A STEREOPHOTOGRAMMIC SYSTEM TO POSITION PATIENTS FOR PROTON THERAPY Neil Muller Department of Mathematical Sciences, University of Stellenbosch, Stellenbosch, South Africa Evan de Kock, Ruby van Rooyen and Chris Trauernicht Medical Radiation Group, iThemba Labs, Somerset West, South Africa Keywords: Stereo vision, proton therapy, visually guided robotics. Abstract: Proton therapy is a successful treatment for lesions that are hard to treat using conventional radiotherapy, as the radiation dose to nearby critical structures can be tightly controlled. To realise these advantages, the patient needs to be accurately positioned, and monitored during treatment to ensure that no motion occurs. iThemba LABS uses a fixed beam-line, and moves the patient using a suitable positioning device. In this paper, we discuss several aspects the stereo vision based system used to both determine the position of the patient in the room, and to monitor the patient during treatment. 1 INTRODUCTION Proton therapy is a useful treatment method as the dose distribution properties of the proton beam al- low higher doses to be delivered to the target volume with lower doses to the surrounding tissue. Due to the high cost associated of treatment, it is often reserved for lesions that are difficult to treat with conventional radiotherapy techniques, such as inter-cranial lesions or lesions close to critical structures (see for example (Webb, 1993)). iThemba LABS 1 has been involved with proton therapy for over ten years. Due to cost restrictions, iThemba LABS uses a fixed beam-line to deliver the proton dose, and a motorised chair with 5 degrees of freedom to position the patient (see (Jones et al., 1995)). A second treatment vault is being built that uses a robot-controlled manipulator to position the pa- tient. The position of the patient during setup and treatment is monitored by a number of cameras and stereo techniques are used to calculate the patient’s position. A critical issue is the high accuracy re- quired. 1 Laboratory for Accelerator Based Science 2 PROTON VAULTS AT ITHEMBA LABS The treatment environment (see Figure 1), shows the treatment layout. We have 9 cameras observing the patient during treatment. During a treatment session, at least 3 cameras will be used to calculate the position of the markers on the mask (see Figure 2(a)). The cameras are positioned around the beam isocenter at an average distance of 2m from the isocenter. The positions were chosen to maximise the volume visible to the cameras while not obstructing activities in the vault. The position of the target volume is determined from the CT scan used to plan the treatment. So that the position of the target volume can be related to the mask, the patient is scanned wearing the mask. When the patient is in the reference position for the scan, a stereo system, calibrated to CT scanners coordinate system (see (de Kock et al., 2002)), determines the position of the markers on the mask. Thus the rela- tionship between the target volume and the markers on the mask is known. The stereophotogrammetric (SPG) system is de- signed to be used in both vaults, and the control systems for the chair and the robot are designed to present a common interface to the SPG system. The 538 Muller N., de Kock E., van Rooyen R. and Trauernicht C. (2007). A STEREOPHOTOGRAMMIC SYSTEM TO POSITION PATIENTS FOR PROTON THERAPY. In Proceedings of the Second International Conference on Computer Vision Theory and Applications - IU/MTSV, pages 538-541 Copyright c  SciTePress