PERFORMANCE OF A VIDEO-IMAGE-SUBTRACTION-BASED PATIENT POSITIONING SYSTEM

Purpose: We have developed and tested an interactive video system that utilizes image subtraction techniques to enable high precision patien t repositioning using surface features. We report quantitative measure ments of system performance characteristics. Methods and Materials: Vi deo images can provide a high precision, low cost measure of patient p osition. Image subtraction techniques enable one to incorporate detail ed information contained in the image of a carefully verified referenc e position into real-time images. We have developed a system using vid eo cameras providing orthogonal images of the treatment setup. The ima ges are acquired, processed and viewed using an inexpensive frame grab ber and a PC. The subtraction images provide the interactive guidance needed to quickly and accurately place a patient in the same position for each treatment session,We describe the design and implementation o f our system, and its quantitative performance, using images both to m easure changes in position, and to achieve accurate setup reproducibil ity. Results: Under clinical conditions (60 cm field of view, 3.6 m ob ject distance), the position of static, high contrast objects could be measured with a resolution of 0.04 mm (rms) in each of two dimensions . The two-dimensional position could be reproduced using the real-time image display with a resolution of 0.15 mm (rms). Two-dimensional mea surement resolution of the head of a patient undergoing treatment for head and neck cancer was 0.1 mm (rms), using a lateral view, measuring the variation in position of the nose and the ear over the course of a single radiation treatment. Three-dimensional repositioning accuracy of the head of a healthy volunteer using orthogonal camera views was less than 0.7 mm (systematic error) with an rms variation of 1.2 mm. S etup adjustments based on the video images were typically performed wi thin a few minutes. The higher precision achieved using the system to measure objects than to reposition them suggests that the variability in repositioning is dominated by the ability of the therapist to make small, controlled changes in the position of the patient. Conclusion: Using affordable, off-the-shelf technology, we have developed a patien t positioning system that achieves repositioning accuracy normally ass ociated with fractionated stereotactic systems. The technique provides real-time guidance and can be used to easily and quickly correct pati ent setup before every treatment, thus significantly reducing overall random positioning error. This improved positioning capability provide s the precision required to realize the potential gains of conformal r adiotherapy. (C) 1997 Elsevier Science Inc.