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Verification of Setup Accuracy with 6 Degree of Freedom Couch and CBCT

T Chen*, H Wang, L Hu, J Zhang, D Barbee, J Xue, NYU Langone Medical Center, New York, NY


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: This study was to investigate the setup accuracy for automatic rotational corrections using 6 degree of freedom (6DoF) couch and online cone beam computerized tomography (CBCT).

Methods: A commercial phantom (BAT phantom) with tissue and air equivalent materials was scanned with CT Sim at 1.25mm plane spacing and 0.98mm pixel size. A 3D plan was generated in Eclipse for the spherical target of 3cm diameter located at the center of the phantom. Several lowdensity structures were also outlined as organ at risks (OARs) surrounding the target. The phantom was aligned to the treatment position on the 6DoF couch of a Varian TrueBeam with CBCT verification for both target and OARs outlined before varied rotational errors were introduced in pitch and roll using a titled supporting platform and verified by a calibrated digital level. Following CBCT imaging, the setup corrections were calculated online using the 3D-to-3D auto-matching function on Truebeam and compared against the actual rotational error. We analyzed the deviations at different levels of error.

Results: The auto-matching on TrueBeam appeared of good quality even for low contrast and small structures. Deviations of the rotational corrections calculated with 6DoF from the measured are summarized in Tables I-III. The max difference in rotation is 0.7° for both pitch and roll. However, it also has been observed that there were deviations in every degree of both translational and rotational freedom, which may lead to over-correction in clinic.

Conclusion: 6 DoF couch and auto-matching offers the convenience for automatic corrections of patient setup in conjunction with CBCT imaging. This preliminary study using a phantom revealed the uncertainties of rotational correction by 6DoF without a deformation of the image. Further study is warranted to establish the criteria for the clinical applications of the auto correction with 6DoF and for the necessary physics QA.


Setup Verification, Cone-beam CT


IM- Cone Beam CT: Quality Control

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