This Small Business Innovation Research Phase I project is aimed at the creation of clinically-viable case-based Radiation Therapy Planning Assistant, and a tool for constructing other case-based intelligent assistants for medical tasks. The key innovations are the ability of the tool to reason about medical images and to develop its initial case base from existing archives of medical data, both textual and multimedia. The project will proceed along two fronts. First is the further development and clinical evaluation of the existing Roentgen radiation therapy system (3). By the end of Phase II, CaseBased Systems will have tested this system in a clinical setting and will have developed a personal computer-based version of it. The second direction of development is to generalize the techniques of Roentgen into a case-based reasoning shell. CaseBased Systems anticipates that such a tool will make it build to construct many other case-based assistants similar to Roentgen for other medical domains, particularly those that require visual reasoning, such as interpreting medical images.