By Jim McCarty, M.D. and Jack Gleason, M.D., radiation oncologists at Alliance Cancer Care
What is SBRT?
Stereotactic Body Radiation Therapy (SBRT) is a technique that precisely targets lung tumors, while at the same time uses very conformal radiation beams to avoid normal tissues. The precision targeting and delivery allow the treatment team to safely deliver all the radiation in a shorter number of treatments (3-5 treatments vs conventional schedules of 30-37 treatments). This approach has several important advantages: higher rates of tumor control, a shorter more convenient treatment course, and less risk of side effects.
What technologies are involved?
SBRT requires multiple technological advances:
- 4D imaging for radiation planning and delivery to account for the motion of the tumor during breathing.
- Better shaping of the radiation beam through better robotics and faster computers.
- Imaging at the time of each radiation treatment with the ability to move the table in small precise increments in 6-dimensions (x,y,z and roll, pitch, yaw) to align the target before delivering the radiation.
What is 4D imaging for radiation planning?
One of the difficulties with treating tumors in the lung is that the lung moves with every breath we take. Anything in the lung, like a tumor, also moves with each breath. We need a way to consider that motion. There are many different ways to manage this respiratory motion. At Alliance Cancer Care, we start by using a 4-dimensional CT scan. Most CT scans are 3-dimensional. The computer can recreate a visual representation of a tumor. We can rotate it around to look at it from the top or the bottom or the side. However, that representation is just a snapshot from one moment in time. CT scans do not normally show what happens with breathing.
To plan the radiation, we do a series of CT scans while the patient is breathing. We can reconstruct these to show not only a 3-dimensional representation but show how that structure moves over time while the patient is breathing. This way, we can target not just where the tumor is at any one time, but the volume in which the tumor moves as the patient breathes.
How have better robotics and faster computers helped?
When the beam of x-rays comes out of the radiation machine, the beam is formed into the precise shape of the target of radiation. There are very small, heavy metal leaves in the head of the radiation machine that form this shape by blocking the radiation into the exact pattern we need.
We know that there is an advantage to using multiple beams from different angles. You could imagine, if we only used one beam, then everything in that beam pathway would receive 100% of the dose. On the other hand, if we used four beams, then everything in each beam’s pathway only receives 25% of the dose, except where all four beams collide, at which point the tumor gets 100% of the dose.
In the old days, the radiation machine would move into the first position, then move the leaves into position, then turn the beam on, then turn the beam off, then move to the next position. Doing this would take a couple of minutes for each beam angle. We know that after lying on the x-ray table for 10 minutes or more, patients start to get restless. If they moved around, then we were at risk for the radiation beam missing the tumor. So we knew we only had time to get in six to seven beam angles before the treatment became too uncomfortable.
Over the last decade, robotics and tiny motors have become much more sophisticated. During that same time, computer processing speeds have become dramatically faster. We can now combine these two technological advances to have the machine move from position one to position two in a fraction of a second. This way, we can have the radiation machine treat from hundreds of different beam angles. The x-ray machine can simply keep moving in a circle around the patient, keeping the X-ray beam on the entire time, while the computer and robotic leaves make hundreds of very tiny changes every second. This way, instead of spreading the radiation out over six angles, we can spread it out over 160 angles…. or more.
How do you use daily imaging?
In the old days, there were kilovoltage x-ray machines that made diagnostic x-rays and CT scans for imaging. There were also megavoltage x-ray machines (linear accelerators) that delivered therapeutic radiation for cancer treatment. Nevertheless, you had to build either one machine or the other one. With technological advances, engineers found a way to build a radiation machine that could deliver both kilovoltage x-rays to make diagnostic images and deliver megavoltage x-rays to give cancer treatments. This way, we can have our cancer radiation machine also produce CT scans. This allows us to do a CT scan moments before each treatment to identify exactly where a tumor is at that very moment. We can even perform another 4D CT scan on the treatment table to look at tumor motion during set-up. This daily image guidance has allowed us to not only more precisely treat the tumor, but also better avoid the surrounding normal structures.
How long does SBRT take?
Being able to treat the tumor with such accuracy and precision, while avoiding the normal organs, has allowed us to safely deliver a higher dose during each day’s treatment. Our technical term for each treatment is a “fraction”. The unit of radiation is “Gray.” Historically, lung cancer has been treated to a dose of 60-70 Gray, using 1.8 – 2.0 Gray a day. This would often be done over 30-37 treatments (fractions). With SBRT, we can typically use a much higher dose per day, like 10-18 Gray for each treatment. Because we are using a higher dose per treatment, we do not have to do nearly as many treatments. The most common regimens for SBRT deliver a total of three to five treatments over two weeks.
