What is Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT)?
Imagine throwing a small rock into a calm pond. It will create a splash of water in the center, and then a wave around the splash that gradually, but quickly, disappears. It is indeed the concept of radiosurgery.
Radiosurgery uses a sharp radiation beam to create a defined “splash” of radiation energy accumulating at the site of the tumor, then makes the waves of the radiation dose decrease rapidly, thereby minimizing the radiation effects to the adjacent normal tissue. The “splash” occurs at the radiosurgery target destroying the tumor, while the adjacent normal tissue is not harmed.
Because of the “splash” effect (ie: sharp radiation dose deposit) that occurs exactly on the target tumor, radiosurgery requires precise targeting. The radiosurgery procedure should be precise within 1 mm. More recently, the technique of performing radiosurgery improved to be less invasive but more accurate. It is also being applied to other body sites, such as the lung, head, and neck, liver, pancreas, etc. The application of stereotactic radiation to extra-cranial sites is changing the landscape of cancer treatment and opening exciting options for patients that did not exist fifteen years ago.
How is Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT) Performed?
Radiosurgery delivers a high dose of precise radiation to a target. Radiosurgery uses conformal radiation beams without a scalpel– no incision or anesthesia is required. Since the procedure is performed with the patient awake, it requires the patient’s cooperation and understanding of the procedure. The components of radiosurgery can be summarized in six steps. At each step of the way, proper decision making by experienced team members is important to ensure an optimal outcome. Every tumor is different, therefore radiosurgery must be individually designed for each patient.
1. Evaluation of your condition. This first step is like any other medical or surgical procedure. For radiosurgery, it is important to know if and how the patient will tolerate the procedure while awake. A good understanding of the disease and proper interpretation of the ancillary studies are critical when designing each treatment.
2. Positioning and immobilization. Radiosurgery is an image-guided procedure. Reliable patient positioning and accurate analysis of imaging studies are important. Imaging studies must be properly interpreted, as well as the tumor’s 3-D geometry and potential radiosurgical beam entry and exit. Because the radiosurgical beam is directed from the outside of the body, any patient movements must be minimized. Many positioning technologies, such as an optical device and an image guiding device, are used. Various commercial systems are available, but none are perfect. The experience of the radiosurgery team plays an important role in this step.
3. Stereotactic target localization and respiratory monitoring. Once the position is determined, the target tumor must be precisely located in relation to the surrounding normal tissue and body. In the field of radiation oncology, this step has been called “simulation.” Since the radiation calculation formula is based on CT parameters, CT simulation is usually used. Three-dimensional stereotactic parameters are obtained at this time. Some measurements are performed in relation to body positioning. In the case of tumors that may move with your breathing, respiratory gating and monitoring will also be performed in a computerized method. Your tumor position will be checked according to your breathing cycle.
4. Radiosurgical beam design and QA. This step requires sophisticated 3-D graphics of the tumor and computerized calculations. For better visualization of the target, the CT image is fused with other images including MRI, PET, and functional imaging. The radiosurgery beam is designed using techniques of 3-D shaping, dose painting, and intensity modulation by precise control of the micro-Multi-Leaf Collimator (mMLC). Of course, beam entry and exit, as well as critical structures in the path, are checked. Once the design is complete, extensive quality assurance will be performed under a ‘dry-run’ of your treatment.
5. Treatment. The patient is brought to the radiosurgery suite for re-positioning exactly the same as the initial immobilization using image-guidance and stereotactic localization parameters. The radiosurgical beam is delivered to the tumor by the robotic movement of treatment equipment. During this session, only the patient is allowed in the treatment room and constant monitoring and motion tracking are performed by remote controllers. Radiosurgery does not require hospitalization. Patients usually resume pretreatment activities on the following day.
6. Follow-up program. The goal of radiosurgery is different for each patient and each tumor. It varies from stopping tumor growth to eliminating the tumor altogether. Follow-up includes regular clinical exams and imaging studies. Stony Brook Medicine has a dedicated tumor board for outcome evaluation.
Understanding Radiosurgery Equipment
Understanding the various types of radiosurgery equipment available is important when making a decision about treatment, but the equipment or computer will not cure the tumor. It is more important for the radiosurgery team to understand the behavior of the tumor and to know how to use it for your tumor. Each radiosurgery system has some different technical features, and more advanced systems are continually being developed. We will explain the various types of radiosurgery equipment commercially available:
• Gamma knife (Elekta): This system uses many gamma rays (201 Cobalt-60 radioisotopes), which converge to one area. As the isotope decays over time, the treatment time gets longer. Since this is the first generation, this commercial name has been synonymous with radiosurgery. The gamma knife is used for treating tumors in the brain and head region only. Perfexion is the newest version of the gamma knife.
• X knife (Radionics): The first step in the technological progress of radiosurgery after the gamma knife, the x knife system uses various sizes of cones to produce sharp radiation beams from the existing linear accelerator radiotherapy machine.
• Novalis (BrainLab): This radiosurgery system is the product of several major technical developments. The linear accelerator produces radiation and conforms the beam along the tumor shape every 2.5-3 mm using micro-MultiLeaf Collimator (mMLC). This also uses automatic patient positioning and robotic couch movement. The movable part within the patient’s body is tracked during the radiosurgery procedure. The treatment only takes 20-30 minutes.
• Cyberknife (Accuray): This system uses a movable linear accelerator, and has one radiosurgery cone that robotically moves to deliver many radiation beams within the target. This system also tracks movable parts within the patient’s body during treatment. There also are many moving parts within the equipment.
• Edge (Varian): This is a new unit with the utmost combination of technical improvements in achieving accurate body positioning and motion tracking as well as radiation beam delivery with high-definition, a multi-leaf collimator (HD MLC, 2.5 mm thick), and an image-guidance system. It also incorporates a GPS system for tracking and monitoring internal organ movement.
• Tomotherapy: This system works similarly to obtaining a cross-sectional CT image. It uses high-energy x-rays and delivers intensity-modulated radiation beams to the target.
• Proton Beam Therapy: It is unique because radiation stops at the designated depth, but the entrance radiation dose tends to be higher when the tumor is sizable. Its biological effect is the same as other radiosurgery treatments, but it is much more expensive.
What matters most for the patient is the accumulated experience, knowledge, and teamwork of the radiosurgery team, not what kind of radiosurgery system you use. The best treatment comes from the appropriate and logical interpretation of the tumor, the clinical condition of the patient, and finally coming to the right decision about radiosurgery for the needs of the individual patients. Imagine you have a very nice car, driving through rough roads with dangerous curves. Who would you choose to drive the car, a novice, or an experienced driver? Our radiosurgery/SBRT team consists of highly experienced experts in the field of oncology, neurosurgery, general surgery, orthopedics, pulmonology, etc. They will discuss your individual situation and make the best recommendation for you. In addition, highly trained physicists, therapists, and nurses will assist with your treatment process.
Our commitment is to provide you with excellent service. If you have any questions regarding stereotactic radiosurgery, please call 631-444-2200.