Stereotactic radiosurgery (SRS) was created by a team of neurosurgeons and physicists in Sweden about 50 years ago in an effort to deliver radiation to precise targets in the brain while minimizing injury to adjacent areas.
It uses sophisticated, 3-D computer imaging to precisely focus photon beams, delivering highly-concentrated radiation to a precise target in one session. Stereotactic radiosurgery is not technically surgery because there is no incision involved. In addition, general anesthesia is not required for adults undergoing this treatment. SRS distorts and destroys the DNA of tumor cells, similarly to other forms of radiation. Accordingly, these cells lose reproduction abilities and die.
Staged radiosurgery, or fractionated stereotactic radiosurgery (FSR), is a process through which the total dose of stereotactic radiation is divided into multiple smaller doses of radiation, on different days of treatment. Typically, this occurs in 2-5 sessions. In some cases, staged treatment kills the tumor while seemingly decreasing possible side effects in comparison to single-dose radiosurgery.
To direct radiation, a frame is placed on a patient’s head. A doctor administers local anesthetic and the frame is secured to the skull by four sterile pins. Because of this, the only discomfort occurs during the administration of a local anesthetic. The pins do not cause pain if the area is well anesthetized. Additional analgesic medication like morphine can be administered intravenously to provide comfort during the procedure.
A CT scan is obtained with contrast, later being utilized or merged with an MRI displaying the brain, which is usually obtained prior to a procedure. The CT complements the MRI. utilized together, the two scans improve the procedure’s accuracy. Children and some adults may require general anesthesia in order to perform this procedure, causing an anesthesiologist to be required. The treatment team is composed of many specialized medical professionals: dosimetrist, a radiation oncologist, neurosurgeon, radiation therapist, medical radiation physicist, and a radiation therapy nurse.
The neurosurgeon and radiation oncologist are the primary caregivers, being responsible for the safe and effective administration of radiation and completion of the procedure. They analyze the images, formulating a radiation plan with the rest of the team. Once finalized, this radiation is administered. The patient just needs to lie on a table. While they lie down, the head frame is fixed to an apparatus on the table. To this point, the radiation beams are directed. Usually, the treatment time for one lesion or tumor is in the range of 30 minutes. At times when greater than one tumor is targeted, this can take longer. Around the conclusion of the procedure, the frame is removed, the pin sites receive a clean dressing, and the patient is discharged, placed in the care of a family member or friend.
Steroid medication can be administered as Decadron prior to the procedure and then continued for a brief period of time following the procedure. A routine follow-up with the neurosurgeon and radiation oncologist is usually arranged. Additionally, a follow-up MRI of the brain is scheduled at the three-month mark to assess its effectiveness.
The alternative for patients with greater than four metastatic tumors in the brain is called whole-brain radiation therapy. While this proves useful for killing tumor cells and providing this coverage to the entire brain, it can affect normal brain cells and cause both cognitive and intellectual decline; hence, this treatment is reserved for those circumstances in which SRS is not an option.
In certain cases, SRS can supplement or boost the effects of whole-brain radiation. In other cases, SRS is advisable. However, the location of a tumor can be in close proximity to a critical structure like the optic nerves. In these instances, the radiation can be divided into fractions, delivered in the same precise manner. This treatment is called stereotactic radiotherapy (SRT), instead of SRS. The other difference between SRT and SRS is that with SRT, a mask made of a thermoplastic material can be utilized rather than the frame, as the patient must undergo several treatment sessions.
Usually, SRS is reserved for tumors that are less than three centimeters in maximal diameter. The most common application of SRS in practice is for the treatment of metastatic brain tumors. Another commonly utilized application of SRS is the treatment of small tumors, which arise from the vestibular nerve called the vestibular schwannoma (acoustic neuroma).
Stereotactic radiosurgery is another noninvasive treatment option for many patients who have conditions like trigeminal neuralgia, arteriovenous malformations (AVMs), arteriovenous fistulas, and various intracranial tumors.
ARTERIOVENOUS MALFORMATIONS AND ARTERIOVENOUS FISTULAS
An AVM is a tangle of abnormal, poorly formed blood vessels that have higher bleeding rates than normal vessels. AVMs can occur anywhere in one’s body, but brain AVMs present substantial risks, especially as they bleed. Dural AVMs occur in the covering of the brain and are an acquired disorder, which can be triggered by an injury.
