Overview
The following
is an advanced-level, step-by-step description of stereotactic radiosurgery
(SRS) and fractionated stereotactic radiotherapy (FSR) procedures for
the body performed in a linear accelerator (LINAC). For basic-level
information, see Introduction to
Radiation Therapy.
Stereotactic
radiosurgery (SRS)
Stereotactic
radiosurgery is a form of external beam radiation that delivers a high
dose during a single session to shrink or destroy tumors and diseases
of the body (Fig. 1). Because a single radiosurgery dose is more damaging
than multiple fractionated radiotherapy doses, the target area must
be precisely located and completely immobilized with a stereotactic
body frame. Any tumor, lesion or malformation to be treated with radiation
is called a target. Patients spend one day at the treatment center while
the target is located stereotactically, a treatment plan is developed,
and radiation is delivered.
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Figure
1. Before radiosurgery, a treatment plan is developed to shape
the radiation beam to the exact shape of the arteriovenous malformation
(AVM) and minimize exposure to normal brain tissue. The colored
rings represent the radiation dose level. Over several months to
years after radiosurgery, the AVM vessels close off, effectively
removing the lesion. |
Fractionated
Stereotactic Radiotherapy (FSR)
Radiosurgery
treatments given over multiple visits are called fractionated stereotactic
radiotherapy (FSR). Until recently, fractionation was not possible using
stereotactic techniques because there was no way to keep the rigid frame
in place after the first treatment session. Repositionable body molds
and frames, along with x-ray and infrared positioners, ensure treatment
accuracy, making multiple radiosurgery sessions possible. FSR offers
the precision of stereotaxy for those who have lesions near critical
structures, such as the spinal cord, that cannot tolerate high doses.
Patients spend the first day at the treatment center while the target
is located stereotactically and a treatment plan is developed. They
will return daily for several weeks to receive fractions of the complete
dose.
Am
I a candidate?
You may
be a candidate for SRS or FSR if you have a:
- Primary
spinal tumor
- Spinal
tumor metastasis
- Lung
tumor
- Prostate
tumor
- Kidney
tumor
- Liver
tumor
- Gynecological
tumor
- Pancreas
tumor
- Colon
/ Rectum tumor
SRS and
FSR may be used alone or with other treatments such as surgery, chemotherapy
or immunotherapy. Radiosurgery can be used when a tumor or malformation
is first diagnosed or has recurred after previous treatment; or it can
be used as a supplement, commonly called boost therapy, to other treatments.
Once your
condition has been diagnosed, your doctor will discuss all treatment
options and may recommend a consultation with a radiation oncologist.
The surgeon and radiation oncologist will work together to choose the
best type of radiation for your particular tumor or lesion, explain
the treatment process and describe some possible side effects. Once
you have decided to go ahead with treatment, you will need to sign a
consent form. The doctor may also send you for a special MRI scan for
use during radiation treatment planning.
What
happens before treatment?
Come to
the hospital or outpatient center the morning of the procedure and check
in with the receptionist when you arrive. Dress comfortably and bring
a book or something else to keep you busy during the waiting periods.
You may also bring a friend or a relative, who may stay with you during
the day.
The nurse
or radiation therapist will escort you to a patient holding room, where
you will need to change into a gown.
What
happens during treatment?
Step
1. Attach stereotactic frame
For SRS and FSR, you lie in a stereotactic body frame. A bean bag-like
cushion is vacuum-molded to your body’s exact shape to ensure accurate
positioning during treatment. It does not interfere with breathing and
is minimally confining. The body cushion and frame do not require any
hardware or incisions (Fig. 2).
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Figure
2. A body frame is a custom-formed vacuum cushion that molds
to the exact contours of your body. It attaches to the table and
positions you during treatment. |
Step
2. CT or MRI localization
You then undergo an imaging scan using either computerized tomography
(CT) or magnetic resonance imaging (MRI) or both. The fiducials on the
body frame show up on the scan and help pinpoint the exact three-dimensional
coordinates of the target within the body. After the scan you may get
up from the body frame.
Patients
receiving FSR typically go home after the localization scan. The doctors
continue with step 3 (treatment planning), and the patient returns within
a week or so to begin treatment. In contrast, SRS patients are taken
to a private room and given a light breakfast while they wait for the
treatment plan to be determined so that radiation can be delivered on
the same day.
