 February 18, 2002 |
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What is Radiosurgery?Radiosurgery is non-invasive and nonsurgical. Some patients with brain tumors are candidates for this type of treatment. Radiosurgery uses multiple accurately focused beams of gamma radiation which intersect at a single point. Each of these beams has relatively low energy. However, the energy at the center point (where all of the beams intersect) is very high.In practice, the patient's head is restrained in a stereotactic frame and positioned so that the tumor is centered in the spot where all of those beams intersect. In this way the tumor receives a great deal of radiation while the surrounding brain tissue receives very little radiation. Each low energy radiation beam passes harmlessly through scalp, skull and overlying brain but all beams focus on the tumor. Radiosurgery is totally non-invasive except for the application of a stereotactic frame. The Leksell Gamma Knife (LGU)There are two methods for performing radiosurgery. One employs the Gamma Knife. The Gamma Knife was invented by Dr. Lars Leksell in Stockholm about 30 years ago. It was originally designed to make small areas of destruction (lesions) deep in the brain in order to treat pain conditions and movement disorders. Then people started using it to treat small inoperable arteriovenous malformations of the brain. Finally, it was used to treat small benign tumors at the base of the skull. All of these were good indications for this technology. Since the proliferation of Gamma Knives in this country, which began in 1986, the indications for radiosurgery have expanded with attempts to treat other types of tumors such as metastatic tumors and small high grade gliomas.A modern Gamma Knife contains 201 Cobalt60 radioactive sources. These are arranged in a spherical housing. The housing provides lead-lined recepticles for each radioactive source and a narrow opening where each beam of gamma irradiation escapes. These openings, called collimators, direct the beam to the isocenter of the spherical housing. Then a spherically shaped inner collimator helmet narrows the beams even further. The Leksell Gamma Unit (LGU)typically has 4 collimator helmets which differ only in the size of the opening in the collimator (4mm, 8mm, 12mm and 18mm). Database AcquisitionThe patient's head is fitted into a stereotactic frame which is used during CT and/or MRI data base acquisition and during the actual radiosurgical procedure. The stereotactic CT or MRI database defines the tumor as a three dimensional object in space.Treatment PlanningThe imaging data from the stereotactic CT and/or MRI is then transferred to a treatment planning computer system. Here the surgeon, the radiation oncologist and the radiation physicist simulate various dose plans using different collimator sizes and added "shots". A shot is one radiosurgical treatment; this produces an essentially spherical volume of radiation. However, few tumors are exactly spherical and other "shots" may be necessary to treat different parts of the tumor. A computer adds up the radiation from all of these "shots" and this results in a radiation dose field who should have the same shape as the tumor. The computer then prints out the stereotactic coordinates for each "shot" (X, Y and Z)and the time that the patient will be in the gamma knife to receive the prescribed radiation during each "shot".Radiosurgical ProcedureThe rest is pretty simple. The patient, whose head is still in the stereotactic frame, is moved to the couch of the Gamma Knife. The X,Y and Z adjustments ae made on the stereotactic frame. The appropriate collimator helmet has been chosen and fastened to the head of the couch. The patient's head is hen fixed to the inside of the collimator helmet. The radiation dose time is entered into the control unit of the Gamma Knife. Everyone (except the patient)leaves the Gamma Knife room and a lead-line door is closed. All the surgeon has to do now is push a button.After the button is pushed a series of events take place. The doors on the Gamma Knife unit open. The couch with collimator and patient are then moved automatically into the Gamma Knife. The collimator helmet locks into place and a timer starts counting the seconds. When the prescribed amount of time for the radiation dose delivery has passed, the couch with collimator helmet and patient is automatically withdrawn and with doors to the Gamma Knife close. Adjustments are then made for further "shots". The patient is removed from the collimator helmet and new X,Y and Z adjustments on the stereotactic frame. Sometimes the collimator helmet is cjhanged for another collimator helmet having aperatures of different sizes. Once these tasks have been completed, the process described above begins again. Gamma Knife versus LINACThere is usually confusion about the "best" method for radiosurgery: Gamma
Knife or LINAC. Radiosurgery with the Gamma Knife (LGU) is probably the same as
radiosurgery delivered with a linear accelerator (LINAC). IN LINAC radiosurgery,
the linear accelerator head moves around its isocenter in a spherical arc. Here
a patient's head is also fixed in a stereotactic frame which is used to position
the head so that the tumor is in the center of the intersecting beam arcs. In
LINAC radiosurgery, the beam is constantly moving but there is only one beam
which intersects itself from many different directions. In the Gamma Knife there
are many beams from fixed radiation sources which intersect in the center of the
unit. In the LINAC, the radiation beam is produced electronically by a linear
accelerator. In the LGU, the radiation is produced from decay of a Cobalt-60
radiation source. Both produce the same type of energy. Patient SelectionAlthough the idea of noninvasive "surgery" may sound appealing, radiosurgery
is appropriate for only a small percentage of brain tumors. In general, appropriate
tumrs are relatively small (less than 3 cms in diameter) and geometrically regular.
This means that the tumor should be relatively spherical, ovoid or cylindrical.
Tumors that are irregular in shape i.e. star shaped ar crescent shaped tumors
are not good candidates for radiosurgery because it is virtually impossible to fit a
radiation dose volume to the tumor without also delivering a lethal dose of radiation
to the surrounding brain tissue. In addition, there is a tumor size limitation.
Tumors larger than 1 inch in diameter are probably not appropriate for radiosurgery for two
important reasons: First, tumors larger than this must be treated with two or
more "shots". This diffuses the radiation dose so that the surrounding brain
gets radiated - not in a fractionated way (see below) but in a single session.
This can potentially damage the surrounding brain tissue.
Radiation NecrosisThe second reason that large tumors are not good candidates for radiosurgery is that the risk of radiation necrosis increases with the size of the tumor radiated. Radiosurgery turns a live tumor into a dead tumor (radiation necrosis). The dead tissue must then be carted away from the brain by an inflammatory reaction. The bigger the mass of dead tissue the greater the inflammation which can make the patient sicker and dependent on high doses of steroids in attempts to keep the swelling under control. This usually takes place between 6 and 18 months after the radosurgical procedure. Sometimes patients require open surgery to remove this mass of dead tissue because the mass of the dead tumor plus the mass of the surrounding edema (swelling) is causing an elevation of intracranial pressure and/or neurologic deficit.Which tumors are best treated with Radiosurgery?Radiosurgery is usually not the "last chance" for patients with large recurrent glioblastomas. It is also not appropriate for infiltrating glial tumors. The best candidates for radiosurgery are patients harboring small, geometrically regular, slow gowing tumors in which the risk of removing these tumors is unacceptable. These tumors include small acoustic neurinomas usually in elderly or medically unstable patients, some pituitary tumors, small meningiomas-especially those at the skull base (the cavernous sinus, for example), and small geometrically regular gliomas (this is usually considered "experimental" and many of these cases are usually done on a protocol). Selected patients with metastatic tumors are also good patients for radiosurgery. Very vascular metastatic tumors such as renal cell carcinoma do especially well with radiosurgery. In addition, patients with multiple metastatic tumors have been treated with multiple shots of radiosurgery to treat each tumor. This may be found to have some advantages over whole brain irradiation which is te standard treatment for multiple metastases. |
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