Feb 25, 2010
MR-guidance: the next step for IGRT?
Image-guidance has become an invaluable tool within radiation therapy, with recent years seeing a shift towards incorporating image-derived, patient-based information into the treatment process. Alongside, the industry has witnessed the emergence of advanced radiotherapy systems with integrated imaging capabilities.
Image-guided radiotherapy (IGRT), and the evolution of the technique over the last 20 years, was the subject of David Jaffray's keynote presentation at last week's SPIE Medical Imaging Conference. "We've seen a transition from a technique-based approach to employing 3D information in creating the intervention," explained Jaffray, head of radiation physics at Princess Margaret Hospital in Toronto, Canada.
In the opening address of the meeting's "Physics of Medical Imaging" track, Jaffray described how the introduction of highly conformal radiotherapy modalities increased the need for precision targeting. And considering the fact that patients move and targets morph over the course of fractionated therapy, co-location of target and therapeutic dose distribution poses a significant technical challenge.
"It's not just a case of getting it right once, it must operate reliably over several weeks," he explained. "In-room image guidance is a necessary development in the push towards dose escalation and normal tissue avoidance in radiotherapy."
Imaging provides accurate measurement of patient position within the treatment room and enables verification of patient set-up against planned geometry. Highly integrated IGRT systems are available and in clinical use, with substantial deployment of volumetric IGRT seen over the past five years. At Princess Margaret Hospital, for example, online cone-beam CT guidance is now used for around 70% of cases.
Jaffray discussed the close relationship between imaging and quality assurance, noting that the most impact could likely be upon lung cases treated with stereotactic body radiotherapy. Here, online image guidance with cone-beam CT can almost completely eliminate target positioning errors.
As well as increasing the accuracy of patient set-up and radiation delivery, Jaffray suggested that imaging information can be used to redesign the treatment plan, on the basis of therapy-induced changes, for example. He cited an example study comparing radiotherapy for head-and-neck cancer with and without adaptive treatment planning. Weekly adaptation delivered a better dose to the target, as well as enabling the use of tighter margins and thus substantially lowering the parotid dose.
"Adaptive replanning provides a huge opportunity to reduce the amount of normal tissue irradiated, while retaining minimum target dose," Jaffray noted.
So what's next for IGRT? According to Jaffray, the future may lie in the integration of MR-guidance. MRI provides excellent soft-tissue contrast without exposing the patient to ionizing radiation. It also offers physiological measurement capabilities. Potential drawbacks include the effect of magnetic field on dose deposition, as well as system expense, Jaffray noted. "We've seen a great interest develop in the joining of MR systems and radiotherapy," he said.
Several MR-guided radiation treatment systems are currently under development. For example, Gino Fallone's team at the Cross Cancer Institute in Edmonton, Canada, has designed an instrument in which a linac and a 0.2T MRI magnet are mounted on a gantry that rotates around the patient. In this "rotating-biplanar" geometry, the beam and field are fixed with respect to each other. In March of last year, Fallone's team demonstrated the first MR imaging during photon irradiation.
An alternative design, and an approach being investigated by Jan Lagendijk and colleagues at the University Medical Center Utrecht in the Netherlands, is to fix the magnetic field with respect to the patient. The Utrecht system uses a higher field strength of 1.5T. The researchers have already demonstrated that MR imaging with the radiation beam switched on does not degrade the performance of either the linac or the MR scanner.
Jaffray also mentioned the Renaissance system under development by Jim Dempsey at ViewRay (Oakwood Village, OH). This device combines MRI with gamma radiotherapy, using three 60Co sources with multileaf collimators. Elsewhere, Tomas Kron at the Peter MacCallum Cancer Centre in Melbourne, Australia, is also working on a cobalt-based treatment system, integrating MR guidance with helical tomotherapy delivery.
At the Princess Margaret Hospital, meanwhile, Jaffray and colleagues are employing active MR guidance for radiotherapy using a Siemens 1.5T MRI on rails. Here, the MR system is moved into the radiotherapy treatment room, imaging is performed, and the MRI is then removed. "We can do this today," he pointed out.
Jaffray concluded that MR-guided radiotherapy appears to be feasible, both from an engineering and a physics perspective. Ongoing challenges include establishing the predictability of dose delivered in a magnetic field, as well as logistical and safety issues. "I think we'll see MR-guided radiation treatment systems on the market within a few years," he predicted.
About the author
Tami Freeman is editor of medicalphysicsweb.