Ultrasound image guided patient setup for prostate cancer conformal radiotherapy

The radiotherapy planning procedure is achieved using images obtained from computed tomography (CT) or magnetic resonance imaging (MR). These images are realised before the treatment which is performed in several sessions over several weeks. At the beginning of each session, the patient has to be positioned on the treatment couch under the linear accelerator in the same position as during MR or CT imaging and planning, and the organs are assumed to be in the same place. Currently, the methods used for this repositioning are based on the external anatomy of the patient and assume that the internal structures do not move. In this study, we present a new approach, suited to clinical practice, for the automatic repositioning of patients in prostate cancer radiotherapy. It is based on localization by ultrasound images and optical stereolocalization and on a matching with images regenerated in the planning volume. The method exploits a statistical model of the prostate to automatically extract its contours. The first tests in conditions of a radiotherapy session show that the method is able to obtain a patient setup with an accuracy of about 1.4mm.