Impact of Kilo-Voltage Cone Beam Computed Tomography on Image-Guided Radiotherapy of Prostate Cancer

Abstract

The purpose of this study is to evaluate the dosimetric impact of kilo-voltage cone beam computed tomography (kVCBCT) on image-guided radiotherapy (IGRT) of prostate cancer with Monte Carlo method. In this study, the 10 MV photons from the treatment head and the 125 kV photons from the on-board imager of a Varian Trilogy linear accelerator has been modeled with EGS4 Monte Carlo code. Using multiple source models as beam input, Monte Carlo dose calculations were performed with the EGS4/MCSIM on the real patient anatomy and beam setup. 3D dose distributions were compared and dosevolume histograms (DVH) were analyzed to assess the impact of kVCBCT doses on the patients. Our DVH analyses indicated that during a typical IGRT treatment of prostate cancer, the 10 MV photon beams deposited the most doses to the prostate and the adjacent critical structures and the kVCBCT-induced excessive doses were most significant on the anterior and posterior sides of patient anatomy (about 3.5% of prescribed dose) and at the femur heads (about 3%). Despite the added CBCT doses, the doses to the femur heads, bladder and rectum were still below the limit. Due to kVCBCT, the testicular dose has increased to about 1.2 Gy, up by 330% compared to the regular IMRT without kVCBCT. Reducing the kVCBCT field span from 30 cm to 15 cm in superior-inferior direction will cut the testicular doses from 4.2 cGy to 0.4 cGy per scan. Hence, it is essential to understand the risks of excessive doses to the testes and other critical organs and choose appropriate scanning protocol when using kVCBCT for patients undergoing image-guided radiation therapy of prostate cancer.