By: Dr. Alishba Batool
Stereotactic Radiosurgery
Stereotactic radiosurgery is a very precise form of therapeutic radiation that can be used to treat abnormalities in the brain and spine, including cancer, epilepsy, trigeminal neuralgia, and arteriovenous malformations. It was conceived by a team of neurosurgeons and physicists in Sweden about 50 years ago to deliver radiation to precise targets in the brain while minimizing injury to adjacent areas. SRS works by distorting and destroying the DNA of tumor cells. As a result, these cells lose their ability to reproduce and die.
What is stereotactic radiosurgery, and how is it used?
It uses sophisticated, 3-D-computerized imaging to precisely focus photon beams, delivering a highly concentrated dose of radiation to a precise target in a single session. It carefully aims beams of X-rays at abnormal tissues through the skin from multiple directions. The X-ray beams distort or destroy the DNA of cells in abnormal areas, so they are unable to reproduce and grow. With optimal treatment, the abnormal tissue becomes inactive and gradually shrinks. Fractionating the treatment allows for high doses to still be delivered within the target while maintaining an acceptable safety profile. The principles of cranial SRS, namely high-precision radiation where delivery is accurate to within one to two millimeters, are now being applied to the treatment of body tumors with a procedure known as stereotactic body radiotherapy (SBRT). SRS and SBRT rely on several technologies,, such as image-guided radiation therapy (IGRT) which uses medical imaging to confirm the location of a tumor immediately before, and in some cases, during the delivery of radiation. SRS and SBRT are important alternatives to surgery, especially for patients who are unable to undergo surgery and for tumors and abnormalities that are: hard to reach, located close to vital organs, and subject to movement within the body.
What are the side effects?
The following side effects are typically temporary and get better within a few weeks:
● fatigue
● skin irritation at the treatment site
● hair loss at the treatment site
● headache
● neurological symptoms, such as seizure, numbness, tingling or weakness
● hair loss in the treatment area
● mouth problems and difficulty swallowing
● eating and digestion problems
● diarrhea
● nausea and vomiting
● headaches
● soreness and swelling in the treatment area
What happens before the procedure?
Stereotactic radiosurgery takes a team approach, involving specialists in radiation oncology and neurosurgery. A dosimetry expert determines how the beams should be directed and a physicist calculates the amount of radiation the patient should receive, balancing risk and potential effectiveness. A CT scan is obtained with contrast administration and is then merged or fused with an MRI of the brain which is usually obtained before the procedure. The CT complements the MRI, and together, the two studies improve the accuracy of the procedure. A key element of planning the procedure is maximizing the treatment of abnormal tissues while protecting healthy tissues around the abnormal area. Steroid medication in the form of Decadron may be administered before the procedure and then continued for a brief time following the procedure. Sometimes a person suffers from claustrophobia.
What happens during and after the procedure?
Local anesthetic is administered, and the frame is secured to the skull by four sterile pins; the only discomfort is during the administration of the local anesthetic. One exception is that some patients who are having treatments to the brain may see flashes of light while the machine is on, even with their eyes closed. When the head frame is removed, there may be some minor bleeding from the pin sites that will be bandaged. In most cases, radiosurgery and SBRT patients can resume all of their normal activities within one or two days.
Are there some complications?
Complications from radiosurgery are few such as pin-site bleeding or infection is rare, and swelling around a tumor may occur, which is why Decadron is administered. In rare instances, a seizure may occur; these are generally brief and self-limiting.
One late complication that may be seen is known as radiation necrosis: tumor cell tumor led by radiation but inadequately cleared by the body. In some instances, this can cause further brain swelling requiring an additional or increased dose of Decadron. In refractory cases, hyperbaric oxygen may be administered, or surgery considered to remove dead tissue. This is why it is important to have close follow-up with a neurosurgeon or radiation oncologist.
It is also possible for a tumor to recur in a different part of the brain, as SRS only targets a very focal area; in such instances, the SRS treatment may be repeated on the new areas of tumor growth. Due to limitations imposed by the overlap of radiation beams coming in from different directions, it is generally recommended to limit the number of tumors treated in one session to four. At some centers, patients who have Parkinson’s disease, epilepsy or some form of psychoneurosis (such as obsessive-compulsive disorder) may be treated on an experimental basis with stereotactic radiosurgery. More recently, with the advent of frameless techniques, stereotactic radiosurgery is being used for spine lesions, more frequently metastatic lesions and less often, benign spine tumors.
