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Tri-State Regional
Cancer Center
706 23rd St.
Ashland, KY 41101
(606)-329-0060
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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 typically associated with
radiotherapy, such as nausea, red skin, or hair loss.
Why fractionation?
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
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 the present, no published studies
confirm this hypothesis. |
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