Patients included in our phase II trial of PBI with interstitial HDR brachytherapy are at low-risk of relapse. Implants are positioned during surgery or postoperatively, within 12 weeks. Treatment schedule is 4 Gy twice a day, with a time relapse of at least 6 hours between each fraction, for four days, for a total dose 32 Gy. Before post-operative implantation, all patients undergo a breast computed tomography (CT) scan to visualize and expand the excision cavity and obtain the planning target volume (PTV), i.e. the lumpectomy cavity plus a margin of 1-2 cm around it. Through virtual simulation with a 3D treatment planning system (TPS), we define the implant catheter position. After implantation, in all patients a breast CT scan checked implant geometry. On CT images transferred to a TPS the surgical cavity is outlined and expanded, the PTV defined, and inactive and active source lengths measured.CT-based 3D software is used to identify and reconstruct the catheters, outline and expand the surgical cavity and obtain the PTV. Dwell positions are activated for each catheter, and inactive and active lengths defined.Prescribed dose was 85% of the basal dose. Dwell weights are optimised in the basal points applying the volume and distance method and best values were chosen independently of strategy. A dose-volume histogram that records the volume covered by 100% and 150% of the prescribed dose (V100 and V150) was obtained for each patient. DHI, defined as V150 - V100/ V100 (7), is recognized as the most suitable quality index.