The Impact of Residual Geometric Inaccuracies on Normal Organ Doses in Image GuidedRadiation Therapy of Prostate Cancer Using On-Board Kilovoltage Cone-eam Computed Tomography

Introduction: The aim of this retrospective study was to evaluate the variations in delivered dose to the bladder, rectum, and femoral heads of prostate cancer patients during a course of treatment by imageguided radiation therapy (IGRT). Materials and Methods: Overall, 15 patients with prostate cancer were selected and. Each week, for each patient five consecutive cone beam computed tomography (CBCT) images were taken after bony anatomy alignment by using two orthogonal radiographic images, as well as CBCT images. Dose distributions and dose volume histograms (DVH) for all the original and CBCT plans were obtained. Maximum, as well as mean doses and volumes of the bladder, rectum, and both femoral heads were recorded for each CBCT plan and compared with the original CT plan. For all the studied body parts, the differences in DVH between CBCT plans and original CT plan were calculated and compared.          Results: Considering all the 75 CBCT images for the 15 patients, average of changes in mean doses and volumes were 17.8%, 41.8%, 7.1%, and 36.8% for the bladder and rectum, respectively. There was a significant (P 0.05) negative correlation between mean bladder dose and volume, while a weak and positive correlation was found between mean dose and volume of rectum in our patients. Conclusion: Our results showed that changes in volumes of the bladder and rectum alter their received interfactional mean doses. Further attention to the volume variations of the bladder and rectum during a radiotherapy course is recommended for more accurate IGRT treatment.