Overview of Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)
Radiation therapy is a treatment method used to destroy unhealthy tissue such as tumors, while minimally affecting the healthy tissue that surrounds it. Since 1951, a technique called stereotactic radiosurgery (SRS) has been used as an alternative to a more general method of radiation treatment for the brain.
SRS works by using multiple high-energy beams of x-rays, gamma rays or protons, which all come together on a specific treatment area that can be seen on an x-ray. This technique allows the treatment to be shaped very precisely, so it matches the area that needs to be treated. Because multiple beams are used that all intersect at the treatment area, a high dose of treatment can be given to the precise area, while the normal brain tissue nearby gets a much lower dose of radiation.
This means that the treatment can be very closely tailored to fit the exact area that needs treatment, allowing the energy to rapidly dissipate or spread out as soon as it passes the edges of the treatment area. This helps keep the normal tissue safe by sparing it from the treatment. It also limits any toxicity and side effects while keeping the treatment safe.
How does stereotactic radiosurgery work? It causes damage to the DNA of the cells in the area being treated by creating ions and free radicals, which are molecules that can cause damage inside cells. The main target is the cells lining the inside of blood vessels in the area. This process also involves cell death, dysfunction in tinier blood vessels, and an immune response from a type of white blood cell called a T-cell. If you were to look at a sample of tissue under a microscope after treatment, you’d see a strong response from the body’s immune system and serious damage to blood vessels in areas that have responded well to the treatment.
Why do People Need Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)
Stereotactic radiosurgery is a type of radiotherapy that targets small, precise areas in the body. It’s a good treatment option for patients with brain tumors, including those that have spread (metastasized) from other parts of the body or those that are resistant to traditional full-brain radiation therapy. This treatment is especially helpful for people who have one or more cancer cells in hard-to-reach areas of the brain.
Brain metastases, which are cancer cells that have spread to the brain from other parts of the body, are quite common in adults who have cancer. This type of radiosurgery can be used alone or in combination with other treatments, including surgery, to treat single or multiple areas affected by cancer. However, the role of combining stereotactic radiosurgery with full-brain radiation therapy is still unclear. While some studies show benefits in controlling the spread of the cancer, the results have been conflicting regarding overall survival and cognitive function.
Meningiomas and vestibular schwannomas are types of tumors that grow outside of the brain tissue and have clear boundaries, making them suitable for stereotactic radiosurgery. Studies have shown that this procedure can control the growth of these tumors with minimal side effects.
For pituitary adenomas, which are tumors that originate from the pituitary gland located at the base of the brain, surgery may not always be able to completely remove the tumor. In such cases, stereotactic radiosurgery can be a suitable option. The procedure has shown high rates of controlling the tumor growth and low rates of side effects.
Stereotactic radiosurgery is also used in treating vascular disorders in the brain, such as arteriovenous malformations (AVMs), which are tangles of abnormal and poorly formed blood vessels. It can effectively treat these conditions, especially when they are not suitable for traditional surgery.
Moreover, this type of radiosurgery can be used to treat functional disorders, like severe pain, epilepsy, and movement disorders. For example, it’s been used to treat trigeminal neuralgia, a facial pain disorder, with good results. However, some patients require repeated treatments which can result in trigeminal dysfunction, causing issues with facial sensitivity and movement.
Stereotactic radiosurgery can also be used to treat spinal and paraspinal lesions. It’s particularly helpful for treating metastatic spinal tumors that can cause pain. It’s an effective treatment option with high rates of pain relief and tumor control.
When a Person Should Avoid Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)
Stereotactic radiosurgery (SRS), a type of radiation therapy that focuses high-power energy on a small area of the body, may not be suitable if the area to be treated is too large or if there are too many areas to be treated. SRS is usually used for small areas, typically less than about an inch and a half. However, there is a trend towards using it more often for larger areas.
