Optimized water phantom brings streamlined commissioning to ring-shaped radiotherapy systems
Original article published in Physics World
The THALES 3D SCANNER from LAP has been specifically designed for acceptance, commissioning, and quality assurance of bore-type image-guided linacs such as Varian’s popular Halcyon machine
After 30 years of outright domination in the radiotherapy suite, conventional C-arm beam-delivery systems are now being joined by bore-type machines that look more like a CT or MR scanner. Initially driven by the emergence of MR-guided radiotherapy, which can only be realized with a ring-shaped design, image-guided radiotherapy machines with the same geometry have found favour in the clinic for their speed, simplicity, and improved patient comfort. Early adopters of Varian’s?Halcyon?system have even reported a?70% reduction in energy usage?compared to a previous installation of the vendor’s more traditional TrueBeam radiotherapy machine.
But the more confined geometry of a ring-shaped design demands a different approach to quality assurance. Take, for example, the water phantom that is used by medical physicists to record accurate dosimetry measurements when commissioning a new radiotherapy system, or for periodic checks of the radiation beam profile during the machine’s lifetime. “Medical physicists want independent QA and verification tools to confirm that the radiotherapy system is working as it should,” comments Thierry Mertens , business development manager for healthcare at?LAP, a company that specializes in laser systems and radiotherapy QA. “A water phantom provides rigorous beam data and beam model visualizations to verify that the system is delivering the correct dose to the patient.”
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While water phantoms originally designed for conventional linacs can be used with ring-shaped systems, Mertens says that their large size and weight adds unnecessary complications to the measurement workflow. For a start, extra care must be taken to ensure the phantom does not collide with the sides of the machine as it moves into the opening, while additional corrections need to be made to compensate for the heaviness of the water tank on the couch.
In contrast, the?THALES 3D SCANNER?has been designed from the outset to validate bore-style machines such as the Halcyon, as well as Varian’s new AI-enabled?Ethos?adaptive radiotherapy system. For the medical physicist that means greater workflow efficiency: the compact system works seamlessly with the smaller 3D volumes of these machines, with extra guidance plates to enable different types of detectors to be incorporated into the device. The motorized phantom offers an automated set-up that takes less than 15 minutes, as well as pre-defined measurement sequences to help the medical physics team complete their test routines more quickly. Dedicated software is also provided for acquiring and analysing the data, with a rolling upgrade programme to improve the functionality in response to customer feedback.
“LAP has carefully considered the design to make the device as easy to use as possible,” says Thierry Gevaert , professor at the Vrije Universiteit Brussel and deputy head of the radiotherapy department at?UZ Brussel?(UZB) in Belgium. “We use a water phantom a few times a year, and with this system there is no need to relearn the measurement process each time. Many of the steps are automated, so we don’t need to waste time finding the right position of the chamber before starting the measurements.”