Target position variability throughout prostate radiotherapy

Evaluation of the validity of a convolution method for incorporating tumour movement and set-up variations into the radiotherapy treatment planning system

Modern radiotherapy techniques have developed to a point where the ability to conform to a particular tumour shape is limited by organ motion and set-up variations. The result is that dose distributions displayed by treatment planning systems based on static beam modelling are not representative of the dose received by the patient during a fractionated course of radiotherapy. The convolution-based method to account for these variations in radiation treatment planning systems has been suggested in previous work. The validity of the convolution method is tested by comparing the dose distribution obtained from this convolution method with the dose distribution obtained by summing the contribution to the total dose from each fraction of a fractionated treatment (for increasing numbers of fractions) and simulating random target position variations between fractions. For larger numbers of fractions (approximately or > 15) which are the norm for radical treatment schemes, it is clear that incorporation of movement by a convolution method could potentially produce a more accurate dose distribution. There are some limitations that have been identified, however, especially in relation to the heterogeneous nature of patient tissues, which require further investigation before the technique could be applied clinically.

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 motion. This was verified with simulations based on multiple computed tomography (CT) data for 15 patients treated in supine position. Apart from the planning CT scan, three extra scans were made during treatment. The clinical target volume (CTV) and the rectum were outlined in all scans. Lateral portal images were simulated from the CT data and difference images were calculated for all possible combinations of CT scans per patient; each scan was used 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 wall in the reference scan were calculated. These distances were correlated with the "true" rectum wall shifts (determined from direct comparison of the rectum delineations in reference and repeat scan) and with CTV movements determined by three-dimensional chamfer matching. Gas pockets occurred in 23% 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 lateral diameter > 2 cm. When gas pockets were visible in the repeat scan, rectum 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 detected from gas pockets also correlated significantly with the anterior-posterior shift of the center of mass of the CTV (r=0.88). In conclusion, the simulations showed that lateral pelvic images contain more information than the 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 ventral edge of the gas pocket.