From OncoLog, July/August 2009, Vol. 54, No. 7/8
The Gamma Knife Perfexion’s eight automated sector drive motors allow for three sizes of gamma radiation beams, and the large collimator treatment volume facilitates treatment in such difficult locations as the upper cervical spine (left) and skull base (right). Photographs courtesy of Elekta, Inc.
Frontiers in Radiosurgery
By John LeBas
Multiple brain metastases, deep or enmeshed intracranial tumors, and tumors of the skull base and upper cervical spine can be difficult to treat, owing mainly to the delicate adjacent anatomy. Conventional therapies are sometimes too risky or of limited effectiveness.
Fortunately, patients with such lesions may benefit from stereotactic radiosurgery, which selectively targets tumor tissue to receive highly focused radiation with the help of computer-assisted treatment planning. At The University of Texas M. D. Anderson Cancer Center, the Division of Radiation Oncology in conjunction with the Departments of Neurosurgery and Head and Neck Surgery is now offering advanced stereotactic radiosurgery with the Leksell Gamma Knife Perfexion (Elekta, Stockholm, Sweden) system.
Like the earlier-generation stereotactic radiosurgery system it replaced, the Perfexion system integrates imaging and customizable dosing parameters to treat intracranial tumors with minimal damage to healthy tissue. However, the new device is fully automated and has improved treatment-planning capabilities, which will allow more efficient treatment of patients, treatment of more lesions per patient, and treatment of more anatomically complex locations than the earlier model.
With this technology, the multidisciplinary oncology team can create a radiation target that fits the lesion almost perfectly. Using a computer that displays images of the target in multiple planes, the physicians direct a cursor to “drop shots,” or place ray paths, around the tumor. From this interaction, the software computes a three-dimensional field that can be finely adjusted for size, dose, and position. After the treatment plan is finalized, the patient is secured into the treatment machine, and a customized dose of radiation is delivered according to the exact specifications of the field created by the team.
The primary appeal and clinical importance of the Gamma Knife lie in its precision and ability to safely reach deep tumors; its treatment efficiency is potentially very high. While surgery is the most common treatment approach for tumors in and around the skull, surgery is not feasible in all cases. “Some tumors aren’t optimally treated with surgery, some need radiation therapy after surgery, and some cannot be treated with chemotherapy,” said Franco DeMonte, M.D., professor and medical director of M. D. Anderson’s Brain and Spine Center. “For those tumors, the Gamma Knife may be a good option.” Surgeons at M. D. Anderson partner closely with their radiation oncology colleagues when treating patients with the Gamma Knife.
Dr. Eric L. Chang (front) and Dr. Franco DeMonte create a customized stereotactic radiosurgery treatment plan using the Gamma Knife Perfexion system.
The precision of the Gamma Knife results from a combination of attributes. The treatment plan is created using high-resolution maps based on magnetic resonance imaging (MRI) and computed tomography (CT) scans. The patient’s head is secured in a frame to create a stereotactic targeting space. And the gamma radiation beams are shaped by channels as small as 4 mm, allowing tumors of various shapes and sizes to be effectively treated with minimal danger to surrounding healthy tissue.
The Gamma Knife directs multiple beams of gamma radiation at individual doses that are considered clinically insignificant. However, the sum of those small doses as they intersect at the target is clinically significant. As the physician drops shots and adjusts the eight sector collimators during planning, the computer calculates the exact amount of radiation that will be built up at each point within the target. The approach spares healthy tissues that lie in the paths of the beams while allowing a tumor-killing level of radiation to be delivered within a well-defined area.
In addition to the 4-mm size, the Gamma Knife’s tungsten collimators offer 8-mm and 16-mm shots emitted by 192 cobalt-60 sources in the machine. The machine uses four pins to hold the skull in position; the slightest movement could mean the difference between hitting or missing the tumor, and a miss could deliver damaging radiation to healthy tissues instead. “It’s a one-shot deal. There’s only one chance to get it right since you can’t take back the treatment once you give it,” said Eric L. Chang, M.D., associate professor and director of M. D. Anderson’s central nervous system stereotactic radiosurgery program. “That’s why the delivery needs to be robust and highly precise.”
