Stereotactic radiosurgery is the very precise delivery of radiation to a brain tumor with sparing of the surrounding normal brain. To achieve this precision, special procedures for localization of the brain tumor are necessary. These tools include the stereotactic frame, the CT or MRI scanner, a computerized system for calculating the radiation dose to the brain tumor, and a precise system for delivering the radiation to the brain tumor. Stereotactic radiosurgery offers an important alternative to more invasive treatments for many brain tumors. The role of radiosurgery vs. surgery is determined by many factors. These include the size of the brain tumor, location, how rapidly the symptoms arose, how ill the patient may be (If the patient is very ill, surgery may offer more rapid resolution of the tumor), and the histology (type) of the brain tumor.
How is Stereotactic Radiosurgery different from conventional radiotherapy?
Conventional radiotherapy is a very useful treatment modality for many brain tumors. This modality is characterized by 1.) Large volumes of irradiation (sometimes including a large volume of normal brain) and 2.) Fractionation. Fractionation means that the treatment is divided into multiple smaller doses (fractions) of radiation. The reason for fractionation is to improve the radiation effect on the brain tumor while minimizing the effect on the normal brain. Normal brain tolerates small, daily doses of radiation relatively well. The brain tumor does not tolerate the small daily doses, resulting in control. By exploiting this difference in response, the fractionated treatment can be very effective in reducing or even eliminating the brain tumor while sparing the normal brain. This concept of fractionation is also very important for radiosurgery, as is discussed below.
For Stereotactic Radiosurgery the "simulation" is a special scan. The simulation allows creation of the images that are used for the treatment planning. The simulation shows the exact relationships of the target to the surrounding normal brain. The simulation involves creation of the custom-fitted "mask," positioning of the patient in the scanner, and then the scan of the brain tumor itself.
The Mask
Prior to the simulation scan, the special plastic "mask" is made. The mask is made of "thermoplastic." This material is soft and pliable when warm, but is hard after it cools. This material is used to create a means for reproducibly positioning the head prior to the scan. This material is perforated so that the patient can see outside the mask. The mask does not cover the mouth or the nose, so that breathing is not impeded.
The Scan
With the mask in place the scan is obtained. A "localizing ring" is attached to the base ring that holds the mask system. No attachments to the skin are made. All hardware is external to the patient, without incisions, anesthesia or drugs. For the scan, the mask provides an "external frame of reference" for the subsequent radiation treatment planning (please see below). This means that the location of the frame is shown on the scan along with the intracranial tumor. Therefore, both the frame and the brain tumor can be simultaneously visualized and precisely localized for the subsequent treatment planning and treatment of the brain tumor.
Dedicated Scanner
This "simulation" is a brain scan that is done in our department using a dedicated scanner. This special device is dedicated to the treatment planning for radiosurgery. This "simulation" lasts about 2 and one-half hours for the patient, who can then return home afterwards. No hospitalization is necessary for this non-invasive procedure. After the simulation the treatment planning begins.
Once the scans showing the brain tumor and the frame are acquired, the images are transferred to a computer workstation. There, the brain tumor is outlined and the treatment planning begins. The radiosurgeon has several variables that must be carefully integrated for a successful plan. These include:
Tumor Volume
The volume of the treated brain tumor is important. The size can determine the schedule for fractionation (number of treatments, the dose per treatment, and hence the total dose that is both safe and effective). The larger the brain tumor, the more desirable is "fractionation" (multiple smaller treatments (fractions) rather than one big one). This is because for any treated tumor the "shell" of normal tissue just outside the tumor volume will receive some portion of the dose. For larger tumors, this "shell" volume increases rapidly as a function of tumor diameter. The fractionation spares this "shell" of normal tissue much more effectively than the single "shot" techniques such as the gamma knife.
Tumor Shape
The shape of the brain tumor (regular vs. irregular) can affect the "homogeneity" of the dose in the brain tumor. The dose to the tumor should be as uniform as possible with very low dose to the surrounding normal brain. Using the larger beam shaping devices (collimators) that are available with the fractionated stereotactic radiosurgery (FSR), the homogeneity is much higher for larger brain tumors when compared to the cobalt-based systems (gamma knife). In particular if normal tissues such as the cranial nerves (facial or trigeminal for example) pass through the brain tumor, the homogeneity is critical to preserve normal function. The most striking example is preservation of facial nerve function for the acoustic neuroma and trigeminal (facial sensation) for the meningiomas of the skull base and cavernous sinus.
Proximity of Normal Structures
For brain tumors that are close to the optic chiasm (crossing of the optic nerves, just above the pituitary) (pituitary tumor, for example) or for tumors having normal nerves pass through their center (acoustic neuroma and meningioma of the cavernous sinus or skull base, for example) the fractionation is even more critical. This allows preservation of the function of the facial nerve, trigeminal nerves optic nerves and other cranial nerves while killing the tumor (please see "fractionation" below).
Targeting
The radiosurgeon selects the position within the brain tumor that will be the center of the arc of rotation of the linear accelerator. This is the "isocenter." For each isocenter, the diameter of the beam that best conforms to the brain tumor can be selected. Metal tubes called "collimators" of different diameters shape the beam. The collimators can be combined to yield very precise coverage of the brain tumor. The dose plan is developed on the computer, checked by the physicist, and tested on the accelerator using a phantom to confirm the correct position of the dose and size of the dose.