An arteriovenous fistula (AVF) is an abnormal channel or passage between an artery and a vein. It causes a disruption of one’s usual blood flow patterns. A person can be born with a congenital AVF, or they can develop an acquired fistula after birth. This is typically prompted by an injury damaging to the arteries.
Even though the effects of SRS on tumor tissue can be seen a few weeks post-procedure, it can take up to two years to see the repercussions of SRS on an AVM. When SRS is utilized to treat an AVM, a doctor obtains an angiogram to improve the accuracy of targeting the critical portions of the lesion. With AVMs, the nidus (the most concentrated portion of the AVM) is the most crucial target. It is preferable to keep the size of the target at a size less than 3.5 centimeters. If the AVM nidus is larger than that, then SRS can be divided into more than one session to avoid potential complications. Another distinction with AVMs is that in some cases, prior to SRS, embolization of the AVM can be carried out to reduce the blood flow within it.
Stereotactic radiosurgery can be a treatment option for certain patients with some metastatic brain tumors arising from other parts of the body (such as the lungs). It may also be helpful for these primary brain tumors:
- Anaplastic astrocytomas
- Pineal tumors
- Pituitary adenomas
- Vestibular schwannomas (acoustic neuromas)
Stereotactic radiosurgery works in the same way as other radiation treatment forms. The tumor is not removed, but radiation distorts the DNA of the tumor cells. Resultantly, these cells lose their reproduction abilities. After treatment, benign tumors can shrink over a period of 18 months to two years. Malignant and metastatic tumors can shrink even more rapidly, sometimes within a couple of months.
Meningioma is another tumor type responsive to SRS. Although surgery is the optimal treatment for meningiomas, those that involve critical structures like the cavernous sinus, or those that are located in areas of the brain where safe surgery to remove them is impossible, SRS is quite useful. It can also be utilized to treat meningioma residue after surgery, especially if there is growth observed in the remaining tumor. In select cases, with careful consultation between the physicians caring for a patient, SRS can be utilized for primary brain tumors, such as gliomas, especially when they recur following standard radiation or chemotherapy.
At times, trigeminal neuralgia (tic douloureux) is described as the most excruciating pain known to humanity. The pain typically involves the jaw and lower face; however, it sometimes affects the area around the nose and above the eye. This stabbing, intense, electric shock-like pain is instigated by the trigeminal nerve’s irritation. The trigeminal nerve sends its branches to the forehead, cheek, and lower jaw, usually with the pain limited to one side of the face.
Stereotactic radiosurgery delivers a single, extremely concentrated dose of ionizing radiation to a precise target located at the trigeminal nerve root. Through a period of time and as a result of radiation exposure, the slow formation of a lesion in the nerve disrupts the transmission of pain signals to the brain. Nevertheless, it is crucial that the procedure be carried out by a team experienced with this SRS application, for the target is at the brainstem and injury may occur, given the high dose of radiation deployed.
There are not many radiosurgery complications. Pin-site bleeding and infection are rare. Swelling around a tumor can occur, but that is why Decadron is administered. Rarely, brief and self-limiting seizures can occur. Caregivers or family members must ensure that the patient is safe and call 911 with their physician’s information, bringing them to the hospital for further care. An anti-seizure medication is provided, usually being quite effective in controlling further seizures.
One potential, late complication is radiation necrosis: dead but inadequately cleared tumor cells from the radiation. In certain cases, this can prompt further brain swelling, requiring additional or increased doses of Decadron. In refractory cases, hyperbaric oxygen can be administered, or surgery considered to remove dead tissue. This complication is why close follow-up with a neurosurgeon or radiation oncologist is crucial.
A tumor may also recur in another section of the brain, as SRS simply targets a very focal area. In these cases, SRS treatment can be repeated on the new tumor growth areas. Because of the limitations imposed by the overlap of radiation beams flowing in through various directions, it is usually recommended to limit the number of treated tumors to four per one session. Sometimes, a neurosurgeon and radiation oncologist will extend that number, but only when provided with careful consultation and consideration for the tumor type, patient’s age, location of the tumors in the brain, and prior treatment history.
At some centers, patients with Parkinson's disease, epilepsy, or other forms of psychoneurosis (e.g. obsessive compulsive disorder) can be treated on an experimental basis with stereotactic radiosurgery. More recently, with the advent of frameless techniques, stereotactic radiosurgery is being utilized for spine lesions, more frequently metastatic lesions, and, less typically, benign spine tumors.
