Roentgen is a case-based aid to radiation therapy planning. It relies on an archive of past therapy cases to suggest plans for new therapy patients. Roentgen supports therapy planning by: 1) retrieving the case which best matches the geometry and treatment constraints of the new patient; 2) tailoring the plan to the specific details of the patient; 3) evaluating the results of applying the plan; 4) repairing the plan to avoid any discovered faults in treatment results. This final plan is the system's suggestion to the human planner. Roentgen breaks new ground in solving problems in a domain dominated by spatial reasoning and the satisficing of constraints.