Filmless methods for quality assurance of Tomotherapy using ArcCHECK

[Gantry QA Using Three-dimensional Diode Array Detector on O-ring Linear Accelerator]

PURPOSE

It is necessary to perform gantry quality assurance (QA) in high precision radiotherapy. However, the O-ring type linear accelerator (Halcyon) does not have a light field and laser as a reference of isocenter point. The aim of this study is to investigate the usefulness of a three-dimensional diode array detector for gantry angle QA, and an O-ring type linear accelerator.

METHOD

The gantry angle and rotational center were verified using the ArcCHECK 3D diode array on the general linear accelerator (TrueBeam) as a reference and Halcyon. The gantry angles were measured at 0, 90, 180, and 270°. The accuracy of the gantry rotational center was evaluated using rotational irradiation in the clockwise and counterclockwise directions between 181° and 179°.

RESULTS

The QA system with ArcCHECK was able to apply on the TrueBeam and Halcyon. As a result of the accuracy of the gantry angle, the maximum error of value calculated from ArcCHECK was 0.1° compared with the nominal gantry angle of Halcyon. As a result of the accuracy of the gantry rotation isocenter of Halcyon, the distance between the isocenter and the gantry rotation center was 0.45 mm and 0.41 mm in the clockwise and counterclockwise directions, respectively.

CONCLUSION

The QA system with ArcCHECK was useful for the gantry angle and the rotation center accuracy on the Halcyon.

A novel method for monitoring the constancy of beam path accuracy in CyberKnife

The aim of current work was to present a novel evaluation procedure implemented for checking the constancy of beam path accuracy of a CyberKnife system based on ArcCHECK. A tailor‐made Styrofoam with four implanted fiducial markers was adopted to enable the fiducial tracking during beam deliveries. A simple two‐field plan and an isocentric plan were created for determining the density override of ArcCHECK in MultiPlan and the constancy of beam path accuracy respectively. Correlation curves for all diodes involved in the study were obtained by analyzing the dose distributions calculated by MultiPlan after introducing position shifts in anteroposterior, superoinferior, and left–right directions. The ability of detecting systematic position error was also evaluated by changing the position of alignment center intentionally. The one standard deviation (SD) result for reproducibility test showed the RMS of 0.054 mm and the maximum of 0.263 mm, which was comparable to the machine self‐test result. The mean of absolute value of position errors in the constancy test was measured to 0.091 mm with a SD of 0.035 mm, while the root‐mean‐square was 0.127 mm with a SD of 0.034 mm. All introduced systematic position errors range from 0.3 to 2 mm were detected successfully. Efficient method for evaluating the constancy of beam path accuracy of CyberKnife has been developed and proven to be sensitive enough for detecting a systematic drift of robotic manipulator. Once the workflow is streamlined, our proposed method will be an effective and easy quality assurance procedure for medical physicists.