Purpose: In this study we evaluated the day to day prostate displacement during radiation therapy by using implanted radiopaque fiducials and daily image guided position verification.
Methods: The data of 10 patients that received radiation therapy to the prostate were analyzed. Three fiducial markers were implanted in the prostate before treatment initiation for everyday verification of the target's position. Daily X ray images (kilovolt/KV films) of the pelvis were acquired for verification and were matched with baseline images produced during treatment preparation using bony structures and fiducials as landmarks. We calculated the mean difference between the two methods and the prostate displacement derived from these measurements.
Results: A total of 208 KV films were obtained. Our results showed a non-uniform prostate motion, with most of the displacements observed in the caudal direction followed by anterior, posterior, cranial, right and left. The mean target motion in each of the above directions was 3.5 mm, 3.5 mm, 3.3 mm, 3.9 mm, 2 mm and 2.4 mm. Based on the cumulative frequency of the target's displacement, a margin of 8 mm, 7mm, 5 mm, 4 mm, 9 mm and 7 mm in the anterior, posterior, left, right, cranial and caudal direction respectively would account for 95% of prostate's motion, provided that every day KV image guidance is performed.
Conclusion: A non-isotopic margin of 8 mm, 7mm, 5 mm, 4 mm, 9 mm and 7 mm around the prostate can be considered safe for treatment delivery.