Fractionated Stereotactic Radiosurgery
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 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.
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. Malignant gliomas have been treated with radiosurgery
at the time of recurrence. See
the New Approaches to Brain Tumor Therapy web site (NABTT).
Many benign tumors are successfully treated
with radiosurgery. These include the vestibular schwannomas (acoustic
neuromas), meningiomas and pituitary adenomas. For the vestibular schwannomas,
fractionated stereotactic radiosurgery (FSR) offers sparing of the facial
motor and sensory nerves. 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, FSR is particularly
useful. For the pituitary adenomas, FSR 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 benefit from FSR include
the hemangioblastomas, chordomas, low grade (pilocytic) astrocytomas,
hemangiopericytomas, and others.
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 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.
Today, because of noninvasive fixation
devices, it is no longer mandatory that stereotactic radiation be delivered
in a single treatment. Because the treatment plan can now be reliably
duplicated day-to-day, multiple fractionated doses or fractionated stereotactic
radiation can be delivered. The main advantage of fractionation is that
it allows higher doses to be delivered to the tumor because of increased
tolerance of the surrounding normal tissues to these smaller fractionated
doses. In other words, while single-dose stereotactic radiation takes
advantage of differences in the pattern of radiation given, fractionated
stereotactic radiation takes advantage of not only the pattern, but more
importantly of the differing radiosensitivities of normal and surrounding
tissues. Another advantage is so-called ”iterative” treatment, meaning
the shape and intensity of the treatment plan can be modified during the
course of therapy.
Three Dimensional Treatment Planning
One recent technological advance in stereotactic radiation is the development
of 3-dimensional images of the tumor and surrounding tissues. Sophisticated
software and workstations take 2-mm cuts from either CT or MRI scans and
converts them into 3-dimensional images. Three-dimensional treatment planning
delivers a high-precision dose to the tumor with normal tissue sparing
and is better than that achieved with 2-dimensional planning. Notably,
continued evolution in this technology will directly translate to improvements
in accuracy and predictability of 3-dimensional stereotactic radiation.
Currently all Johns Hopkins acoustic neuroma FSR treatments use fusion
of MRI and CT images to achieve very high sensitivity and precision of
target delineation.
Top
 How
is Treatment Planned?
Once the scans showing the brain tumor and the mask has been 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 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, 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.
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.
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.
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.
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What Is The Simulation
Scan?
 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.
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.
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. 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 planning and treatment of the brain
tumor.
This "simulation" scan is done
using a dedicated scanner. This special device is specifically used for
the treatment planning of radiosurgery. The "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.
Top
 How
is Treatment Administered?
After having the simulation scan, and the treatment planning is completed,
the patient then returns for the treatment. 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 with no scaring. 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.
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What Devices Are
Used?
 Linear
Accelerator
The linear accelerator or "LINAC"
easily treats very small and very large tumor volumes by treating over
time during cell division. When treating over time, it is called fractionated
stereotactic radiosurgery, or FSR.
A LINAC produces radiation that is referred to as high energy Xray, and
has been used in cancer treatment for years. It is a general purpose,
precise and accurate radiation delivery machine that is often used in
radiosurgery. To be used in radiosurgery, a LINAC's hardware must be upgraded
with precision bearings and various sized precision collimators (devices
that modify the size of the radiation beam). LINAC technology is most
often used in multi-session, smaller dose treatments so that healthy surrounding
tissue is not damaged from too high a dose of radiation. LINAC technology
is also able to target larger brain and body cancers with less damage
to healthy tissues. Precise techniques
using one-session Gamma Knife machines and other one-session LINAC technology
are best utilized within the brain. The
most common uses of LINAC stereotactic radiosurgery are for the treatment
of metastatic cancer, some benign tumors and some arterio-venous malformations.
 Gamma
Knife
The gamma knife is really not a knife at all. For this type of procedure,
201 highly-focused x-ray beams make up the "knife" that "cuts"
through diseased tissue. This leading technology makes it possible for
physicians to reach even the deepest recesses of the brain and correct
disorders not treatable with conventional surgery. The Gamma Knife uses
precisely targeted beams of radiation that converge on a single point
to painlessly "cut" through brain tumors, blood vessel malformations,
and other brain abnormalities.. As a result, patients have less discomfort
and much shorter recovery periods.
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