Intensity modulated radiotherapy (IMRT) is a new technology in external beam radiotherapy that allows the shaping of individual radiation fields. High doses to sensitive organs surrounding the tumour can thus be avoided. However, the standard application of many radiation fields together can create complex dose distributions. The inverse treatment planning process is based on trial-and-error and unable to account for the many trade-offs that have to be made regarding dose distribution to tumour and organs. In this book Ines Winz shows how an interactive decision support system can guide the treatment planning process. Based on multi-criteria optimisation the planning software pre-calculates Pareto-optimal treatment plans which are stored in a database. The therapy planner is guided in navigating interactively among these plans by a number of search methods and sensitivity analysis. 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.