For a brain tumor called glioblastoma, a trial was conducted to compare the effectiveness of SRS with external beam radiotherapy and chemotherapy versus just radiotherapy and chemotherapy. The trial found no differences in overall survival rates, the control of the disease at its original location, or in improvement of quality of life. In addition, a low percentage of patients (4%) suffered from serious side effects. Currently, SRS is not widely recommended due to the lack of conclusive evidence.
The use of SRS for a procedure called pallidotomy, a surgical procedure to alleviate involuntary movements or rigidity in Parkinson’s disease, resulted in many complications, including visual field defects. As a result, it’s not commonly used.
Equipment used for Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)
Stereotactic radiosurgery, which uses focused radiation beams to treat conditions like tumors and abnormalities in the brain, can be performed using different types of devices. These devices can vary based on the kind of radiation used and the method of focusing radiation on the target area.
One type of radiation used is ionizing radiation, which can create charged particles when it passes through matter. This radiation can come in different forms, including rays like radio waves and X-rays. For stereotactic radiosurgery, only X-rays, which have enough energy to create charged particles, are used. These X-rays are generated either from isotopes (currently, cobalt-60) or X-ray machines known as linear accelerators.
The first type of device for stereotactic radiosurgery, introduced in 1967, used focused gamma beams produced by radioactive cobalt. It involved 192 individual sources of radiation arranged in a specific manner. The decay of cobalt generates strong ionizing radiation. These cobalt sources are organized to focus the beams at a single target – a technique that is primarily used for treating conditions inside the skull.
In the early 1980s, a new method using linear accelerator radiosurgery was introduced. This device produces high-energy X-rays by accelerating electrons against a metal target. It’s a flexible tool as these X-rays can be focused and can be used to target areas both inside and outside of the skull.
A spin-off of linear accelerator technology is intensity-modulated radiation therapy (IMRT). It varies the shape and intensity of the beam, making it possible to deliver non-uniform doses of radiation, thereby minimizing harm to healthy tissue. This method is useful for treating areas outside of the skull and the spine as well.
Another way is using protons, particles found in the nucleus of an atom. Devices based on synchrotrons or cyclotrons produce proton beams, which deliver energy as they pass through tissue. Because protons can be precisely controlled, they can deposit the bulk of their energy directly at the target. However, due to cost and space requirements, proton-based stereotactic radiosurgery is less common.
Choosing between these different methods often comes down to the expertise of the doctor and the availability of the necessary equipment. To this day, there is no clear-cut clinical trial data to say one method is superior to the others.
Determining the right amount of radiation (dosimetry) to use during stereotactic radiosurgery is a critical yet challenging process. Multiple factors contribute to this challenge, including the small field of treatment, changes in the particle properties, and the technical difficulties associated with containing and measuring the radiation.
Who is needed to perform Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)?
When dealing with conditions that impact the brain, a team of different medical specialists is usually needed. These include brain surgeons, also known as neurosurgeons, who specialize in operations on the brain and spine. Radiation oncologists are doctors who use radiation to treat cancer. Neuroradiologists specialize in diagnosing and treating illnesses in the brain and spine using medical imaging technologies.
Also, medical physicists work to ensure that the right amount of radiation is used during treatment. A dosimetrist is another key team member, responsible for calculating the correct dose of radiation for your treatment. Then there’s the radiation therapist, who operates the machines that give radiation treatment.
Finally, nurse specialists, particularly those who specialize in cancer care (also known as oncology nurses), provide you support, advice and care throughout your treatment. Together, this team works to offer the best possible treatment and care for your condition.
How is Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT) performed
Stereotactic radiosurgery is an outpatient procedure, which means you won’t have to stay overnight in the hospital. It’s a type of radiotherapy that can help treat some medical conditions by directing high-powered x-rays at a small spot in the body. Unlike traditional surgery, it doesn’t involve cutting into your body, so you won’t need general anesthesia, meaning you won’t be put to sleep.