Sometimes because of the tumor location, we have to be even more careful to avoid normal neighboring structures. We may use a relatively lower dose per day over 10 fractions, instead of four to five, but they are still given over two weeks. Ten fractions always sound like more radiation than four to five fractions, but it is the same overall amount of radiation. It is just a matter of how much of the total dose is given with each fraction. Either way, with SBRT, we are looking at two weeks of radiation instead of seven weeks of radiation.
For each day, the total time in the clinic is likely going to be 30-45 minutes. The total time on the x-ray machine is probably 10-15 minutes. Any medical clinic can run into delays or get behind schedule, but typically, each day’s treatment is very quick.
Who is a candidate for SBRT?
SBRT is primarily used for stage I or early-stage II lung tumors, which means small to medium-sized tumors in the lung without lymph node involvement or invasion into the central part of the chest. The central part of the chest is called the mediastinum and represents a challenge for both surgery and radiation. There are many important and sensitive structures in the mediastinum, including the esophagus, the heart, the trachea (aka, the windpipe), big blood vessels, etc. There are also lymph nodes in the mediastinum, and sometimes these lymph nodes have lung cancer in them. In the best-case scenario, cancer surgeons do not just cut out the cancer itself, but they remove some surrounding normal tissue just to be sure to get any little roots or seeds that may be extending off the tumor. It is exceptionally difficult to do that in the center part of the chest because of all of these sensitive structures. SBRT has a similar problem. It is difficult to do high doses of radiation per treatment to the mediastinum because these critical structures are more sensitive to radiation. When cancers involve the more central part of the chest, we usually deliver conventional fractionation over six to seven weeks, instead of SBRT. In summary, SBRT is best when the tumor is out in the lung, is away from the mediastinum, and does not have any spread to lymph nodes in the more central part of the chest.
Is SBRT better than surgery?
It just depends on the situation. As of 2021, surgery remains the “gold standard” for treating localized lung cancers. Some patients, however, are not good candidates for surgery. Sometimes their lung function is so impaired that it is not safe to take them to surgery or to remove a lobe of their lung. Sometimes the tumor is just not in a good location to allow for safe removal. Sometimes the patients are just not willing to undergo surgery. In all those situations, using SBRT as a nonsurgical approach is the best option.
But what about cure rates? If you read ten different studies looking at early and localized lung cancers, you will get ten different cure rates. In the most general sense, though, surgery can control 95% or more of localized tumors. SBRT should be able to control 90% or more of localized tumors. (You can almost certainly find individual studies that have higher control rates for either modality.) When using SBRT as a nonsurgical option, you likely have equal or at least very similar local control, and it allows a noninvasive treatment.
How do you know if it worked?
After completion of SBRT, we will do a chest CT. This will typically be two to three months after radiation, but this can vary. With SBRT, we treat both the tumor and a bit of a rind around the tumor. Because of this, the area of change the radiologists see on the first follow-up CT or two may look larger. This does not mean the tumor is larger, it just means we have treated a slightly larger volume around the tumor and we are looking at scar tissue. Again, this type of treatment works 90 – 95% of the time. When the radiation has successfully killed off a lung tumor, the first or second CT scan may look slightly bigger due to scar, but after that, the CT scans all look about the same and it stabilizes. In the small minority of cases in which the SBRT does not work, then at some point, a CT scan will show a larger tumor. A follow-up CT scan may show it to be even larger still, or a follow-up PET scan may show the tumor to still be active. In those situations, we move on to the next available treatment. Interestingly, even in patients who are cured with SBRT, the lung tumor typically does not disappear on imaging. It will typically stay there as a dead scar that is visible on imaging. In this way, the patient can be cured of lung cancer but never have a “normal” chest CT again in the same way that a woman can be cured of breast cancer but never have a “normal” mammogram after treatment. The mammogram or the chest CT will show scarring at the treatment area. Essentially, we do not need the scans to be normal, we just need them to show no growth at the treatment location and no new changes anywhere else.
What are the other names for SBRT?
SBRT is also called Radiosurgery or Stereotactic Ablative Radiation (SABR).