Quantifying Imaging Radiation to Patient from Optimized Procedures with LIVE System during SBRT Lung Treatment

Abstract

Tumor control and normal tissue complication probabilities are known to be highly correlated to the target localization accuracy in radiation therapy. The Limited-angle Intra-fractional Verification (LIVE) system was developed to track tumor movement for patient positioning verification during SBRT. However, the 4D MV/kV imaging involved results in additional radiation dose to patients. As such, the purpose of this study is to evaluate the additional imaging radiation dose from optimized MV/kV image acquisition in the LIVE system and to determine if it exceeds the AAPM TG-180 report image dose threshold. A medical linear accelerator with a fully integrated system for image guidance was assessed. Monte Carlo simulated kV and MV beams were calibrated and then used as incident sources in an EGSnrc Monte Carlo dose calculation in a CT-image-based patient model. In three representative lung SBRT treatments evaluated in this study, tumors were located in the patient’s posterior-left lung, mid-left lung and right upper lung. The optimized imaging sequence comprised of arcs ranging from 2-7, acquired between adjacent 3D/VMAT beams, with multiple simultaneous kV (125 kVp) and MV (6 MV) image projections in each arc, for different optimization scenarios. The MV imaging fields were generally confined to the treatment target, while kV images were acquired with a normal open field size and full bow-tie filter. In a 7-arc image acquisition case (highest imaging dose scenario), the maximum kV doses to 50% of the tissue volume (D50 from DVH’s), for spinal cord, right lung, heart, left lung and the target, were 0.4 cGy, 0.4 cGy, 0.6 cGy, 0.7 cGy and 1.4 cGy, respectively. The corresponding MV imaging doses were 0.1 cGy to the spinal cord, right lung, heart and left lung, and 11 cGy to the target. In contrast, the maximum radiation dose from two cases treated with two field VMAT and 2-arc image acquisitions, is approximately 30% of that of the 7-arc acquisition. We have evaluated the additional radiation dose resulting from optimized LIVE system MV/kV image acquisitions in two best (least imaging dose) and one worst (highest imaging dose) lung SBRT treatment scenarios. The results show that these MV/kV imaging doses are comparable to those resulting from current imaging procedures used in IGRT and are within the dose threshold of 5% target dose as recommended by the AAPM TG-180 report.