SU‐E‐J‐148: Dosimetric Changes During the Course of Image‐Guided Radiation Therapy of Prostate Cancer

Purpose: This work investigates the dosimetric changes due to inter‐factional organ motion during the treatment course for IMRT of prostate cancer with in room CT guidance. Methods: Fifteen prostate cancer patients were recruited for this retrospective study. For each patient, MR and CT images were fused. IMRT treatment planning was performed on the simulation CT images. Inter‐fractional organ motion during the course of treatment was corrected using a Siemens CT‐on‐rails system. The prostate, rectum, bladder and femoral heads were outlined on both the original treatment plan and the subsequent daily CT images for 98 treatment factions by the same investigator. Dose distributions on these daily CT images were recalculated with the isocenter shifts relative to the simulation CT images. The doses from the subsequent daily CTs were co‐registered and compared with the original doses planned on the simulation CT. Results: Our results based on 15 patients (98 daily CT sets) with anatomical matching (i.e., matching the prostate contours from simulation CT scans with the prostate on daily CT‐on‐rails scans) showed that 7.1% of the treatment fractions exhibited poor target coverage (Dmin < 65Gy). For critical structures, 27.6% and 26.5% of the treatment fractions violated our rectal criteria of V65 < 17% and V40 < 35% for the rectum and 10% and 6% of the treatment fractions violated our bladder criteria of V65 < 25% and V40 < 50% for the bladder, respectively. Conclusion: The IGRT procedure for target localization using 3D imaging for prostate cancer is still not ideal if only anatomy matching is used. Future studies are warranted for improvements in target dose coverage using adaptive therapy through re‐planning or other target localization techniques such as that to match prescription isodose surfaces with target volumes to correct for both translational and rotational organ motion.