Because the Gamma Knife can so precisely treat small lesions without damaging normal brain tissue, the device is ideally suited for the treatment of patients with multiple brain metastases. Frequently, patients with brain metastases have already received whole-brain irradiation, and further whole-brain irradiation may carry an increased risk of toxicity. Gamma Knife radiosurgery, however, can obviate the need for repeating whole-brain irradiation, and some patients may be able to avoid whole-brain irradiation altogether by receiving Gamma Knife radiosurgery as initial therapy.
At M. D. Anderson, patients with difficult tumors in and around the skull are evaluated in a multidisciplinary fashion. “A multidisciplinary approach is essential to ensure that we choose the best treatment modality and produce the best possible plan for each patient,” Dr. Chang said. That approach carries into treatment planning for Gamma Knife sessions, involving specialists in radiation oncology, neurosurgery, head and neck surgery, and radiation physics.
The Gamma Knife Perfexion system, available for about 3 years, has been tested in controlled studies at other institutions. Experience from those studies suggests its treatment potential is greater than that of the earlier radiosurgery system. “The earlier system, which was 10 years old, was not fully automated. As a result, it was just not efficient to use it to treat more than four lesions per patient at a time,” said Almon S. Shiu, Ph.D., professor and supervising radiation physicist. “We believe the new system is the most automated radiation therapy system available, which will allow us to target a greater number of lesions.”
The new machine automatically positions the radiation sources at the correct collimator channels; in the old device, the channels had to be manually configured to customize the gamma rays’ path for each patient. With the automated system, Dr. Chang said, multiple targets in the brain can be treated more efficiently.
The Gamma Knife system integrates imaging studies and computer-calculated dosing fields (seen as circles) to allow precise targeting of tumors.
The automation and configuration of the Gamma Knife will also allow the treatment of different types of lesions, including acoustic neuromas, meningiomas, pituitary tumors, and other benign brain tumors; skull base tumors; malignant gliomas; and possibly tumors in the upper cervical spine. On the opening days in the Gamma Knife suite, specialists successfully treated two skull base tumor patients—one with a recurrent nasopharynx carcinoma and one with a recurrent salivary gland cancer in the skull base—who previously would not have been candidates for radiosurgery because the earlier-generation machine could not reach the skull base.
The integration of CT and MRI studies, which are essential to developing the treatment plan, has also been improved with the Perfexion system. The previous version of treatment planning software did not allow the treatment plan to be worked up until the day of the treatment, after the patient was secured in the frame. The new planning software does allow the flexibility of preplanning, which could improve operational efficiency—important since about 300 patients are treated each year, and the number is expected to grow.
The advances in the technology translate primarily into a wider range of treatment options for patients with brain, skull base, and upper cervical spine tumors. “We wanted to be able to offer all modalities to the patient and improve continuity of care,” Dr. DeMonte said. “It is important to offer choices in treatment, and for many patients, the risk-benefit ratio and preferences will support one choice or another. For example, an elderly patient with a skull base tumor may not be a candidate for surgery, but that patient may be successfully treated with the Gamma Knife.”
Paul Gidley, M.D., an associate professor in the Department of Head and Neck Surgery who specializes in the treatment of acoustic neuromas, said similar choices will apply to some patients he sees. “The same complications—namely, facial weakness or paralysis and hearing loss—can occur whether we treat acoustic neuromas with surgery or radiation therapy,” he said. “However, for patients who are infirm or have other medical problems and thus cannot tolerate surgery, Gamma Knife may be an excellent choice.” More specifically, Gamma Knife radiosurgery is an option for medium-sized (2.5 cm) and smaller acoustic neuromas or those that have regrown after surgery.
The advantages for patients with brain metastases are important when considering that such cases are increasing in both complexity and number. “We started out using radiosurgery for treating patients with single brain metastases, but treatments have evolved to the point where we are now routinely treating newly diagnosed brain metastasis in patients with up to three or four tumors,” Dr. Chang said. “Cancer patients in general are surviving longer as more effective systemic treatments become available, but as survival increases, patients are also developing more brain metastases. To help those patients over the long term, we need to safely and efficiently treat brain metastases when they appear while preserving neurocognitive function and minimizing any damage to the brain. Radiosurgery is also chemo-friendly, allowing patients to return to their chemotherapy program with minimal delay.”
For more information, visit the Brain and Spine Center at www.mdanderson.org, or contact Cindy Kizer, R.N., at 713-563-2405.