Detection of internal organ movement in prostate cancer patients using portal images

Previous research has indicated that the appearance of large gas pockets in portal images of prostate cancer patients might imply internal prostate mo tion. This was verified with simulations based on multiple computed tomogra phy (CT) data for 15 patients treated in supine position. Apart from the pl anning CT scan, three extra scans were made during treatment. The clinical target volume (CTV) and the rectum were outlined in all scans. Lateral port al images were simulated from the CT data and difference images were calcul ated for all possible combinations of CT scans per patient; each scan was u sed both as reference and repeat scan but gas pockets in the reference scan were removed. Gas pockets in a repeat CT scan then show up as black areas in a difference image. Due to gravity, they normally appear in the ventral part of the rectum. The distances between the ventral edge of a gas pocket in a difference image and the projection of the delineated ventral rectum w all in the reference scan were calculated. These distances were correlated with the "true" rectum wall shifts (determined from direct comparison of th e rectum delineations in reference and repeat scan) and with CTV movements determined by three-dimensional chamfer matching. Gas pockets occurred in 2 3% of cases. Nevertheless, about 50% of rectum wall shifts larger than 5 mm could be detected because they were associated with gas pockets with a lat eral diameter >2 cm. When gas pockets were visible in the repeat scan, rect um wall shifts could be accurately detected by the ventral gas pocket edge in the difference images (r = 0.97). The shift of the rectum wall as detect ed from gas pockets also correlated significantly with the anterior-posteri or shift of the center of mass of the CTV (r = 0.88). In conclusion, the si mulations showed that lateral pelvic images contain more information than t he bony structures that are normally used for setup verification. If large gas pockets appear in those images, a quantitative estimate of the position of prostate and rectum wall can be obtained by determination of the ventra l edge of the gas pocket. (C) 2000 American Association of Physicists in Me dicine. [S0094-2405(00)00203-0].