MRI-Guided Radiation Therapy
By Kathy Hardy
Vol. 15 No. 9 P. 20
Precise adaptive radiation therapy guided by real-time MRI images could prove a significant advance in radiation oncology. Integrating those two tools is the driving force behind the MRIdian system, a combined real-time MRI and radiation therapy delivery process developed by Cleveland-based ViewRay. The system received 510(k) clearance in 2012 and is now in use in several locations, including the Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine in St. Louis, where the first system was installed and patients first underwent radiation therapy in January 2014.
Similar in concept to the Accuray Tomotherapy System that integrates CT imaging and radiation therapy, the MRIdian system integrates MR imaging and radiation therapy. MRI guidance can provide image-guidance both before therapy and also concurrent with treatment without exposing the patient to the additional ionizing radiation of CT scans. Physicians using the system say it gives them a real-time view of what’s happening in a patient’s body at the time of each treatment, taking advantage of MRI’s strength in imaging soft tissue. On-board imaging at the time of treatment enables adjustment of the targeted radiation treatments, minimizing the possibility of damaging healthy organs located near a tumor.
“MRI-guided imaging improves the localization for delivery of radiation dose to the target vs the surrounding tissue,” says Jeffrey R. Olsen, MD, an assistant professor of radiation oncology at Barnes-Jewish Hospital and Washington University School of Medicine. “From a patient perspective, the treatment experience is similar to standard radiation treatment delivered with a linear accelerator. The difference compared to current standard treatments with the MRIdian system is the soft tissue imaging. Current standards of localization using a technology called cone beam CT are good for bone visualization but more challenging for soft tissue localization.”
In addition to targeting, the system offers physicians an assessment of whether the tumor has reduced or grown in size before each treatment.
“With real-time MRI images we can see what we’re dealing with over the course of the patient’s treatment,” says Clifford G. Robinson, MD, an assistant professor of radiation oncology at Barnes-Jewish Hospital and Washington University School of Medicine. “You can see changes in the soft tissue and then modify the treatment to adjust for those changes. This is an important point with this technology. It allows you to see shifts in the tumor over time, with real-time MRI imaging.”
The 510(k) clearance permits the system to be used with radiation therapy in any part of the body where other forms of radiation therapy are prescribed, according to Michael Saracen, senior director of marketing for ViewRay. He says one of the first treatments delivered by the MRIdian system was stereotactic body radiation therapy for lung cancer. In this instance, high radiation doses were delivered to small, well-defined tumors to kill cancer cells while minimizing exposure to surrounding healthy organs. While approval is broad with its 510(k) substantial equivalency clearance, radiation oncologists are still investigating the system’s potential uses.
“This is a novel system, and we have no precedent for its use,” Robinson says. “We’re still identifying who would benefit from this system. When we see someone in consultation now that we would have previously used a CT scan for imaging guidance, we now think about their compatibility with MRI coils and whether they’re a candidate for this technology.”
Robinson’s focus at the Siteman Center is on patients with brain and lung tumors. Before each treatment, he obtains changes in tumor volume and position in real time using the scanner. Patients are screened the same way they would be if the physicians were considering an MRI for other purposes, he says. Patients undergo low-resolution scans to verify positioning followed by high-resolution scans to see the needed views of where the tumor is located, he says.
Soft Tissue Imaging
“For soft tissue, the view is out of this world,” Robinson says. “Being able to track tumor movement is not novel in and of itself. But I would have used fiducial markers in the past. In this case, the tumor is the fiducial.”
Cancers are highly dynamic systems that can undergo significant changes over the treatment course. In general, cancer treatment therapy is administered according to a fixed, linear protocol. However, as the concept of personalized medicine spreads into practice, the idea of managing a patient’s treatment based on his or her individual characteristics instead of a standard of care determined by outcomes in a larger patient group is gaining more attention. That’s where users of this MRI-guided system see the real-time view playing a valuable role in individualizing a course of treatment that best suits their patients’ needs.
“By using MRI at the time of treatment, physicians now have the ability to process the image and calibrate the radiation dose, providing adaptive therapy,” Saracen says. “MRI takes into consideration changes in anatomy that could be caused by things like a full bladder or tumor size changes. You’re not treating to a plan that was created last week.”
For Olsen, soft tissue imaging is at the core of his practice. At the Siteman Center, his focus is in the abdominal region, treating patients with gastrointestinal, musculoskeletal, colorectal, esophageal, and pancreatic cancers. He says these areas of the body are also where physicians see a high degree of tumor motion and some of the greatest areas of change. The same is true for the lungs and surrounding areas where respiratory motion can complicate treatment.
