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Last Updated: 1/9/2004
WHAT IS STEREOTACTIC RADIOSURGERY?
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 are necessary. These tools include the stereotactic frame, the CT or MRI scanner, a computerized system for calculating the radiation dose, and a precise system for delivering the radiation.
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:
- Large volumes of irradiation (sometimes including a large volume of normal brain) and
- 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 tumor while minimizing the effect on the normal brain. Normal brain tolerates small, daily doses of radiation relatively well. The tumor does not tolerate the small daily doses, resulting in control of the tumor. By exploiting this difference in response, the fractionated treatment can be very effective in reducing or even eliminating the tumor while sparing the normal brain. This concept of fractionation is also very important for radiosurgery, as is discussed below.
How is precise localization achieved?
For Stereotactic Radiosurgery the stereotactic frame provides an "external frame of reference" for the subsequent radiation treatment planning. This means that the location of the frame is known, and, via the CT or MRI scans both the frame and the brain tumor can be simultaneously visualized and precisely localized for the subsequent treatment planning on the computer.
How is the dose planning performed?
Once the CT or MRI scans showing the tumor and the frame are acquired, the images are transferred to a computer workstation. There, the tumor is outlined and the treatment planning begins. The radiosurgeon has several variables that must be carefully integrated for a successful plan. The dose to the tumor should be as uniform as possible with very low dose to the surrounding normal brain. The radiosurgeon selects the position within the 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 tumor can be selected. Metal tubes called "collimators" of different diameters, usually from about 13 mm to 34 mm in size, shape the beam. The collimators can be combined to yield very precise coverage of the 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 dose.
How does the patient receive treatment?
The patient then returns to the treatment area, is positioned on the treatment table and receives the treatment. In our institution, almost all 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 single dose modalities. The patient feels nothing as the beam treats the tumor. Usually there are none of the side effects usually associated with radiotherapy such as nausea, red skin or hair loss.
The rationale for fractionation of radiosurgery is the same as that for conventional radiation: It results in the highest "therapeutic ratio" (highest killing of tumor cells with the lowest effect on normal brain). We know that conventional radiation could never be done in a single fraction, and we have therefore taken advantage of the benefit of fractionation for the radiosurgical cases.
Which tumors can Stereotactic Radiosurgery treat?
Radiosurgery can successfully treat many different 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). The following images show the treatment planning for a solitary brain metastasis from adenocarcinoma of the breast. The images show the precise distribution of radiation dose limited to the tumor and sparing the surrounding normal brain. The second panel shows the absence of the tumor on the post-treatment MRI obtained three months after radiosurgery
(Click Image For Larger Version)
Computer Display of Treatment Planning: Lines Encircling the Tumor Show Relative Dose (RAD).
MRI Three Months After Radiosurgery for Brain Metastasis Shows Tumor Is No Longer Visible.
Many "benign" tumors can be 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. For the meningiomas that are difficult to remove because of location, 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).
What are the different types of machines for Stereotactic Radiosurgery?
The 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. 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.
The cobalt source machines 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. Further, the cobalt source machines have smaller collimators that may render larger tumors more difficult to treat with a homogeneous dose of radiation.
The 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. Proton beam therapy is useful for many skull base tumors and vascular malformations of the brain.
The Peacock system uses "inverse" treatment planning to make a very conformal distribution of the radiation dose in the tumor. It works in a way similar to a CT scanner to precisely determine the amount (weight) for each of many small beams that irradiate the target. This system also allows fractionation.
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.
Johns Hopkins' Radiosurgery
for Brain Metastases"
J. Neuro-Oncology 37:79-85, 1998.
""Stereotactic Radiosurgery for Human Glioma:
Treatment Parameters and Outcome for Low vs. High Grade"
Journal of Radiosurgery 1:3-8, 1998.
|For More Information, Contact:|
|Daniele Rigamonti, M.D.|
|Director, Stereotactic Radiosurgery at Johns Hopkins|
|The Weinberg Building|
|401 N. Broadway|
|Baltimore, MD 21231|
|web site: www.hopkinsmedicine.org/radiosurgery|
|contact name: Tammy Cuda|