Personalized Prone Immobilization for Accelerated Partial Breast Radiotherapy

INCLUSION Criteria: * Written informed consent according to ICH/GCP regulations before registration and prior to any study-specific procedures * Upfront breast conservative surgery * Radiotherapy planned within 12 weeks after surgery * ECOG/WHO performance status ≤2 * Age ≥40 year For invasive breast cancer only: * Staging pT1-T2, N0M0 (Tumor size ≤3 cm) * If no sentinel lymph node biopsy performed, patient must be clinically N0 * Negative margins (no tumor on ink) For ductal carcinoma in situ only: * Staging pTis (tumor size ≤3 cm) * Negative margins (≥2 mm) EXCLUSION Criteria: * Patient of male sex * Inability to obtain informed consent or adhere to the CIP * Breast skin damage that could prevent VENUS SHELL™ usage according to protocol timeline (e.g. presence of any skin or soft tissue wound that failed to show evidence of progression towards healing, clinical signs of active infection at or immediately adjacent to the intended intervention site, etc.) * Known germline BRCA1/2 mutation * Unable to lay prone * Unable to undergo imaging by CT scan * Documented hypersensitivity or allergy to medical plasters, acrylonitrile butadiene styrene * Any serious underlying medical, psychiatric, psychological, familial or geographical condition, which in the judgment of the investigator may affect patient compliance or place the patient at high risk within the study. * Pregnancy or breast feeding * Claustrophobia * Patient with a body weight exceeding 120 kg * Patient with pendulous breasts that come into contact with the CT table, preventing adequate space for shell usage * Presence of any cardiac implantable electronic device (CIED), including but not limited to a pacemaker, implantable cardioverter-defibrillator (ICD), cardiac resynchronization therapy (CRT) device, or other electronically active implant.