks. Acute and late toxicities were scored according to the RTOG Common Toxicity Criteria. Cosmetic outcomes were assessed using the Harvard criteria. No major perioperative complications were recorded. Circumscribed wound infection occurred in one patient (4.3%). Only 3 patients (13%) experienced acute skin toxicity Grade 1. We observed no teleangiectasias or pigmentations. The cosmetic outcome at last follow-up visit was rated as excellent/good, in 82.6%, fair, in 13% and poor in 4.4% of patients, respectively. There was no evidence of disease recurrence after median follow-up of 43. 4 months. Systematic integration of the perioperative fractionated 3D CT-based HDR brachytherapy as a boost for patients with breast cancer after BCS is feasible and seems safe. It might be beneficial especially for women with high risk of local recurrence.