Step
3. Treatment planning
Information about the target’s location, volume and proximity to
critical structures is gathered by the CT scan and transferred into
the treatment planning computer system. In some cases MRI images also
are sent electronically to the system. The software uses the CT or MRI
images to form a 3D view of your anatomy and the target. Using the software,
the team (radiation oncologist, surgeon and physicist) determine the
radiation prescription:
- appropriate
radiation dose
- number
and angle of treatment arcs
- size
and shape of the beams to exactly match your tumor or target
It is crucial
that the dose be applied only to the affected area. By using numerous
beams, the radiation dose of normal tissue is minimized. All beams meet
at a single point, where the target is located. At the center the single
beams add up to a very high dose of radiation.
Step
4. Position the patient
Once the LINAC is calibrated and prepared for your specific treatment
plan, you lie in the stereotactic body frame. The stereotactic body
frame is secured to the treatment table. Alignment lasers and localizing
x-rays help the radiation therapist position you correctly (Fig. 3).
Infrared markers are placed on your skin near the target area. These
markers are detected by infrared cameras and compared to the body position
in the treatment planning computer (Fig. 4). Stereoscopic x-rays are
taken and compared to the treatment plan. Any misalignments are detected
and corrected by a computer-controlled motorized tabletop.
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Figure
3. The custom-molded stereotactic body frame is secured to the
table, precisely positioning the target in the treatment field.
X-ray positioners, which are mounted on the ceiling, take stereoscopic
x-rays of your anatomy and compares them to the position in the
treatment planning software. |
|
Figure
4. The therapist uses infrared body markers, which are detected
by the infrared cameras, to compare your body position to the computerized
treatment plan and verify correct positioning. |
Step
5. Treatment
Once exact positioning is confirmed, the therapist leaves the room and
operates the LINAC machine from the control room. The treatment team
can watch you through video monitors and speak to you over an intercom.
The LINAC and treatment table periodically move to deliver radiation
beams from one or more directions.
The LINAC
machine is large and makes noises as it moves around your body. Its
size and motion may be intimidating at first; it may pass close to your
body, but will not touch you. Treatment may take 30-60 minutes or longer,
depending on the number of targets.
What
happens after treatment?
Step
6. Remove stereotactic frame
After treatment the radiation therapist releases the stereotactic body
frame from the table. You may then change back into your clothes, gather
your belongings and go home.
If you
received FSR, the custom body mold is stored at the center for your
next treatment session. You will return each day at your scheduled time
to repeat steps 4 through 6 until all fractions of the complete dose
are delivered.
What
are the results?
Following
SRS or FSR treatment, CT, MRI, or angiography scans will be taken periodically
to look for signs of response. Several months may pass before the effects
of treatment are visible.
What
are the risks?
Side effects
vary depending on the tumor type, total radiation dose, size of the
fractions, length of therapy, and amount of healthy tissue in the target
area. Some side effects are temporary and some are permanent. Generally,
patients may experience fatigue, skin irritation around the target area,
and hair loss (see Introduction
to Radiation Therapy).
On rare
occasions, the radiation dose can cause a build up of dead tumor tissue,
called radiation necrosis, several weeks to months after treatment.
Dead or necrotic tissue can become toxic to surrounding normal tissue
and swelling may occur. Treatment for radiation necrosis may include
steroid medication, hyperbaric oxygen treatments, or surgical removal.
Sources & links
If you
have more questions, please contact Precision Radiotherapy at 513-475-7777.
Links
National Cancer Institute www.cancer.gov
International
Radiosurgery Association www.irsa.org
American
Brain Tumor Association www.abta.org
www.radiologyinfo.org
www.oncologychannel.com
Glossary
benign: not cancerous.
chemotherapy: treatment with toxic chemicals (e.g., anticancer
drugs).
fractionated: delivering the radiation dose over multiple sessions.
lesion: a general term that refers to any change in tissue, such
as tumor, blood, malformation, infection or scar tissue.
linear accelerator (LINAC): a machine that creates a high-energy
radiation beam, using electricity to form a stream of fast-moving subatomic
particles.
malignant: cancerous.
metastatic: cancerous tumor that has spread from its original
source through the blood or lymph systems.
radiation: high-energy rays or particle streams used to treat
disease.
stereotactic: a precise method for locating deep brain structures
by the use of 3-dimensional coordinates.
target: area where the radiation beams are aimed; usually a tumor,
malformation, or other abnormality of the body.
tumor: an abnormal growth of tissue resulting from uncontrolled
multiplication of cells and serving no physiological function. A tumor
can be benign or malignant.
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