SU‐E‐T‐818: Analysis of Internal Margin Based on Organ Deformation for Prostate IMRT

Purpose: The purpose of this study was to predict organ deformation and the possible CTV‐ITV reduction when adopting daily IGRT for prostate cancer patient. Further, to identify any intra‐fraction time trends in organ deformation. And to analyze internal margin for prostate IMRT based on organ deformation. Methods: It was intended to investigate 30 patients treated with prostate IMRT. The pre‐disposal in radiotherapy was implemented by instructing each patient to drink 500 cc of water each time after defecation and urination. CTimages for radiation treatment planning were acquired 3 times at 10‐minute intervals for 30 minutes after drinking water. CTimages were transferred to treatment planning system. One physician contoured the prostate, rectum, and bladder for all images. The acquired three‐dimensional contour data of the organ were transferred to a customized data processing program. Then, the prostate, rectum and bladder were divided into eight segments. These regions were analyzed with respect to deformation of the entire set of contour points in the 3‐dimensional outline surface in the region of each of the organs. Results: The deformation of the prostate tended to become amplified in the extension direction accompanied by an increase in deformation of the bladder to the anterior and superior direction. But there were not correlated the rectum deformation. The internal margins which not considered organ deformation, were 2.27 mm, 1.76 mm, and 0.99 mm in the L‐R, S‐I, and A‐P direction respectively. However, respect to the organ deformation, these margins were 1.27 mm, 1.56 mm, and 1.53 mm respectively. Conclusions: This analysis based on organ deformation was found to be useful for determination of internal margin. And it will be possible that the extra margin is reduced.