Launched in April of this year, TrueBeam's key selling point is its integration of respiratory gating, imaging and treatment delivery – enabling treatment of a moving target with high speed and accuracy. The system can deliver gated RapidArc radiotherapy, which synchronizes imaging with dose delivery during a continuous rotation around the patient.
"TrueBeam has been in development for over seven years. At the time of launch, more than 200 people were working on the programme," explained Corey Zankowski, senior director, product management, at Varian. "TrueBeam was designed from the ground up to be an image-guided, intensity-modulated machine. It's a completely different linac design."
There are eight TrueBeam systems already in operation and several others under installation. Zankowski also pointed out that, despite its very recent release, TrueBeam was the subject of several presentations at the AAPM meeting's scientific conference.
One of TrueBeam's significant features is its high-intensity mode, which offers a dose delivery rate of up to 2400 monitor units per minute – more than double the maximum output of previous Varian systems. Delivering a higher dose per fraction allows for faster treatments, enabling complex treatments - such as those using high dose per fraction with image guidance and gating - during standard time slots, as well as leaving less time for tumour motion during dose delivery. The high-intensity mode can deliver 18–20 Gy in around seven minutes, while gating the beam, says Zankowski.
The system also boasts a newly designed X-ray tube that can generate cone-beam CT (CBCT) images using, on average, 25% less X-ray dose to the patient for the same image quality, and in 60% less time. According to Varian, the dose advantage comes primarily from a patient-specific scatter correction during reconstruction, while the time savings come from streamlining the overall workflow and tight integration of the machine.
Another benefit is TrueBeam's integrated image guidance, which enables both kilovoltage (kV) and megavoltage (MV) imaging to be performed mid-treatment with sub-millimetre accuracy. "The technology creates opportunities to treat new patients and use new techniques," Zankowski explained. "One example is intrafraction motion review, which acquires kV radiographs during treatment, synchronized with the respiratory cycle."
Forward looking
The introduction of new treatment methodologies is enabled by TrueBeam's Developer Mode, a non-clinical mode designed to facilitate collaboration with clinicians and physicists developing new treatment and imaging techniques. Varian's Michelle Svatos explained how this feature enables users to try out novel ideas – implemented via BeamXML code – without impacting the system's clinical performance.
Finally, Gig Mageras from the Memorial Sloan-Kettering Cancer Center (New York, NY) discussed the use of TrueBeam in respiratory-correlated CBCT for pre-treatment imaging. Memorial Sloan-Kettering currently treats around 20 patients per day with TrueBeam – including complex cases.
"As is well known, respiratory motion affects CBCT quality," Mageras told attendees at the symposium. "Motion over a one-minute acquisition leads to image blurring, and reduced tissue boundary visibility makes localization difficult." He cited two methods that can potentially improve this situation.
Firstly, respiratory correlated CBCT, which combines continuous slow gantry rotation with beam gating using Varian's RPM gating system. This approach reduces image blurring but also reduces contrast and, as such, is largely limited to use in the lung. Secondly, Mageras described gated CBCT, in which both gantry rotation and kV imaging are gated. This technique results in better quality images, but when used on previous linac systems that weren't designed to do this, produces mechanical stress on the gantry.
He went on to describe preliminary studies of these methods on the TrueBeam system, using a Quasar respiratory motion phantom (from Modus Medical Devices of Canada). Software is already available in developer mode on TrueBeam to allow acquisition of respiratory correlated CBCT. This approach reduced motion blurring compared to standard CBCT. However, gated CBCT – implemented in Developer Mode – both reduced blurring and enhanced image quality, effectively freezing the motion.
"Gated rotation appears to improve the image quality," Mageras concluded. "So I'll be pushing Varian to get gated rotation CBCT into clinical mode at some point."