Frameless Stereotactic Radiosurgery
What is frameless stereotactic radiosurgery?
Cancer doctors have been using stereotactic radiosurgery (SRS) to treat cancer in the brain for many decades. They can kill or shrink tumors without the risks of open skull brain surgery by delivering a single high dose of radiation precisely to the tumors to cause them to shrink and sometimes disappear. The same technique is sometimes used to treat other conditions in the brain, such as arteriovenous malformations, and some neurological conditions.
What’s new about the frameless SRS?
Until recently, patients receiving SRS would have a head frame attached to their skull with pins and then fastened to the treatment table. This kept the patient’s head perfectly still during imaging and treatment. The frame prevented the beam from damaging healthy brain tissue but it could be uncomfortable and sometimes stressful for patients.
Now, therapists at the UW Health Radiation Oncology Clinic can fit patients with a custom-fitted, plastic mask that holds their head in place during treatment. A new radiation therapy machine specifically designed to conform to small targets allows therapists to target the tumors without the immobilization frame.
Are there other benefits?
Frameless SRS is not only more comfortable, it also reduces treatment time. With the frame-based system, simulation, treatment planning and treatment needed to take place on the same day. As a result, patients would be in the hospital with the frame attached as they waited for radiation oncology staff to create the final treatment plan. With the frameless system, simulation, planning and treatment occur on different days. This reduces the time constraints on treatment planning, which enables treatment of more complex cases (multiple metastases) that might otherwise require whole brain radiation, a procedure that may have greater side effects than targeted SRS.
“For patients with brain metastases, the frameless system is valuable because these can be sick patients. This frees them from having an invasive surgical procedure, which improves their quality of life,” says Dr. Andrew Baschnagel, radiation oncologist. “We also can get them treated more quickly and move them on to other therapies, such as chemotherapy or immunotherapy, without delay.”
What about children who need treatment?
Zac Labby, PhD, a medical physicist and co-leader of the frameless SRS program, says it will also improve treatment of pediatric patients.
In the past, young children often needed general anesthesia to tolerate having a head frame in place. Children are more likely to tolerate SRS wearing a mask. If they do struggle with the mask they can be given conscious sedation, which is less onerous than the full sedation often required with the frame-based system.
How does frameless SRS work?
Like other types of radiotherapy, SRS works by damaging the DNA of targeted cells, which causes them to lose the ability to reproduce. The major difference between standard radiotherapy and SRS is the dose. SRS uses a single high dose of radiation rather than a fractionated dose administered over multiple sessions. Because of the high dose, precision is especially important. The clinic’s Varian TrueBeam STx is designed to treat small targets, with radiation beam that conform to the shape of the tumor.
What is treatment like for patients?
Before treatment, a mask is made that precisely conforms to the size and shape of the patient’s face. The mask attaches to the treatment table to immobilize the head. During the first visit, the patient gets a CT scan that uses an intravenous contrast agent to provide high-quality images for use in the treatment planning process.
On the day of treatment, the patient is again immobilized with the mask for a second CT to verify alignment—one of many quality assurance measures. The mask has cutouts that expose the patient’s nose and brow to provide reference points for a patient position monitoring system. This technology, known as AlignRT, uses three, ceiling-mounted cameras to create a 3D model of the patient in real time. The system continuously compares these 3D images to the optimal positioning determined during treatment planning to ensure treatment accuracy. If the target area moves out of a predetermined tolerance range during treatment, the system automatically shuts off the beam.