In radiotherapy, the delineations of the volume of the macroscopic spread of the tumor as the Gross Tumor Volume (GTV), and the surrounding volume of the microscopic spread of the disease as the Clinical Target Volume (CTV), are key tasks for a high quality treatment plan. In order to design a software that also supports cognition, software designers need a deeper understanding of the physicians’ cognitive processes and medical context. This paper presents a research about identifying main medical factors relevant for the delineation in the radiotherapy context as a first step in deepening the understanding for software design. Using two discussion formats with six radiation oncologists, we identified 29 medical factors regarding the delineations of tumorous volumes, categorized into: treatment context, tumor context and tumorous areas. In addition, the role of multimodal images, dose planning, and future wishes have been elaborated. These findings could support the software designers in using Evidence Based Software Engineering approach. It is expected that software designed based on the results presented here, is tailored towards the medical context and cognitive needs of radiation oncologists in the delineations of brain tumor, and therefore will improve the effectiveness and efficiency of their work.