What is ExacTrac and why does it matter for my SRS treatment?

August 31, 2022

By Vince Ulizio, Medical Physicist at Alliance Cancer Care

For high-dose radiation treatments called stereotactic radiosurgery (SRS), one extremely important aspect of treatment is positional accuracy.  To safely deliver high-dose treatment, you must be in the exact position intended to hit the tumor and avoid normal tissues.  Our Alliance Cancer Care physicists and therapists run many tests throughout the day, month, and year to ensure that the machine itself and the tools it uses are treating the exact spot as expected.  Even with all these tests, possible patient movement with breathing and swallowing still needs to be accounted for during treatment, so this is where the ExacTrac system is crucial in delivering state-of-the-art radiation treatment.

The ExacTrac system makes submillimeter adjustments that are almost imperceptible.  With the size of some brain lesions, these adjustments may be small, but they can be the difference between an accurate treatment and partially missing the designated treatment area.  ExacTrac will force the alignment to be within 0.5mm, which is about the thickness of a notecard or fingernail, and about ¼ the size of the tip of a pen.  Adjustments even smaller than this can be made and these adjustments are routinely made during treatment.

When we begin your treatment, you will lie down on the treatment table and X-rays are taken to get your position perfect.  Once your initial setup is complete, treatment starts and the table moves and the treatment machine will rotate around you.  These movements can lead to slight position changes that need to be corrected.  Even if the whole system is shown to keep an accurate position while rotating, those tests do not account for all the variables during treatment.  With the combined effect of machine motions and the length of treatment (approximately 20 minutes), your body motion is inevitable.  Therefore, these changes also need to be adjusted to keep your treatment on target.  We use constant imaging and adjustments with ExacTrac to keep your treatment safe and effective.

The ExacTrac system allows us to take images at any time during radiation treatment and makes tiny adjustments as necessary to keep the tumor perfectly in position for accurate radiation.  ExacTrac images are performed immediately before radiation is turned on for treatment, so we have confidence you are positioned correctly.  The system can also image during your radiation treatment.  While the radiation is being delivered, a quick confirmation image or multiple images can be taken to double-check that no movements have occurred.  If your positioning is not accurate, the radiation will turn itself off to prevent incorrect treatment.

The system also monitors the BBs that are attached to your mask, as an additional way to track your position at all times during the treatment session.  This system can also turn off the radiation if movement is detected.  Your therapists have a monitor on their computer screen, which can indicate the direction of the shift and the absolute measurement in millimeters.

Many studies have shown that the longer the treatment session takes (the longer you are on the treatment table) the more motion occurs, and thus the more corrections that would be needed.  The ExacTrac system keeps treatment times short by performing very quick alignment images.  Without this system, we would have to perform longer imaging techniques, with less confidence that nothing has moved between that image acquisition and the radiation treatment completion.

ExacTrac also comes with a specialized mask system that differs from other treatment types. The mask is designed for alignment that is more accurate and enhances patient comfort, which leads to less movement during treatment.  It also has special parts that allow small adjustments to help the mask fit as well as possible.

This system is designed to work for areas that receive very high dose treatments.  The main sites that are utilized with ExacTrac are sites within the brain and sites within the vertebral bodies of the spine.  These areas can be treated with very high doses and often within just one treatment session, increasing the importance of positional accuracy.  The ExacTrac system is especially good at imaging bony anatomy, so alignment is performed using the skull and the vertebral bodies.

This system helps us to deliver accurate and safe high-dose radiation treatments to patients in a relatively quick manner, but our technology and equipment are always improving.  Our next set of upgrades will make treatment accuracy even better, with a system for 4D thermal tracking of the patient’s heat signature, or external body.  This will allow even more accurate positioning and data to ensure the tumor is in the correct location.  These upgrades potentially lead to smaller volumes of treatment as well, since the positioning is so accurate, and this ultimately leads to less normal tissue being irradiated and even safer treatments.