Ultrahypofractionated Versus Normofractionated Sequential Boost After Whole-breast Radiation Therapy in Patients Treated With Breast-conserving Surgery for Breast Cancer

The ULTIMO trial is a monocentric, prospective, randomised controlled, open-label, phase III interventional clinical trial, with a non-inferiority design, in patients with breast cancer undergoing breast conservative treatment (BCT). After giving informed consent and verifying eligibility, data collection starts and patients will be randomized in one of the following treatment arms: * Standard treatment arm: Breast Conserving Surgery (BCS) takes place, followed by the SoC hypofractionated Whole Breast Radiation Therapy (WBRT) and SoC normofractioned sequential boost (normSEB) of 10Gy over 5 fractions. * Experimental treatment arm: Breast Conserving Surgery (BCS) takes place, followed by the SoC hypofractionated WBRT and experimental ultrahypofractionated sequential boost (ultSEB) of 6Gy in a single fraction. The primary objective of the ULTIMO study is to assess whether the ultSEB RT boost protocol is non-inferior to the current SoC normSEB RT boost protocol. This assessment will be based on the cosmetic outcome of the breasts, while also taking into account quality of life (QoL), the frequency and intensity of (S)AEs of interest (breast pain and fibrosis), and oncological survival.

Inclusion Criteria: * Breast cancer patients referred for WBRT + boost to the lumpectomy cavity * Patients ≥18y * Karnofsky Performance Score \>70%, or ECOG \