SU‐GG‐T‐377: Evaluation of Cone Beam CT in Prostate IMRT

Purpose: The purpose of this study was to evaluate the volume changes of target and critical organs using cone‐beam CT (CB‐CT) and to investigate their effects on the organ doses for prostate IMRT cases. Previously the feasibility of CB‐CT based treatment planning was demonstrated by researchers (L. Lee et al., 2006; S. Yoo et al., 2008, Int. J. Rad. Onc. Biol. Phys.) Method and Materials: Ten prostate IMRT patients had daily onboard imaging and weekly kV CB‐CT using a Trilogy system for treatment position verification. The latest CB‐CT data were imported into Eclipse treatment planning system and used for drawing CB‐CT based contours for prostate, rectum, and bladder. The volume of the organs in CB‐CT were measured and compared to the organ volumes from the initial CT images. Organ contours were exported to the primary IMRT plan and the plan was rerun using CB‐CT based volumes with the same field parameters. The volume and mean dose changes were measured. Results: The average interval between the first CT and latest CB‐CT was 29 days. The mean volume changes of the prostate, rectum and bladder were −8.3%, 10.8%, and −5.6%, respectively between the first CT and latest CB‐CT for 10 prostate IMRT cases. The dose coverage ratios of CB‐CT based volumes to primary volumes were 99.9%, 111.7% and 100.6% for prostate, rectum and bladder, respectively. Conclusion: The prostate volume got decreased about 8% (n=10) after a month based on kV CB‐CT and the dose coverage remained the same. However, in the average approximately 10% increase in rectal volume and dose were observed. The greater mean volume was measured for bladder with CB‐CT, however, the mean dose was almost equal to the primary plan. Therefore, based on the observation, updated volumes with CB‐CT can be used for improving the rectal dose as needed.