Short treatment time and low toxicity make accelerated partial breast irradiation (APBI) an attractive approach in treatment of early-stage breast cancer with favourable prognostic factors. Increased rates of in-breast relapse, observed with "short-range" APBI techniques, motivates the investigation of external beam radiotherapy (EBRT). In prone EBRT target definition can be optimized using the pre-operative MRI-scan. We will investigate prone MRI-multidetector-CT non-rigid co-registration in defining the surgical borders, tumour bed and glandular tissue as clinical target volumes (CTVs) and validate the imaging and delineation process on Thiel-embalmed cadaver phantoms. In a phase I-II trial, we will test the hypothesis that more accurate CTVs will be suitable for hypofractionated APBI (5 x 5.7 Gy to the tumour bed) in the setting of topographical dose-painting (range 5.0-5.7 Gy/fraction in the glandular breast tissue). This aproach is expected to combine low toxicity with low relapse rates in the breast.