Obi system

Data from forty real cases involving 40 patients, including ten head cancer patients, ten chest cancer patients, ten abdominal cancer patients, and ten pelvic cavity cancer patients, were collected at the Affiliated Hospital of the Armed Police Logistics College, Tianjin, China (It has been renamed Characteristic Medical Center of PAP) from December 2012 to December 2016. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the ethics committee of the Affiliated Hospital of the Armed Police Logistics College (No.PJHEC-2016-A18). Informed consent was not required since this was not a treatment study. The planning design and dose calculation were performed on the Varian Eclipse (version 10.0, Palo Alto, CA, USA) treatment planning system (TPS). CIRS 062 and Catphan 504 phantoms were used as in the tests (19 (link)). CBCT image scattering correction was implemented in the CITK software based on Qt 5.6, EGSnrc, the RTK open-source software platform and the MITK open-source software platform (27 (link)). The CBCT slice images were reconstructed online by the Varian OBI system, and the projection images were exported at the same time.

Similar to section “CBCT of a HexaCHECK MIMI Phantom,” the isocentricity test can also be done using 2-dimensional kV imaging of a custom-made phantom. The HexaCHECK phantom provided by the Protura vendor at the time of installation can be used for commissioning, whereas the custom-made phantom can be used as a crosscheck of HexaCHECK and established baselines for subsequent routine QA tests when HexaCHECK phantom is not available in the clinic. The custom-made 7.5 × 7.5 × 7.5 cm cube with a 6-mm sphere at the center of the phantom was used for virtual and physics graticule alignment checks. The 2D kV images were taken with and without known rotations, such as 2° pitch and 1° roll. The isocenters with crosshair display on the kV images were compared with and without rotations. Figure 5 shows the display of Protura workstation that is interfaced with the Varian OBI system. The tolerance of crosshair centering was 1 mm. Figure 6 shows that the Protura system could accurately correct to the couch rotation position (2° pitch and 1° roll) executed with the isocenter as the virtual reference point. In routine monthly QA, a different combination of pitch and roll can be tested besides the commissioning values. For example, any 2 combinations of pitch, roll, and yaw could be sufficient if on a rotational basis (ie, different combination in even months and odd months).

The MC simulation workflow is shown in Figure S2. This operation mainly includes three parts (1). The pCT images and CBCT registration information are obtained, and the pCT images are input. Immediately, each pCT image is preprocessed by the ‘ctcreate’ subroutine of EGSnrc to obtain the initial voxels. The registration information is combined, and the initial voxel is corrected (2 (link)).The scanning model and MC optimization parameters are input, based on which the MC simulation settings are preprocessed (3 (link)). The CBCT modeling parameters and detector modeling parameters are input, and the ‘BEAmnrc’ subroutine is used to construct the CBCT simulation system. Finally, four subroutines are used to perform the simulated imaging, scatter estimation, energy deposition and dose deposition. As shown in Figure S3, first, downsampling of the scattering estimation and projection data is performed. Then, pixel calibration is conducted to establish the relationship between them. The scattering estimation is then extracted from the projection images. Finally, the projection images are upsampled and imported into the Varian OBI system.