What are the benefits of SRS?
This technology makes it possible for neurosurgeons to reach the deepest recesses of the brain and correct disorders not treatable with conventional surgery. Since there is no incision, there are minimal surgical risks and little discomfort. Adult patients may be lightly sedated but are awake throughout the procedure. Hospitalization is short and at most, requires an overnight stay. The majority of patients are treated on an outpatient basis. As a result, patients experience less discomfort and have much shorter recovery periods than having undergone conventional surgery.
What equipment is used?
● The Gamma Knife® uses 192 or 201 beams of highly focused gamma rays all aiming at the target region. The Gamma Knife is ideal for treating small to medium-sized intracranial lesions.
● Linear accelerator (LINAC) machines, prevalent throughout the world, deliver high-energy X-rays, also known as photons. The linear accelerator can perform SRS on larger tumors in a single session or during multiple sessions, which is called fractionated stereotactic radiotherapy.
● Proton beam or heavy-charged-particle radiosurgery is in limited use in North America, though the number of centers offering proton therapy has increased dramatically in the last several years.
How is the procedure performed?
Stereotactic Radiosurgery Using the Gamma Knife: Gamma knife radiosurgery involves four phases: 1. Placement of the head frame, imaging of the tumor location, computerized dose planning, and radiation delivery. In the first phase, a nurse will place a small needle in your hand or arm to give you medications and contrast, if needed, for imaging. A neurosurgeon will use local anesthesia to numb two spots on your forehead and two spots on the back of your head. A box-shaped head frame will be attached to your skull using specially designed pins to keep your head from moving within the frame until the treatment session is finished. This lightweight aluminum head frame is also a guiding device that makes sure the Gamma Knife beams are focused exactly where the treatment is needed. Next, you will be taken to an imaging area where an MRI scan will be performed to show the exact location of the tumor in relation to the head frame. In some cases, a CT scan may be performed instead of, or in addition to, an MRI scan.
Radiosurgery Using the Linear Accelerator: (LINAC) SRS is similar to the Gamma Knife procedure and its four phases: head frame placement, imaging, computerized dose planning, and radiation delivery. LINAC technology is much more common than Gamma Knife technology. Unlike the Gamma Knife, which remains motionless during the procedure, part of the LINAC machine called a gantry rotates around the patient delivering the radiation beams from different angles. When SRS is delivered using CyberKnife, a robotic arm moves the compact LINAC around the patient under image guidance.
SBRT: For some technologies like the CyberKnife that base image guidance according to an x-ray-based solution, you may be asked to have a fiducial marker placed in or near your tumor for selected cases. Placement of the fiducial marker is almost always an outpatient procedure. Next, the radiation oncologist will determine the method of aligning your body with the beams from the linear accelerator, which is called a simulation. Immobilization devices are often used to align patients very precisely and make sure they remain still during simulation and treatment. After an immobilization device is created for you, a CT scan is performed over the area to be treated. physicians may also perform a ‘4DCT,’ where the CT scan obtains information on how your tumor moves while you breathe. This is very common for tumors in the lungs or liver. The third part of the course is planning. The radiation oncologist will work with a radiation dosimetrist and medical physicist to plan the beam arrangement best suited for your tumor. They may incorporate other imaging techniques, such as MRI or PET/CT. Radiation delivery of SBRT is done on a linear accelerator.
References:
Stereotactic Radiosurgery. (n.d). Johns Hopkins Medicine. Retrieved from: https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/stereotactic-radiosurgery
What Is Stereotactic Radiosurgery, and What Conditions Does It Treat? (n.d.). American Association of Neurological Surgeons. Retrieved from: https://www.aans.org/en/Patients/Neurosurgical-Conditions-and-Treatments/Stereotactic-Radiosurgery
Stereotactic Radiosurgery (SRS) | Stereotactic Body Radiotherapy (SBRT). (2023). Radiologyinfo.org. from https://www.radiologyinfo.org/en/info/stereotactic
