This paper proposes the use of an interactive decision support system to guide the treatment planning process for external beam radiation therapy. Based on multicriteria optimisation our research treatment planning software CARINA calculates efficient (also called Pareto optimal) treatment plans. These are stored in a database and accessed for evaluation by the treatment planner. The interactive component consists of navigation among the pre-calculated plans using free search, fine search and exact search as well as sensitivity analysis, which extracts dose dependence information for all structures from the plan database. As a result, plan quality is improved by finding advantageous trade-offs in competing treatment plans, trial-and-error is avoided, and effectiveness of treatment planning is increased.