Types of Stereotactic Radiosurgery Procedures
Distinct modalities are able to deliver SRS. The most widely utilized ones are the Gamma Knife and LINAC (Linear Accelerator)-based systems like the CyberKnife® or X-Knife®. Each system contains its own advantages and disadvantages. However, they all share certain similarities: the employment of a radiation source, or generator, in order to deliver photons to a target; the use of computerized algorithms and head frames (or custom-fitted thermoplastic masks) to deliver radiation precisely.
After doctors use pins to attach a stereotactic frame to the patient's head, CT (CAT) scan and MRI are utilized to determine the precise location of the tumor in the patient's brain. Lesions ranging from 5-40 millimeters can be treated with the Gamma Knife procedure. If the patient has an AVM, an angiogram and a CT scan are carried out within the frame attached. The Gamma Knife is comprised of a sphere containing 201 Cobolt-60 sources, which target their beams toward the patient's head. The patient's head is placed within a cavity in the instrument, being covered by a helmet that narrows the beams, and shields the head from unwanted radiation. The radiation is regulated by the number of the 201 ports being utilized, the number of exposures, and the head position. Computer-guided dosimetry is targeted to match the lesion. Different beam sizes are available through the employment of different helmets with holes of various sizes. Lesions from 5-40 millimeters can be treated. Surgeons can perform various exposures by readjusting the helmet and head position to achieve different lesion shapes. This procedure takes a doctor about 30 minutes to complete.
The Linear Accelerator’s (LINAC) initial procedure is similar to the Gamma Knife’s method. Doctors utilize pins to attach a stereotactic frame to the patient's head. Later, they use a CT, CAT scan, or MRI to determine the tumor's precise location in the patient's brain. Lesions up to 3.5 cm in diameter can be treated with the LINAC procedure. If the patient has an AVM, an angiogram and a CT scan are carried out within the attached frame. Computer-guided dosimetry is designed to match the lesion. A cone approximating the lesion’s size is placed in the linear accelerator’s collimator. Cones range from 12.5 millimeters to 40 millimeters in size.
The patient is set lying on his back on the linear accelerator’s treatment couch. The head is secured, precluding movement in-treatment movement. Radiation is targeted at the lesion from different directions in the form of arcs. The arc delivers a predetermined amount of radiation. Then, the treatment couch is rotated along with the collimator housing the cone. This sequence continues until the therapy finishes. The number of arcs utilized varies from about four to six, taking about 30 minutes. Some devices like CyberKnife do not use frames, but hold the head in place using masks instead.
Multiple manufacturers make this type of machine, which have brand names such as Peacock®, X-Knife, CyberKnife, Clinac®, etc.
Different health-care centers have differing systems, and their efficacy in the treatment of the tumors and lesions mentioned above is similar. In certain health-care centers, proton-beam therapy can be available. This is a sophisticated radiation form, which uses similar precision targeting as the aforementioned systems. However, it utilizes protons instead of photons. Protons have different physical properties compared to photons . For some tumors, this can be an advantage and referral to a proton beam therapy center is an option.
However, the biological effectiveness of protons is similar to photons. In the case of most brain tumors, standard photon beam therapy is useful and has an excellent track record and is the modality utilized.
BENEFITS OF SRS
This technology allows neurosurgeons to reach the deepest recesses of the brain, correcting disorders untreatable through conventional surgery. Because no incision exists, minimal surgical risks and little discomfort result. Adult patients can be lightly sedated, but they remain awake through the procedure. Hospitalization is short and if needed, requires an overnight stay. The majority of patients are treated on an outpatient basis. Resultantly, patients experience less discomfort and shorter recovery periods than having undergone conventional surgery.
After stereotactic radiosurgery, bandages are placed over the pin sites from the stereotactic frame. The bandages should also be removed the following day. Patients can be observed for a certain time following the treatment before going home. They can also be kept in the hospital overnight for observation. Certain people experience minimal tenderness around the pin sites. Sometimes, swelling also occurs around the pin sites. Most patients can return to their usual activities the next day as long as swelling is not bothersome.
Each situation must be carefully individualized through close consultation between the patient, the neurosurgeon, and radiation oncologist. This is essential to deciding upon the best treatment plan. On a custom basis, the neurosurgeon will wish to see the patient in the office about one month after the procedure. Then, neurological examination will be performed. Typically, a diagnostic test, such as a CT scan or MRI, will also be performed about six months post-procedure to check on the status of the radiated area. These changes can require between one to three years to take effect.