Setup Errors And Dosimetric Impact Of Online Correction Via Cone-beam CT-based Image Guidance For Thoracic Carcinoma Radiotherapy

Objective: To study the role of KV CBCT on the geometrical accuracy of three-dimensional conformal radiotherapy (3D-CRT) and intensity modulated radiotherapy (IMRT) and to analyze the effect of the setup error on the physics dose of targets and peripheral organs at risk (OAR) for thoracic carcinoma.Methods and materials: 23 patients with thoracic carcinoma were choosed at random. Varian-IX linear accelerator on-board imaging system (OBI) was used to acquire CBCT scans in three-dimensional conformal radiotherapy or intensity modulated radiation therapy before delivery. The right- left (x), superior-interferior (y), anterior-posterior(z) set-up datas of patients can be obtained from the tomography images automatically restructured by the system. And then CMS TPS was used to analyze the physics dose of targets and peripheral organs if the isocenter was replace by the actual location which obtained from the CBCT scan.Result: According to 182 CBCT scans the systemic±random error on x,y,z directions were ( -0.181±3.3349 ) mm, ( -0.429±5.3993 ) mm, (-2.637±2.1518)mm respectively and the absolute maximum was 13mm, 21mm, 8mm respectively. Within 3mm and 5mm of which accounted for 64.8%,50.5%,48.9% and 93.9%,71.4%,81.9% respectively. There was only 1.6%,8.2%,0% if setup errors more than 10mm. Target dose reduced with respect to treatment plan, mean and standard deviation of treatment dose discrepancy from the plan were 89.7%±10.5%, 97.9%±17.9%, 95.5%±5.0%, 98.8%±1.2% for PTV D(95%), PTV D(mean), GTV D(95%), and GTV D(mean). Mean and standard deviation of the difference in normal organ dose were 102.7%±6.7%, 103.1%±8.7%, 105.0%±15.3%, 110.6%±40.0%, 103.0%±9.9%, 98.1%±6.8% for the volume of lung accept 5Gy, 10Gy, 20Gy, 30Gy, the mean dose of lung, the maximum dose of 1cc spinal cord. The difference was significant between PTV D(95%). Then patient were divided into three group, setup error on x,y,z directions less than 3mm, within 3mm and 5mm, more than 5mm. The deviation was significant in the group which discrepancy more than 5mm.Conclusion: It is critical important for the first CBCT before treatment. The delivery of this group is not highly accurate. It was more obvious in the superior-interferior directions. There was significantly difference if setup errors more than 5mm in any direction. However, treatment dose to normal tissue can vary regardless of the correction. Normal tissues should be considered during target registration, according to target proximity.