The Breast Cancer Online Workshop is a comprehensive case-based course aimed to promote the use of volume-based planning in breast cancer. As delineation of target volumes is a critical part of radiation therapy planning workflow, the course focuses on hands-on interactive experience with leading experts in the field of breast cancer and radiation oncology. Participants are asked to delineate a case according to ESTRO breast cancer delineation guidelines, and during the course the participants' delineations are compared to the teachers’ delineations while discussing the differences. The course aims to reduce variability and inconsistency in delineation and provides tools to identify the volumes on imaging, mainly CT simulation. At the workshop you will learn about elective nodal irradiation, tumour boost volume, and how delineation matters for the dose to organs at risk, toxicity, and radiation treatment planning. The workshop is a great opportunity to learn tricks and tips for target volume definition and delineation in breast cancer, and discuss clinical challenges in radiation therapy for breast cancer.