Effects Of The Setup Error On The Dose Distribution For Carcinoma Of Head And Neck With Intensity Modulated Radiation Therapy By Cone-beam CT

Objective:To evaluate the effect of the setup error on the physics dose distribution of targets and peripheral organs at risk (OAR) undergoing the intensity modulated radiation therapy for carcinoma of head and neck. Methods:This study includes two stages:1) 15 patients with carcinoma of head and neck treated by Varian IGRT. The On-Borad-Imager of Varian 23EX linear accelerator was used to collect the images of Cone Beam Computerized Tomography (CBCT) after initial setup,after re-positioning and after radiation delivery,3CBCT scans were collect everyweek for one patient and there were 15 patients received CBCT scans A total of 240 CBCT scans were obtained,which were matched with the positioning CT images before radiation treatment through target center.The Varian software was used to calculate the error after initial setup,after re-positioning and after radiation delivery in x (left-right), y (cranial-caudal) and z (ventral-dorsal) direction,all the initial setup errors were corrected.Note all the setup errors,Then the treatment planning system of Eclip se7.3Version was used to study the effects of the setup error on the dose distribution undergoing the radiation therapy for carcinoma of head and neck. Results:1)15 patients received a total of 80 CBCT scans after initial setup. The systematic±random error in x, y,and z direction was (0.6750±2.0238),(0.3375±1.5089),(0.4500±1.7495) mm, respectively.All the setup errors were corrected, and after the correction both the systematic errors and random errors were obviously lower than those before correction.2)The extent of dose variation for GTV, left lens, right lens, spinal cord,and brainstem was-1.82%～1.21%,-11.81～14.26,-12.08～17.25,-3.94%～12.57%,-6.76～12.05%。Conclusion:This study shows as follows:1) Real-time validate in patients with carcinoma of head and neck by CBCT can reduce the setup error. And the setup error between the treatment is small.2)The setup errors were small in IMRT for carcinoma of head and neck,but it resulted in decreased coverage of the target and increased doses to the organs at risk.The OBI-CBCT system could revise the setup error in real time,which could reduce the systematic and random error and improve the accuracy of setup and dose of targets and OAR. As a result, it could provide better quality assurance in radiation therapy.