Guy Jones, M.D., and Juno Choe, M.D., Ph.D., are the treating radiation oncologists at Tri-Cities Cancer Center, a community cancer center in Kennewick, Washington, USA, serving a growing community of over 300,000 in the eastern part of the state. Recently, Dr. Jones discovered that a patient he had treated for lung cancer was subsequently referred elsewhere—to a distant site in Spokane, Washington, some 140 miles away—to receive a Gamma Knife® radiosurgery for trigeminal neuralgia. “The patient’s referring physician had no idea that we perform these treatments right here in Kennewick using LINAC technology,” Dr. Jones said. “People treat with the machines they have, and doctors refer to the centers they know.”
Guy Jones, M.D., treating radiation oncologist at Tri-Cities.
Dr. Jones feels certain that, given the option of being treated locally on the Edge™ system versus traveling 140 miles for a treatment that is more invasive and takes longer, patients would choose the Edge system every time. The trick is getting referring physicians familiar with how the Edge system can benefit their patients by providing an excellent radiosurgery platform closer to home.
“SRS with Gamma Knife is a long process,” explained Dr. Choe. “A patient might be asked to show up at six in the morning, when a neurosurgeon bolts a headframe to the patient’s skull. Patients then get an MRI scan that is used for radiosurgery planning. They usually schedule three to four SRS patients at a time, so treatment might not take place until late in the day. Patients might have the headframe bolted to their skulls for up to 14 hours.”
Juno Choe, M.D., Ph.D., treating radiation oncologist at Tri-Cities.
Contrast this with a LINAC-based SRS treatment: “During one visit, we make a stereotactic mask for the patient and do a simulation CT scan—that process usually takes less than an hour, typically about 40 minutes. Then we take the mask off and the patient goes home,” Dr. Jones said. “We do all our planning offline, and when the patient comes back, the treatment process takes about 40 minutes with the mask on. If you give patients an option between having a head frame bolted onto their skull for 12 hours, versus a non-invasive thermoplastic mask for 40 minutes, most will choose the latter. For the facility, there are other downsides to the Gamma Knife technology. It’s expensive and it doesn’t treat outside the head, limiting its utility. And there’s the cost and hassle of replacing sources. It takes a lot of financial support to keep those systems up and running and you need a security clearance to operate one because of the cobalt sources,” Dr. Jones added.
Dr. Jones and Dr. Choe worked together to establish a program for treating trigeminal neuralgia on their Varian Edge system, which was installed in 2015.
“I arrived at Tri-Cities with diverse experience using various technologies to treat trigeminal neuralgia,” says Dr. Jones. “At one center we used a LINAC with frame-based positioning; at another site we used Gamma Knife, which was also frame-based; and later I treated trigeminal neuralgia framelessly on a Cyberknife® machine. I felt certain we could establish a comparable program at Tri-Cities using the Edge system,” Dr. Jones said. “It was a little bit of a leap, but I felt it was a well thought out decision. We ran a lot of end-to-end technical testing on our machine before we treated any patients. At this point, we’ve treated about 50 cases of trigeminal neuralgia on the Edge system.”
The Tri-Cities protocol for trigeminal neuralgia involves the use of five arcs. “Our organ-at-risk avoidance strategy is mainly that we don’t put any of the arcs through the eyes. We found that it takes about 40 minutes per patient on the treatment table, including positioning, imaging, and beam-on time.”
Trigeminal neuralgia treatment plans involve delivery of 75-90 Gy as the maximum dose along the trigeminal nerve root, as it exits the brainstem, according to Dr. Choe. “My goal is generally that half of the maximum dose—say 40 Gy—falls within a circle that is about five millimeters in diameter. That’s the area we’re treating, and we want it to be centered precisely on the nerve root and not elsewhere.”
This requirement that the nerve be targeted with great precision means that careful image-guidance processes are used to set the patient up for treatment. “The treatment mask is not an absolute fixation device,” said Dr. Jones. “Things do not line up perfectly when we set the patient up based on marks on the mask. We use the cone-beam CT functionality built into the Edge system to move the robotic couch to get the trigeminal nerve or the target as close as possible to where we want it to be. The accuracy of the cone-beam CT system is critically important and, on the Edge, it is super precise. We’ve done testing to verify that the entire system is accurate to less than one millimeter. Ultimately, the resolution of the diagnostic imaging—usually MRI—is what limits us in terms of precision.”
Next, the therapist uses the six-degrees-of-freedom couch to correct positional errors along the X, Y, and Z axes as well as any pitch, roll, and yaw rotational errors. “We get the patient’s trigeminal nerve root to within a millimeter of where it needs to be,” said Dr. Choe.
And even after a near-perfect set-up, there is more to do to ensure targeting accuracy. “The patient can physically move within the mask, so we use the Optical Surface Monitoring System (OSMS) to look at the patient’s face,” Dr. Jones said. “If they flinch, or cough or move in any way, the beam shuts off immediately and we re-setup the patient. We repeat our imaging and adjust the couch parameters once again. Worst case scenario, it extends the treatment by a little bit, but this is critical for us to have confidence that we are hitting our target.”
Outcomes are about what you would expect for these types of treatments, no matter the technology used. “In the literature, outcomes are all similar regardless of whether you treat using a Gamma Knife machine or a LINAC,” Dr. Choe said. “Typically, you see about a 60% complete pain response rate. Another 10% of patients have a major response and 10% have a minor response. So that’s an overall response rate of about 80% or so, and our results at Tri-Cities are in line with this.”1,2,3,4
Clinicians at the Tri-Cities Cancer Center keep the Edge machine busy with a wide variety of other radiation treatments when it is not being used to treat trigeminal neuralgia, including more traditional 3D conformal and IMRT cases in addition to SRS and SBRT.
“We treat everything,” said Dr. Choe. “Lung is a huge SBRT application for us, and we treat a lot of early-stage lung cancer as well as metastatic disease in the lung. We do SBRT for liver tumors, and we treat a lot of kidney and adrenal gland cases. We treat recurrent tumors in the pelvis and head and neck region with SBRT, as well as tumors in the spine. We also offer SRS treatment for a wide variety of malignant and benign tumors within the brain and skull base.”
For lung cancer patients, target motion is carefully measured with 4D CT imaging at the time of simulation. "Respiratory gating is used for select cases, but it can increase the treatment time by three-to-four-fold. We more commonly use an abdominal compression belt to apply gentle pressure. This can often reduce tumor motion down to less than a centimeter in many cases. We treat the whole range of motion of the tumor with a margin of approximately 5 millimeters. This allows us to treat a small volume of lung tissue and still finish quickly. Patients certainly appreciate the faster treatment times, and it adds quite a bit to patient comfort and tolerance of the procedure.”