Purpose: The uRT-linac 506c (United Imaging, China) is equipped with diagnostic-quality 16-slice CT imager, with the coaxial integrated design, sag may occur due to the couch transfer between CT imaging and radiotherapy . The purpose of this study is to explore the couch sag correction accuracy from linac isocenter to CT acquisition positon under different weights.
Methods: Four different loads of 50, 70, 90 and 110 kg were used to simulate the weight of clinical patients, respectively. Design a test plan using QUASAR Penta Guide Phantom in order to simulate the image guiding process. The couch was divided into anterior, middle and posterior area within the CT scan range，the Penta Guide Phantom was placed at each position considering under the same load. Isometric positioning was carried out according to the orthogonal MV Portal image, then we confirm the physical sag value by entering the treatment room after the couch aligned into the CT bore. CT scan with 1 mm slice thickness was obtained and registered with the plan image manually. All three areas have the same verification process and finally the registration differences at the three areas under different loads were compared.
Results: The physical sag difference is among 2mm at different scan positions. With the increase of loads, the difference gradually increases. For the maximum load 110kg the physical sag at anterior, middle and posterior sections is 3.5mm, 2.8mm and 2.2mm, respectively. However, among all registration results, the A/P direction value (with sag correction) did not change by more than 0.2mm.
Conclusion: We believe that the sag is contained in the CT image acquisition due to the coaxial integrated design of uRT-linac 506c. However, studies have shown that the system has a strong sag correction accuracy and will not add this correction to the registration leading to inaccurate treatment.
Funding Support, Disclosures, and Conflict of Interest: Young Scientific Research Program (grant number pumch201910569) of Peking Union Medical College
Image Analysis, Acceptance Testing, Registration