Let’s break down the process, step by step. First, the doctors have to find the exact spot where the treatment is needed. For this, a specialized helmet is attached onto your head that helps them direct the radiation accurately. This helmet is secured with a head frame fixed onto your skull with screws. Don’t worry, a local anesthetic (numbing medicine) is used. If the thought of a frame seems uncomfortable, there’s also a frameless version.
To ensure that the gamma rays (a type of high-energy radiation used in this treatment) are correctly focused, images of your head are taken with the help of an MRI, CT, or angiogram. With this technique, both you and the machine stay still throughout the treatment.
Another approach is called linear-accelerated surgery. In this method, either you or the device (gantry) can move to change the point where the radiation is delivered. It used to require a frame to keep your head still but now, there are frameless options which are more comfortable for the patient.
Once the location has been defined, a personalized treatment plan is designed which uses a computer system. This system uses data like depth doses, tissue maximum ratio (which refers to the ratio of the radiation dose at a certain depth in the body compared to the dose at the surface), off-axis factor, and collimator output factor for each stereotactic collimator (a medical device that helps shape the radiation beam). This design strategy helps adjust the radiation exactly to the shape of the area to be treated, reducing any potential harm to the surrounding normal brain tissue.
After ensuring the treatment plan’s safety, you will be placed on a treatment couch, and the radiation treatment begins. The process can take anywhere between half an hour to three hours. To help minimize potential complications, you might be given a medicine called dexamethasone. Most patients can go home about an hour after the treatment and return to normal activities within a week.
Possible Complications of Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)
Radiation therapy can sometimes cause damage to blood vessels and nerve fibers in the brain, which may lead to tissue decay. Immediate side effects that happen within weeks of treatment include symptoms like a headache, nausea, and vomiting. These are probably due to swelling in the brain and disruption of the barrier between the bloodstream and the central nervous system. Subacute effects, which tend to resolve over six months, come from widespread nerve damage and can show up as sleepiness and fatigue. Later effects, which may happen six months after treatment, result from long-term damage to nerve fibers and cells lining blood vessels in the brain. This type of damage could be permanent and symptoms include progressive memory loss.
In comparison, Stereotactic radiosurgery (SRS), a type of radiation therapy that uses special equipment to position a patient and deliver radiation to a specific area of the brain, has fewer side effects than whole-brain radiotherapy (WBRT). Reacting immediately to SRS, patients usually develop a headache but complications such as infection at the pin site, seizures, or temporary worsening of neurological symptoms are less common (occurring in less than 5% of cases). Late side effects can include radiation decay, swelling of the brain, or new or increasing neurological deficits, but these are rare (occurring in less than 5% of cases). Some research suggests that responses such as brain swelling can depend on the size of the tumor, its location, the dosage of radiation, and other factors. SRS can lead to delayed effects on cranial nerves, particularly those related to vision. Factors that might contribute to this type of injury are the type of nerve, the dosage of radiation, and the length of the nerve treated with radiation.
Factional stereotactic radiotherapy (FSR) is emerging as a promising treatment approach. It comes with a lower rate of radiation decay than SRS and a reduced risk of damage to the optic nerve caused by radiation.
What Else Should I Know About Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT)?
Stereotactic radiosurgery (SRS) is an excellent treatment option for things like brain and spine cancers, original (primary) tumors, blood vessel conditions, and functional issues. This treatment method is evolving and becoming suitable for more conditions.
SRS is a better option compared to whole brain radiation therapy (WBRT), a treatment that is often linked with severe complications related to our nervous system, including issues that can affect your thinking abilities. These issues limit its use.
One of the benefits of SRS over traditional surgery is that it’s less invasive. It’s a process that doesn’t require putting you to sleep with general anesthesia and can be done in a day, meaning you’ll be able to go home after. What is also impressive is the fact that it has short recovery times and can be used to treat multiple problematic areas in one go. It is a useful method for targeting those problem areas that are deep within the body and hard to reach, or those located in critical regions.
