Review of the Medical Physics and Engineering Conference 2019 Poster presentation by Geraldine Verschoor, Nikolaos Margellos, and Will Holmes Smith of Norfolk and Norwich University Hospitals, Norwich, UK. The MPEC 2019 Conference was held in Bristol, UK, in late 2019, and was sponsored by the Institute of Physics and Engineering in Medicine.

Treatment with radiation on a modern linear accelerator relies on the ability to position the patient with incredible accuracy. Therefore it is critically important to verify that the systems used for positioning are all describing the same isocentric point in space as the treatment beam. Treatment couches with 6 degrees of freedom (6DoF) can add additional uncertainty to this system.

The Standard Imaging MIMI (Multiple Imaging Modality Isocentricity) Phantom establishes mechanical stability of the image guidance system by verifying the isocentre described by the MV, kV, CBCT, and other guidance systems is within accepted 1mm tolerances. HexaCheck is an accessory to the MIMI Phantom that allows isocentre rotation using a gimbal system along all three axes of rotation (roll, pitch, yaw) in ±2.5° increments. The gimbal ensures the centre of the phantom will remain at isocentre when there is a rotational offset. This greatly simplifies data analysis and enables commissioning and verification of 6D couch alignment and positioning offsets in a busy clinical environment without the need to write extra software.

In 2018 and 2019, the NNUH commissioned two Varian TrueBeam Linear Accelerators (Linacs) each with a 6DoF couch and reported the following independent study results using the MIMI/HexaCheck to assist with commissioning:

Figure 1: HexaCheck with the MIMI Phantom positioned on the 6DoF couch.

Methods and Materials

First the phantom was CT scanned in the neutral position (i.e. zero rotations) using the H&N protocol and the scan was used as reference for image matching.

Initial tests were carried out to test whether the NNUH’s current method of H&N image matching using kV orthogonal pairs could provide sufficient accuracy for online matching compared to CBCT. The phantom was rotationally offset from the neutral position, then either a CBCT scan or a kV pair was taken and matched to the CT image.

The main part of the commissioning focused on CBCT imaging. The phantom was offset and imaged in every possible combination of rotational offset (27 combinations in total). HexaCheck with the MIMI Phantom positioned on the 6DoF couch.

Online corrections of the 6DoF couch were carried out to correct for the offset, then the phantom was reimaged. The residual error that was recorded gives a measure of the accuracy of the 6DoF couch correction.

Results

The researchers’ initial investigation of the accuracy of using matched pair kV images to correct for rotational shifts showed that the accuracy was significantly lower than using CBCT imaging and not adequate for accurate rotational corrections.

TABLE 1: Corresponding rotational deviations (mean±1SD) deriving from calculations for all 27 different rotational offsets that were tested.

TABLE 2: Corresponding translational deviations (mean±1SD) deriving from calculations for all 27 different rotational offsets that were tested.

The researchers noted the corresponding rotational and translational deviations between the two Linacs presented no significant difference, showing consistency of the results from commissioning procedures and adding value to the accuracy of the Linacs’ 6DoF modality.

In their concluding remarks, the authors observed this commissioning work has shown rotational errors of less than 0.3° and translational errors of less than 1 mm are possible with the 6DoF couch using CBCT imaging. These results are the same order of magnitude as other studies using different phantoms and methods. This level of accuracy is warranted for clinical radiotherapy utilization and the tolerance of the weekly QA of the couch was set at ±0.5°. In contrast, the NNUH’s current method of using kV pairs for H&N was not accurate enough for applying rotational corrections.

For more information, see the full research study here.

Standard Imaging is pleased to present this review of an independent research study using the MIMI Phantom and HexaCheck for commissioning of a 6DoF treatment couch. As a Quality Assurance provider for the Medical Physics community for over 30 years, we pledge to help you in the fight against cancer with all the tools needed to complete your work efficiently and effectively.