The present multicenter, retrospective and prospective observational study, aims to evaluate an ultra-hypofractionated whole breast irradiation schedule (WBI, 26 Gy in 5 fractions), in order to confirm literature data (FAST-F study) in the clinical practice. Patient population included women affected by early stage breast cancer (BC), both invasive and ductal carcinoma in situ, receiving ultra-hypofractionated WBI (with or without a tumor bed boost) after breast conserving surgery (BCS). Main exclusion criteria are mastectomy and regional nodal irradiation. Neoadjuvant and/or adjuvant systemic therapies are allowed. The primary otcome is acute and chronic toxicity evaluation. Secondary outcomes are: overall servival (OS), disease-free survival (DFS), rates of local and loco-regional recurrences, distant metastasis occurrence, cosmetic outcome and quality of life (QoL) assessment. Acute and late toxicities will be scored according to the Common Terminology Criteria for Adverse Events (CTCAE) scale version 5.0. Cosmetic assessment will be graded according to the Harvard scale. Frontal photographs of both breasts will be used to evaluate toxicity and cosmetic results. For QoL assessment the EORTC (European Organisation for Research and Treatment of Cancer), QLQ-C30 and EORTC-QLQ-BR23 questionnaires will be administered.