PLANNING TARGET VOLUME MARGIN DETERMINATION IN VMAT PROSTATE REGION USING CBCT

Abstract

Backgrounds: In advanced radiation therapy technique, the determination of adequate clinical target volume (CTV) to planning target volume (PTV) margin is mandatory to reduce dose and side effect to normal tissue meanwhile increasing the dose to the tumor. Objective:The purpose of this study is to determine PTV margins for prostate region in volumetric modulated arc therapy (VMAT) based on inter and intra-fraction motion using cone beam computed tomography (CBCT) images. Materials and Methods: First, the QA for couch and imaging system were performed. Then 15 prostate patients who treated with TrueBeam linear accelerator were acquired weekly CBCT image before and after treatment and the CBCT images were registered to CT-simulator images with bony anatomy and natural calcium matching. The position deviations from standard image in X, Y and Z directions were recorded. The CTV to PTV margins were calculated using Van Herk’s equation according to random and systematic errors approach. Results: The mechanical test of couch movement was very accurate within 0.2 mm error. The image quality of CBCT with pelvis protocol was good enough for IGRT due to passing all of the Varian criteria needed. The software for image registration was also in good agreement between known shifted values and calculated from the program with the maximum error of 0.6 mm. For clinical application, patient setup variations as inter-fraction motion have greater effect than patient movement during treatment as intra-fraction motion because of the patient fixation used and short time in VMAT treatment. The higher values in random error than systematic error were demonstrated because the high accuracy of machine itself with good IGRT system can reduce the systematic error; in contrast, the random error was unavoidable, especially from the effect of bladder-rectum filling. From 8 mm margin in our routine protocol at King Chulalongkorn Memorial Hospital, the calculated PTV margins in the lateral (X), longitudinal (Y), vertical (Z) directions were reduced to 6.38, 5.24 and 6.33 mm, respectively. The Y direction is less effect from bladder and rectum filling and body change compared to other directions. Conclusion: From our calculated margins, it is possible to reduce the dose to bladder and rectum and improve the target coverage of prostate cancer patients who is treated with VMAT technique.