3D-Scansystem for Patient-Positioning in Radiotherapy

Abstract

C. Gaisberger 1),2), H. Weichenberger 2), H. Deutschmann 1),2), F. Sedlmayer 1),2)

1) Universitätsklinik für Radiotherapie und Radio- Onkologie, Gemeinnützige Salzburger Landeskliniken Betriebsges. m.b.H., Salzburg, Österreich
2) Institut für Technologieentwicklung in der Strahlentherapie, radART - research and development on advanced radiation technologies, Paracelsus Medizinische Privatuniversität, Salzburg, Österreich

Introduction

In radiotherapy it is fundamental to position the patient within millimeter accuracy to irradiate the tumor area exactly. Up to now line lasers and skin tattoos are used to position the patient, but this method has a limited accuracy. To prevent incorrect treatment the patient position must be approximately equal to a reference position, so that healthy tissue is spared. Scientists of the University Clinic for Radiotherapy und Radio-Oncology in Salzburg developed a 3D-scansystem to acquire a surface contour of the patient prior and during the daily treatment session, to ensure the correct position of the patient. Furthermore, the system will help to improve the documentation and allows to implement gating and tracking applications. The patient positioning system is integrated in the In-house developed software “open radART”.

Materials and Methods

The principle of topometry together with the help of structured light is a well proven technique in industry to receive 3D structures. To scan a patient surface, a gray coded and phase-shifted stripe-light is projected by a commercial video beamer. A camera with a well defined position acquires twelve pictures from every different stripe-projection.  Image processing of the decoded images and applying geometrical calculations result a vertex-cloud. With the help of the “Delaunay-Triangulation”, the points are connected to triangles, optimized with VTK and visualized by means of OpenGL. To reduce shadowing effects two systems are used. The resulting 3D scans are optimized for the different scandirections and merged together. During CT-scan acquisition, a reference scan of the patient’s skin is performed. This reference scan can be used for following registration tasks. To equalize two different point-sets the “Iterative Closest Point” algorithm computes a translation vector and a rotation matrix. If the patient position is found to be incorrect, it is possible to reposition the treatment couch by linear accelerator controls. The deviation from the reference value can be shown in a color-coded 3D-representation. During treatment a simplified scan will be performed and compared with the start values. If the patient movements exceed a predefined threshold, the 3D-scansystem detects the movement and interrupts the treatment. So it is possible to prevent the patient from being treated with incorrectly aligned beams. To calibrate the system a 2D-calibration phantom is needed. This moveable phantom is aligned at the LinAc’s isocenter. For calibration of the 3D-scansystem the “Calibration-Method from Tsai” is used. World coordinates from the calibration phantom and CCD coordinates from the detected points are required for this method. It calculates different geometrical parameters of the camera like orientation, position, focus and the radial distortion of the lens. To calibrate the beamer, the same method is used inversely.

Results

The time needed to perform a scan from one side with the highest resolution (~500.000 points) is about 2.2 seconds. Two scans from different sides to reduce shadowing effects double the scantime. To verify the correct patient treatment position during a radiotherapy session, the faster scan with 200 points takes less then 0.2 seconds. The accuracy of the system in the fiel of view amounts to approximately 1mm. In a scan with both systems there will be created a composite structure in a few seconds computing. This structure is generated, while preserving the maximum physical accuracy, at any point of the individual structures.

Conclusion

The 3D-scansystem would offer a multifunctional solution to improve radiotherapy concerning safety and precision. For instance the system can assist to track a lung carcinoma, and therefore safety margins can be reduced. To increase the performance, two systems are installed, in order to reduce shadowing effects. This system will be implemented alternatively to the “classical” patient positioning workflow by means of skin tattoos and line lasers at the University Clinic for Radiotherapy und Radio-Oncology in Salzburg.

 

   
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