Planning Target Volume Management of Uncertainties, Immobilization, image guided and adaptive Radiation Therapy

As a standard, today's radiation therapy is based on CT images which are used for therapy planning. These images are obtained once before therapy starts and serve as a basis to obtain the position and shape of the target volume. As the patient has to be positioned anew for each fraction, deviations of the tumor position relative to the radiation field but also internal motion of the tumor may occur. These deviations lead to uncertainties, which are taken into account by adding a safety margin around the clinical target volume (CTV) to create the planning target volume (PTV). As a standard today, CT-based treatment planning is used, where images are obtained once prior to therapy. The information on tumor position and shape, which is obtained from these images, is used throughout the whole cycle of radiation therapy without any change. This cycle may last several weeks. By repeated imaging of the patient in the treatment position prior to each fraction, the position of the tumor can be assessed and corrected for each fraction. A reduction of positioning uncertainty may be used to reduce the safety margin. This leads to a decreased volume of irradiated normal tissue. A reduced volume of irradiated normal tissue leads to reduced side effects and provides the opportunity of increased tumor control by dose escalation. Before the PTV is reduced, a detailed analysis of the uncertainties for the specific imaging method and radiation technique must be performed.