Condition:
Pediatric Brain & Spinal Tumors
Contents:
Why is the use of radiation problematic in children?
A Partial Solution: Radiosurgery
A New Approach: the CyberKnife
What are the limitations of radiosurgery and advantages of the CyberKnife?
What is the CyberKnife® treatment process for infants and children?
What are the potential complications of radiosurgery?
When to use what: Gamma Knife, CyberKnife or other?
What about tumors of the spine?
Summary
Why is the use of radiation problematic in children?
Treatment of brain tumors in children is complex and requires the expertise of specialists. The usual tools – surgery, chemotherapy and radiation - are the same ones used for adults, combined into a customized plan for the individual child based on his or her specific medical circumstance. Even though the treatment of brain tumors in children is similar to that of adults, there are important differences.
Tumors in children have their own unique patterns of growth, and what is effective for adults may not be as effective for infants and children. Some of the differences are minor; the dose of chemotherapy might be less for children, or the timing of surgery might be different. But a major difference is the use of radiation.
Radiation therapy is an important tool in the treatment of adult brain tumors, since the majority of tumors will either shrink or slow their growth in response to radiation treatments. Years ago, doctors were hopeful that the same would hold true for children, that radiation would be as potent a tool for children as it was for adults. But what happened when children under the age of three years were treated with radiation to the whole brain was tragic.
Their actively growing brains were too fragile for such radiation – within several years, these children suffered from severe developmental delay, serious problems with memory, and loss of cognitive ability, all of which correlated with structural changes in their brain tissue. By later life, many were neurologically devastated.
Children older than five years are less prone to the above complications, so radiation therapy is an important part of their treatment. In contrast, pediatric oncologists prefer to withhold radiotherapy from younger children, relying instead on surgery and chemotherapy.
Sometimes brain irradiation is delayed until the child is old enough to withstand the long-term effects. Unfortunately, this means that these children are denied what would be the very real and important benefits that stem from treatment with radiation.
A Partial Solution: Radiosurgery
In the 1950’s, Swedish neurosurgeon Lars Leksell had an idea that is still alive and growing even today. Why not devise a machine that could focus the radiation on the tumor alone, sparing the surrounding brain from the damaging radiation? His vision was primarily intended for adults. However, such a device held immense benefits for children as well, since the minimal radiation exposure to the surrounding brain would lessen the risk of devastating neurological side-effects.
Leksell’s idea culminated in the development of the Gamma Knife, which is not a knife at all. Instead, it consists of a large spherical metal helmet about four inches thick with multiple holes that guide the radiation to a single area at the helmet’s center. Since this targeted area receives a dose of radiation far in excess to that to the surrounding brain, the desired effect of selectively focusing the radiation is achieved.
The Gamma Knife is used differently than standard machines for radiotherapy. The standard fractionated plan uses small doses of radiation administered daily for many weeks to kill tumor cells without significantly harming normal tissue. In contrast, Gamma Knife treatment consists of a single, powerful radiation dose administered in one sitting.
The intent of Gamma Knife treatment is to destroy everything within the targeted area. Since the Gamma Knife is so precisely focused upon the tumor, Leksell termed this strategy radiosurgery. Over the past 25 years, this approach has proven to be spectacularly successful in treating a variety of adult brain tumors. In fact, many tumors that are resistant to fractionated radiotherapy respond readily to radiosurgery. Such treatment is an effective and widely used treatment for many brain tumors.
But the Gamma Knife and many other forms of radiosurgery cannot be used for young children for one important reason: the application of these technologies requires the attachment of a rigid metal frame to the head, a frame that is too heavy and compressive for the fragile infant skull. For older children with thicker skulls, Gamma Knife and other types of frame-based radiosurgery remain a reasonable option with success rates similar to those
A New Approach: the CyberKnife
Many devices are now available which more accurately deliver radiation without the requirement of a rigid frame. Each of these techniques must somehow immobilize the head with respect to the radiation beam. Some do this with tight bands wrapped around the head or by inserts into the ears; others use facemasks, dental molds, or molds conforming to the skull. Although these devices are commonly used to give fractionated radiotherapy, none offer the high precision required for radiosurgical treatment.
The CyberKnife is the first device capable of radiosurgical precision without the requirement of a head frame. Consisting of a high-energy x-ray source guided by a robot, the CyberKnife achieves the required precision by making updated radiographs of the patient throughout treatment; the system detects patient movement and the robot automatically compensates for it. In fact, extensive recent testing has shown that the precision of the CyberKnife is comparable to that of the Gamma Knife.
This means that radiosurgery with the frameless CyberKnife can finally be offered to infants and young children - patients that have been previously denied the benefits of radiosurgery.
What are the limitations of radiosurgery and advantages of the CyberKnife?
Not all brain tumors are suitable for radiosurgery. For reasons that are not completely understood, the risk of complications begins to rise when a tumor exceeds one inch in diameter or is located in particularly sensitive areas of the brain like the brainstem or thalamus. In these cases, options are limited; a single, large radiosurgical dose may be too dangerous, while repeated, small doses of a fractionated regimen may be ineffective.
As an alternative, many CyberKnife centers combine the best features of these two strategies by offering hypofractionated or staged radiosurgery. This treatment uses a very large dose of radiation and spreads it over 3 to 7 smaller daily doses. The benefit of this approach is that one can safely treat larger brain tumors with large and effective radiosurgery doses.
