Dose calculation accuracy of clinical radiotherapy plans using next generation cone beam computed tomography imaging technology.

Background and purpose: Next generation cone beam computed tomography (CBCT) technology has shown improved Hounsfield unit accuracy over standard CBCT, and comparable to that of fan beam CT simulators (CTsim), potentially allowing direct dose calculation on CBCT. In this study, we evaluated the dose calculation accuracy of clinical treatment plans calculated using this technology, compared with standard CBCT. Materials and methods: Thirty subjects with thoracic or upper abdominal cancer were imaged on CTsim, standard CBCT and next generation CBCT, and treated using breath hold techniques. Both CBCT image sets were rigidly registered to CTsim, and clinical treatment plans were forward calculated on all images. 3D Gamma analysis was used to evaluate CBCT dose distributions relative to CTsim, and DVH analysis compared PTV and OAR dose metrics. Results: The median (±IQR) 3 %/3 mm gamma pass rate was 96.7 ± 3.1 % for next generation CBCT and 93.3 ± 14.0 % for standard CBCT. Next generation CBCT gamma scores were statistically significantly higher than standard CBCT for all gamma criteria. Median DVH metrics were within ± 2.3 % and ± 2.7 % of the corresponding values from CTsim, for next generation and standard CBCT respectively. Standard CBCT showed an underestimation of ipsilateral lung dose for breast subjects, while next generation CBCT did not. Conclusions: Next generation CBCT allows for good average DVH agreement with CTsim, and improved dose calculation accuracy over standard CBCT across 3D dose distributions. While further clinical investigations are warranted, this technology may allow for the use of CBCT in direct-dose calculation in adaptive radiotherapy.