Oct. 28, 2016
For selected patients with neurological tumors, proton beam therapy can be an excellent alternative to conventional photon radiation. Proton beam therapy targets tumors more precisely, limiting damage to surrounding healthy organs and tissue and offering the possibility of using higher doses of radiation for more-beneficial outcomes.
Proton beam therapy treatment room
Treatment rooms for administration of proton beam therapy are available at Mayo Clinic in Rochester, Minnesota, and Mayo Clinic in Phoenix/Scottsdale, Arizona.
Proton beam therapy is now available at Mayo Clinic's campuses in Minnesota and Arizona. Both facilities use pencil beam scanning exclusively and are the first centers in the Midwest and Southwest to offer this advanced proton beam therapy technique. The program's funding mechanism allows Mayo Clinic to offer proton beam therapy to patients at costs similar to intensity-modulated radiation therapy (IMRT).
"Mayo Clinic is providing a very unique approach," says Bernard R. Bendok, M.D., chair of Neurosurgery at Mayo Clinic's campus in Arizona. "With minimally invasive endoscopic and microsurgical skull base approaches, we can reduce tumor volume, and then treat the areas we can't reach safely with proton beam therapy. Our multidisciplinary focus is creating novel treatment pathways that are safer for patients and offer better outcomes."
Enhanced tumor targeting
Conventional photons are high-energy X-rays that completely penetrate tissue and lose some of their energy along their route — resulting in a relatively high entrance dose and some level of exit dose. In contrast, protons penetrate to a limited depth, based on the energy of the beam, and deposit most of their energy at the end of the beam.
"We're able essentially to program the protons to stop at a specific point, so radiation delivery can conform to the tumor," says Elizabeth Yan, M.D., a radiation oncologist at Mayo Clinic's campus in Minnesota.
The pencil beam technology used at Mayo Clinic offers added precision. "The pencil beam delivers protons in a raster pattern at different specified depths, to better conform to the volume of tumor," Dr. Yan says. "With a traditional scattered proton beam, physical shapers are placed in the pathway of the beam before it reaches the patient. This results in slightly more entrance dose before the beam reaches its target depth."
Staining in treatment plans for tectal glioma
Red and orange staining in treatment plans for tectal glioma indicates similar intentional doses using intensity-modulated arc therapy (left) and proton beam therapy (right). The intentional doses (red and orange) are very similar with both techniques. However, the spillage of unintentional low-dose radiation (blue) to surrounding normal brain tissue is quite different.
Proton beam therapy can be particularly beneficial for patients with clival chordoma and chondrosarcoma. "In the past, those pathologies have been pretty resistant to conventional radiation treatment. Proton beam therapy allows us to deliver a higher radiation dose to the tumor without toxicity to the brainstem, optic nerve, pituitary gland and other surrounding structures," notes Jamie J. Van Gompel, M.D., a consultant in Neurosurgery at Mayo Clinic's campus in Minnesota.
Minimizing damage to surrounding tissue can lower the risk of malignancy developing after radiation treatment, making proton beam therapy a particularly helpful option for younger patients and patients with benign tumors. "We feel proton beam therapy also will be very helpful for some arteriovenous malformations and dural fistulas that are inoperable," Dr. Bendok says.
For patients who are candidates for surgery, the option of follow-up proton beam therapy allows Mayo Clinic neurosurgeons to avoid resection in critical areas.
Dr. Van Gompel cites the example of a young woman with a very large pituitary tumor. "We removed about 85 percent of the tumor. But a portion was wrapped around the carotid artery," he says. "We treated the residual tumor with proton beam therapy. Over the next two years, we hope that the functional part of the tumor will stop secreting growth hormone so that the patient can quit taking medications and live a normal life."
Fees equivalent to IMRT
Fees for proton beam therapy are generally far higher than for conventional radiation therapy, due to the high cost of constructing proton beam facilities. However, the $368 million that Mayo Clinic invested in its proton beam program came from the capital budget of Mayo Clinic — a nonprofit foundation — and from benefactors. Neither venture capital nor government funding was used. As a result, Mayo Clinic is able to offer a standard fee for proton beam therapy equivalent to the standard fee for IMRT.
"We want, as much as is possible, to be able to offer proton beam therapy to those patients who would benefit from it, without cost being the prohibitive factor," says Robert L. Foote, M.D., chair of Radiation Oncology at Mayo Clinic's campus in Minnesota and medical director of the Mayo Clinic Proton Beam Therapy Program.
Proton beam therapy is generally well-tolerated, with side effects similar to those associated with conventional radiation. "With proton beam therapy, patients can avoid suffering major complications from extensive surgery and secondary malignancies," Dr. Van Gompel says. "Our patients can go on to live a fairly normal life after having a tumor that might have been fatal 10 or 15 years ago."