“It’s difficult to see specific anatomic changes during treatment with current imaging modalities,” Olsen says. “There is a lot of motion in those areas. MRI enables you to see variability and if you can see that, you can potentially adapt the treatment plan.
“Current treatment planning systems allow generation of precise radiation treatment plans,” he adds. “What has lagged behind, however, is technology to confirm patient positioning at the same level of accuracy, and to evaluate changes in anatomy during treatment. MRI allows improved visualization of soft tissue anatomic change, where we previously had to rely on fiducial markers.”
Olsen also points to visibility issues in the head, neck, cervical area, and spine, where the use of MRI guidance can help overcome a different problem. While there are fewer motion issues in head and neck cancers, treatment is often in close proximity to sensitive neurological structures. “Tumors can adapt to neurological structures such as optic nerves, which are also difficult to visualize with current forms of localization imaging,” Olsen says.
According to Saracen, radiation is used as a treatment method with approximately two-thirds of all cancer patients. During the past several years, he says, radiation has taken on a more curative role, used in conjunction with surgery. Image-guided radiation treatments, such as Gamma Knife, CyberKnife, and other external beam radiation treatment tools, have come a long way but there is still the issue of locating the exact marker placement, he says.
“Before the MRIdian system, there was no way to know exactly where the tumor was located while the [radiation] beam was on,” he says. “Now you can see where to place the beam in real time. With continuous soft tissue imaging, radiation oncologists can see the tumor, monitor where the radiation dose is being delivered, and adapt to changes in the patient’s anatomy.”
Using MRI-guided treatments also reduces additional radiation exposure from CT scans, Saracen says, because a standard radiation treatment course can require numerous additional CT scans.
While MRI-guided imaging is a tool to assist physicians in using radiation therapy to treat patients with cancer, many find that this form of imaging is helping patients in other ways as well.
“The first patient was able to see his ‘movie,’ and he could see what happened during the procedure,” Saracen says. “That gave him the confidence that his cancer was being treated.”
When considering MRI as a guidance tool, Saracen says a major factor was finding a source of radiation delivery that is most compatible with that imaging modality. External beam radiation systems emit a lot of radiofrequency (RF) energy, which create a certain level of “noise” that interferes with MR signals.
“When looking at radiation delivery methods to combine with MRI, we needed to eliminate certain things that would increase the noise interference,” Saracen says. “It would be like trying to listen to crickets at a rock concert.” As a result, ViewRay developers decided to go with cobalt as a radiation source instead of linear accelerators, he says.
“Cobalt emits no RF energy so it doesn’t interfere with the MRI system,” he says. “While traditionally cobalt is not as penetrating as other sources of radiation, the MRIdian system takes that into consideration by using highly activated cobalt, thus pushing it into the realm of linear accelerators.”
In addition, the magnet itself can cause some interference with the radiation beam, Saracen says. The stronger the magnetic fields, the greater the effect on the electrons. However, utilizing a lower field-strength magnet reduces the effect without diminishing the quality of the image guidance. “You’re imaging for tracking and treating the disease in real time,” he says. “This isn’t being used for diagnostic purposes.”
While ViewRay designed the MRI machine used in the system, it uses Siemens’ underlying control systems, according to Saracen. The treatment planning system using a custom designed Monte Carlo algorithm and the software that runs the device were written by ViewRay. This integrated system allows changes to be made “on the fly,” he says.
As with any new technology, reimbursement is an issue. There are two aspects to image-guided radiation therapy: imaging for alignment purposes and delivery of the radiation treatment. For now, those using the system say these aspects of the process are being billed separately. However, Saracen says ViewRay customers have formed a reimbursement consortium that will start looking at new and existing codes for the system as a whole.
This latest evolution of image-guided radiation therapy brings the possibility of more real-time benefits, Robinson says, and could be a precursor to more adaptive planning in radiation therapy.
“I’m anxious to see how this will allow us to adapt the treatment plan in real time,” he says. “That was never possible before.”
Olsen agrees, noting that trials are currently under way that could further demonstrate how MRI-guided radiation therapy may result in more patient-specific treatment, possibly with increased doses more closely focused on specific targets, with improved outcomes.
“Going forward, we need to demonstrate value,” Olsen says. “We are excited regarding this new technology, but plan to be methodical in our evaluation, recording patient outcomes to show that we’re making a difference.”
— Kathy Hardy is a freelance writer based in Phoenixville, Pennsylvania.