The session organizer, Benjamin Fahimian from Stanford University, began by summarizing the problem. Depending upon the site being treated, he explained, motion due to various sources – such as the beating heart, breathing, and skeleto-muscular and gastrointestinal movements – and of varying magnitude can be encountered during treatment.

Approaches used to compensate for such motion include compression, breath hold, gating and dynamic tracking delivery. But to apply these, the target position needs to be accurately monitored as a function of time. Such tracking can be achieved, for example, by imaging radio-opaque fiducials, anatomical landmarks or electromagnetic transponders, optical imaging of correlated surrogates or volumetric imaging within treatment delivery, with some of these methods already available commercially.

"The importance of image guidance cannot be overstated for the accurate delivery of radiotherapy to the body," Fahimian told the delegates.

High-contrast kV imaging

Fahimian went on to describe the use of kilovoltage (kV) X-ray imaging for radiotherapy guidance. He explained that kV images offer high contrast and quality. The main disadvantage is the radiation dose delivered to the patient, as well as the image having a different isocentre to the treatment beam.

Kilovoltage tracking can be performed with a monoscopic set-up – imaging from one direction, using a conventional on-board imager (OBI) – or a stereoscopic geometry – imaging from two directions. Stereoscopic imaging offers the advantage of localizing the target in 3D space through triangulation. To reduce dose, periodic X-ray images can be correlated with continuous optical tracking of external surrogates.

Monoscopic imaging provides a high level of flexibility: enabling triggered kV, continuous fluoroscopic kV, or combined kV-MV (megavoltage), for example. "An intra-fraction monoscopic image from an OBI can be used to verify the expected 2D positions of a target at a particular point in the respiratory cycle," explained Fahimian. "The problem is that you can localize the object in 2D, but where it is in 3D is unknown."

So how can this 3D ambiguity be addressed? One way is to use knowledge from projection images acquired during set-up. Another option is to perform tomosynthesis, by reconstructing images from multiple angles. Alternatively, says Fahimian, don't address it, just use monoscopic kV imaging for verification of 2D location. Indeed, this is actually the most common approach used clinically.

For example, a patient undergoes a 4D planning CT, and software markers are placed at suitable respiratory points. For a gated treatment, a kV image is taken just before the beam is switched on and software markers projected on the beam-level images reveal whether the measured position lines up with the expected location.

Fahimian concluded by re-emphasizing the dose issue. "For a planar radiographic image, entrance dose levels per intra-fraction image range from 0.25–0.5 mGy," he said. "Given this, combination with optical imaging is important to minimize dose, so the two approaches are often coupled together."

Use the MV beam

The second speaker, Ross Berbeco from Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School examined how to go "beyond kV tracking", by using the MV treatment beam and electronic portal imaging device (EPID) to perform cine EPID tracking.

Advantages of MV over kV imaging, he explained, include no additional dose, lower costs and the ability to reconstruct volumetric data. In addition, the beam's-eye-view geometry means that "what you see is what you treat". On the downside, image contrast is lower in MV images, and tracking intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT) treatments is challenging, due to collimator leaves obscuring parts of the target.

Berbeco presented an example MV verification of stereotactic body radiotherapy to the liver, which demonstrated that in-treatment monitoring could capture non-periodic intra-fractional changes. In another study, MV imaging was used to automatically identify and track the motion of multiple regions of a lung phantom with a dynamic multileaf collimator during radiotherapy. One key result of this study, he noted, was the demonstration of sub-millimetre tracking accuracy.

As for future developments: "There are a lot of interesting ideas to deal with the challenges of contrast in MV images," Berbeco explained. One option is to change the radiation source, for example by using a switchable X-ray target that will deliver more low-energy photons, a concept being developed at Dalhousie University. He also mentioned the TumoTrak X-ray system, which places a kV source on the linac head – retaining the beam's-eye-view, though not using an MV beam.

Another way to improve image quality for low-contrast targets is to redesign the detector to collect more photons. With this aim, Berbeco and colleagues are developing an imaging system with high detection efficiency, by using an EPID with multiple layers, each containing a scintillator coupled to amorphous silicon flat panel imager.

"MV imaging provides a cost-effective, simple solution for tracking moving targets," Berbeco concluded. "Future improvements in detectors and sources will improve the image quality."

Tracking transponders

Next to take the podium, Paul Keall from the University of Sydney discussed dynamic target tracking using electromagnetic transponders (or beacons). An electromagnetic tracking system comprises a source array and one or more beacons, which are implanted near to or within the target. The array sends pulses at specific radiofrequencies to selectively excite the beacons, which then decay and return a signal that's detected by the array's receive coils. This signal is then used to localize each beacon position with sub-millimetre accuracy.

The technology is already available commercially. Varian's Calypso system, which uses three beacons and an array located above the patient, has been in clinical use since 2005. Micropos Medical's RayPilot, meanwhile, has one beacon and an array integrated into the couch. "Electromagnetic tracking provides us with a rich knowledge regarding the magnitude and complexity of tumour motion," said Keall. "But implanting markers is a challenge and is probably the largest barrier to widespread implementation."

Keall shared some clinical results, including a study of 41 prostate cancer patients receiving radiotherapy. The study compared guidance using the Calypso system with X-ray localization, and reported good agreement between the two. In another example, 50 lung-tumour patients underwent bronchoscopic implantation of three anchored transponders in or near the tumour. "This work showed that real-time localization and tracking of lung tumours is feasible and provides motion information that can be used for radiotherapy planning and delivery," Keall explained.

Electromagnetic tracking has also been demonstrated in patients with pancreatic cancer and liver cancer. Future developments, Keall predicted, will include miniaturization of the beacons, use in other body sites, and further integration with the radiotherapy system.

Moving to MRI

Last up, Daniel Low from the University of California, Los Angeles, described the newest approach to intra-fraction motion monitoring: MRI-based soft-tissue tracking during radiation delivery. This approach requires specialized radiotherapy equipment, such as ViewRay's MRIdian system, for example. The MRIdian, which integrates a 0.35 T MRI with three cobalt-60 sources, is now installed in three clinics. Another example is the hybrid linac-MRI being developed at the Cross Cancer Institute in Canada.

Low also described the MR-linac conceived at University Medical Center Utrecht and being commercialized with Elekta and Philips. He showed an MRI cine video of pancreas motion, demonstrating the potential for using MRI guidance for real-time motion compensation. "The 4D nature of MRI is clear," he explained. "We can image as much as we want to in 'real time', because MRI doesn't deliver any radiation dose."

He described how MRI could be applied to guide gated treatment, by recording a cine MR image and selecting a key frame (at inhalation, for example) that is most suitable for use with gating. MRI can also provide guidance as to whether a target has moved out of a defined spatial boundary, enabling treatment to be halted if the boundary is crossed.

Looking forward, Low predicts a move towards 3D real-time MR imaging during treatment (requiring more image slices and management of the resulting larger data volume), and potentially also real-time review and monitoring by therapists. "MR-based target tracking is one of the more important features of MR-guided radiotherapy; it offers the potential of more accurate treatments and ultimately enable reduced margins," he concluded. "This is brand new, it's extremely exciting."

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