Author: Yuchao Hu, Yajun Jia, Zhangmin Li, Yong Sang, Jianan Wu, Man Zhao 👨🔬
Affiliation: Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Department of Radiation Oncology, Guangzhou Concord Cancer Hospital, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital， Chinese Academy of Medical Sciences and Peking Union Medical College 🌍
Purpose: To evaluate the imaging dose of 4D-CBCT in patients treated with thoracic radiotherapy.
Methods: A model of the Elekta XVI imaging system was created in TOPAS software. Percentage depth dose (PDD) and profiles were measured and compared with Monte Carlo (MC) calculations to verify the model. Absolute dose measurement using a calibrated ionization chamber was used to calibrate the model. 8 lung cancer patients were selected for imaging dose assessment. For each patient, the imaging doses were calculated using the XVI log file, simulating CT and the XVI model. Organ at risk (OAR) dose metrics and their increases when considering the imaging dose were calculated using MIM software.
Results: Deviations between MC calculations and measurements for PDD and profiles were within 1.1% and 5%, respectively. For a pre-treatment 4D-CBCT scan, the mean lung and heart doses were 9.5±1.4mGy and 9.7±1.4mGy, and the maximum spinal cord, esophagus and ribs doses were 17.6±4.2mGy, 17.1±3.3mGy and 37.8±5.3mGy, respectively. For an in-treatment 4D-CBCT scan, the dose metrics were 5.6±1.8mGy, 5.6±1.5mGy, 10.3±3.5mGy, 10.3±4.3mGy and 22.4±5.3mGy, respectively. For patients receiving 5-fraction treatments and one pre-treatment and one in-treatment 4D-CBCT for each fraction, the increases in lung V20, lung V5, mean lung and heart doses, maximum spinal cord, esophagus and ribs doses were 0.04±0.03%, 0.29±0.23%, 76.3±14.4mGy, 76.5±12.0mGy, 76.3±20.2mGy, 70.5±13.1mGy and 101.2±20.5mGy, respectively.
Conclusion: The imaging doses of pre-treatment and in-treatment 4D-CBCT were calculated using a validated MC model. The OAR dose metrics for in-treatment 4D-CBCT showed a relatively significant linear relationship with the beam MU. There is a significant linear relationship between the increase in OAR dose metrics and the number of fractions. For 5-fraction treatments, the increases in OAR dose metrics were insignificant. There may be an apparent increase in lung V5 when using 4D-CBCT in 25-fraction treatments.