Hypofractionation or staged radiosurgery is relatively new and it has great theoretical appeal. Furthermore, preliminary reports have been encouraging enough to advocate its use in select circumstances. Since children are rarely able to wear a head frame for 3 to 7 days, hypofractionation is not a practical procedure with the Gamma Knife, and of course, younger children cannot wear the frame at all.
In contrast, hypofractionation is easily performed with the “frameless” CyberKnife. Consequently, for children with larger tumors or those located in especially sensitive areas, hypofractionation with the CyberKnife offers an entirely new treatment option.
What is the CyberKnife treatment process for infants and children?
The first step of any radiosurgical treatment is a series of in-depth conversations with your pediatric oncologist, radiation oncologist and neurosurgeon. Once you and your doctors decide CyberKnife radiosurgery is the best option, the treatment process begins by making a custom facemask to help immobilize the head during treatment. The next step involves an outpatient CT scan (the CyberKnife uses CT primarily rather than MRI to track skull position during treatment).
Intravenous contrast is given during this scan to enhance the delineation of the tumor. Infants and young children may require sedation or general anesthesia during this process. Rarely, the tumor cannot be visualized on a CT scan, in which case an MRI scan may often need to be made. If, in very rare circumstances, the tumor cannot be seen on a scan, your doctors will need to decide whether CyberKnife treatment is still possible.
Using the CT scan, your doctors will design a customized treatment plan that instructs the robot how to deliver the radiation. Actual treatment might be given the same day as the scan or a few days later. On the day of treatment, young children and infants will be supported on a form-fitting pillow and given general anesthesia.
The facemask that was made prior to the CT scan is used to hold the head relatively immobile. In addition, throughout radiosurgery the patient is carefully observed. CyberKnife treatment usually last about an hour, but may be longer. If hypofractionation or staged radiosurgery is being used in an infant, general anesthesia is given each day.
Neither the benefits nor the potential complications of radiosurgery are immediate, and it may be several months before results are known. Your doctors will tell you how often your child will need to be seen and when to obtain radiologic studies.
What are the potential complications of radiosurgery?
The chances of serious complications due to radiation are usually significantly smaller than those for surgery, although certainly not zero. Your doctors will tailor this information for you. The following is a generic but only partial list of potential complications.
Since radiation has the potential to damage the brain immediately surrounding a tumor treated with radiosurgery, there is a small but definite risk of destroying brain tissue (radionecrosis). To a far lesser extent, stroke and even death are potential risks.
The most common possible complication is the risk of irritation or swelling of the brain around the tumor, leading to symptoms that depend very much on the location of the swelling. These may include muscle weakness, numbness, speech difficulties, impaired vision or seizures. Rarely are such problems permanent.
More commonly, they are temporary. Furthermore, there is the chance that treatment will not be effective, or that the tumor will swell before it shrinks. Finally, as with any radiation treatment, there is a small risk that the radiation itself will induce a malignant tumor in the future.
When to use what: Gamma Knife, CyberKnife or other?
Deciding between these many options is not easy, the experts do not always agree, and you will need the advice of your doctors. Here is a general guideline:
Children less than three years of age. As discussed earlier, conventional fractionated radiation therapy is usually not a good option for infants since the risk of later complications is high.
However, the availability of the CyberKnife to treat these young patients, either with a single dose or with hypofractionation, represents a significant advance. Although a few centers offer stereotactic radiotherapy – fractionated treatments given over several weeks - with machines that do not require head frames, these machines do not have the precision of the Gamma Knife or CyberKnife.
Such conventional radiotherapy is usually safe but in many cases may not be as effective as single stage or hypofractionated radiosurgery.
Older Children. The options for older children include those for infants as well as conventional radiotherapy and Gamma Knife radiosurgery. Radiotherapy may be given if the tumor is too large for radiosurgery or if the tumor infiltrates into a large volume of brain.
Single dose radiosurgery can be administered with either the Gamma Knife or the CyberKnife, although the CyberKnife may be chosen for many children to avoid the discomfort of the head frame or the use of general anesthesia.
Hypofractionation with the CyberKnife may be chosen even if the tumor is large to take advantage of the high precision offered by this device.
What about tumors of the spine?
Since the CyberKnife does not require a head frame, it can deliver radiation to any area of the body. In this regard, a specific procedure has been developed for radiosurgery of the spine, which uses small fiducials (metal screws) implanted in the spinal bones prior to CyberKnife treatment.
This short operation is generally done on an outpatient basis. Although early experience has been promising, only adults and older children have been treated thus far. Nevertheless, this procedure is technically possible for infants and young children; consult with your doctors and nearby CyberKnife center.
Summary
Radiosurgical treatment of infants with the CyberKnife is appealing and may offer new hope to young patients with serious tumors. However, it is important to recognize that this use of the CyberKnife is new and its development is continuing.
Early experience at the University of Texas Southwestern Medical Center in Dallas has indicated that CyberKnife treatment of infants with malignant brain tumors is safe, and an ongoing protocol at that center is expected to yield more detailed information. Radiosurgery for older children, by contrast, has been studied by many centers over many years and has been shown to offer the same efficacy and safety seen in adults.
CyberKnife radiosurgery for infants and children is a specialized but promising option that will require discussions between you, your doctors, and the team at the CyberKnife center. Although experience is still early, CyberKnife radiosurgery is emerging as a powerful tool for treating infants and children with brain and spinal tumors.