The patient then returns to the treatment area, is positioned on the treatment table and receives the treatment. In our institution, almost all brain tumor treatments are fractionated. Thus, the frame is attached to a rigid plastic mask that precisely contours the facial skeletal features. This allows "repeat fixation" of the patient for multiple, outpatient treatments that result in no scars as with gamma knife treatments. The patient feels nothing as the beam treats the brain tumor. Usually there are none of the side effects usually associated with radiotherapy such as nausea, red skin or hair loss. Most patients carry on their normal daily activities before and after the daily treatment.
The rationale for fractionation of radiosurgery is the same as that for conventional radiation: It results in the highest "therapeutic ratio" (highest killing of brain tumor cells with the lowest effect on normal brain). The brain tumor and the normal tissues respond differently to high single doses vs. multiple smaller doses of radiation. Single large doses can kill more normal tissue than several smaller doses. Multiple smaller doses can kill more tumor cells while sparing the normal tissues. There is no instance in our department in which a brain tumor is treated with a single "shot" of radiation.
Radiosurgery can successfully treat many different brain tumors, both benign and malignant. The malignant tumors treated most often are the "brain metastases" or tumors that have spread to the brain. They are ideal targets, usually spherical, and displace normal brain, rather than "infiltrating" into normal brain. The malignant gliomas have been treated with radiosurgery at the time of recurrence. Our own data show that these results are comparable to those of most other modalities given at the time of recurrence, and have less toxicity. At the time of recurrence, other glial tumors may be successfully treated including the pilocytic astrocytoma and the recurrent "low grade" infiltrating gliomas (Grade I and II).
Many benign tumors are successfully treated with radiosurgery. These include the acoustic neuromas, meningiomas and pituitary adenomas. For the acoustic neuromas, radiosurgery offers sparing of the facial motor and sensory nerves when compared to surgical resection and to single "shot" radiosurgery that is provided by the "gamma knife." For the meningiomas that are difficult to remove because of location near the skull base or cavernous sinus, or for those that are recurrent after surgery and regular radiation, radiosurgery is particularly useful. For the pituitary adenomas, radiosurgery can spare the optic nerve and chiasm as well as the hypothalamus (thus sparing the "releasing hormones" that drive the normal pituitary). Other tumors that can have radiosurgery include the hemangioblastomas, chordomas, low grade (pilocytic) astrocytomas, hemangiopericytomas, and others.
What are the different types of machines for Stereotactic Radiosurgery?
Linear Accelerator
provides very precise, uniform irradiation for stereotactic radiosurgery of brain tumors. Importantly, this device allows "fractionation" of treatment that allows the safe administration of a higher dose of radiation than can be given with the machines using multiple cobalt sources (for example, the gamma knife). The linear accelerator produces radiation having a higher energy than that produced by the cobalt-source machine. Further, the collimators or beam-shaping devices can be larger for the linear accelerators, resulting in much greater uniformity of dose for the larger tumors.
Cobalt Source
machines such as the gamma knife are also very precise. However, because the frame has to be bolted on to the patient's head with metal bolts, fractionation of treatment is not possible. We know that fractionation results in the highest "therapeutic ratio" (tumor killing vs. damage to normal brain). Further, the cobalt source machines have smaller collimators (beam shaping devices) that may render larger tumors impossible to treat with a homogeneous dose of radiation. This inhomogeneity of the gamma knife results in "hot spots" that can kill normal brain tissues. If the hot spot is in the region of the cranial nerves at the base of the skull for a gamma knife treatment, toxicity can result for facial sensation, facial strength, hearing, movement of the eyes and other functions.
Proton Radiosurgery
derives its advantage from the so-called "Bragg peak" that describes deposition of radiation dose from proton beams. As the protons in the beam slow down in tissue, they give up (deposit) disproportionately more radiation per unit of travel. Just before the protons stop, they give up almost all their energy, resulting in a "peak" at that depth in tissue. The depth can be precisely defined by the energy imparted to the proton beam by the cyclotron that produces the beam. The rates of complication for the proton therapy may be higher than for the "x-ray" or "photon" based therapy as given by the linear accelerator or cobalt sources.
What is the utility of combining chemotherapy or radiosensitizers with Stereotactic Radiosurgery?
Now the combinations of radiosurgery and chemotherapy or radiosensitizers are being explored. These combinations may provide additional control of the tumor, but at present, no published studies confirm this hypothesis.
Can radiosurgery be given more than once for the brain tumors?
At the present time and for the foreseeable future this would be difficult to accomplish. For example, we know that 10 years after the irradiation of the pituitary adenoma we can give HALF of the original dose. This is because the tissues at risk (cranial nerves for the skull base meningiomas, for example) recover the tolerance to radiosurgery only very slowly. This is the basis for "fractionation" (see above) that spares these structures and allows a higher dose to the brain tumors during the FSR.
There are no published or anecdotal (hearsay) reports that show second tumors after radiosurgery. The reason may be the focal, local method of radiosurgery, sparing exposure of the surrounding normal tissues. Only a small volume of normal tissue receives radiation. Decades of observation involving thousands of patients may be necessary to determine if there may be an association.
