Room: Exhibit Hall | Forum 4
Purpose: To implement electronic portal imaging device (EPID) in the rotational gamma system (RGS) and investigate its applications on quality assurance (QA) for isocenter and beam collimation check.
Methods: EPID system was integrated in a rotational gamma system, CybeRayTM which is a stereotactic radiotherapy system with multiple radiation sources focusing to the isocenter. EPID was installed in the opposite side of the machine to the treatment head with proper shielding. The systemâ€™s isocenter was checked with EPID by taking images of a small tungsten sphere in different gantry angles. The isocenter location is figured out based on the distance of the sphere center to the beamâ€™s center on EPID images. The beam collimation check was done by take EPID images for all the collimators. Consistency check on beam size and location were performed with template matching/grouping technique and back-tracing algorithms. A software system to perform these tasks was implemented to realize daily QA for both collimator verification and isocenter check.
Results: The isocenter check was compared with film measurement. They consistently agreed with each other with a difference less than 0.5mm. Collimator verification cannot pass for either incorrect collimator type (low similarity) or off-positioned collimator (large offset). It passes only when correct type of collimator is at the right position. For the most common case that collimator is off-position, the system error of the embedded algorithm is less than 0.1mm, indicating recognizing an offset of 0.5mm can be guaranteed. It was further validated by an experiment where four of the seven collimators were purposely offset by 0.5mm. The mean error of the calculated offset is only 0.033mm.
Conclusion: EPID system was integrated in a rotational gamma system successfully. Its applications on automatic isocenter and beam collimation check are feasible and practical. EPID can be used in RGS for automatic QA.