IMRT, IGRT, and Respiratory Gating

For decades, a linear accelerator has been the workhorse of the radiation oncologist to treat cancer through a process called external beam radiation therapy. These linear accelerators speed up electrons to nearly the speed of light inside a vacuum tube and direct this electron beam onto a heavy metal target. What emerges from the other side of the target is a narrowly focused beam of high-energy x-rays, also known as photons (Yes, Star Trek fans, you CAN call them Photon Torpedoes!). These photons can then be directed through a targeting process through normal tissues of your body from many different angles to all converge on the tumor being treated. Thus, some normal tissue is irradiated as the beam passes through it, but this dose is minimal compared to the dose received by the targeted tissues. Early on, we used a process known as 2-D or 3-D treatment planning to help us safely accomplish the delivery of the radiation therapy, and we were even able to custom-shape the edges of the beam using special alloy blocks or metal leaves inside the machine. As computer and machine technology have evolved, we are now able to incorporate new “space age” treatment planning and delivery techniques that go way beyond the 3-D environment. These newest techniques are called Intensity Modulated Radiation Therapy (IMRT), Image Guided Radiation Therapy (IGRT), and Respiratory Gating.

IMRT (intensity modulated radiation therapy) is an advanced system for delivery of external radiation. This technique allows maximum radiation to be delivered to your tumor target and the surrounding areas at risk for harboring cancer cells, while minimizing radiation to your nearby normal organs. IMRT essentially allows us to change the strength of different portions of the beam as it leaves the linear accelerator: If you think of the beam as a flashlight pointed at you, it is as if we can make some parts of the beam brighter, and some dimmer, than others. This flexibility allows us to essentially “carve” dose around vital organs and give a more uniform dose to the tumor while sparing normal tissues from high levels of radiation exposure.
IGRT uses IMRT techniques, but matches these techniques with extremely accurate near real-time localization of your tumor in space. Organs in our body constantly move, which previously meant that a large treatment field needed to be used because from day to day or minute to minute we were never quite sure just where the actual tumor was. IGRT helps pin down precisely where the tumor is that day just prior to or during each treatment. This precision, in turn, lets us make the treatment fields much smaller, thus limiting exposure of normal tissues nearby.
Like standard external-beam therapy, the IGRT-based IMRT or IMRT alone is typically given for only a few minutes a day, five days a week. The radiation treatment is completely painless and non-invasive.

The IGRT (image guided radiation therapy) precision localization capabilities of our machines can be realized in one of three ways. Many of our centers use BAT ® ultrasound technology as part of daily IMRT treatment, especially for patients with prostate cancer. The BAT® (B-mode Acquisition and Targeting) localizer integrates ultrasound imaging with sophisticated robotic computer technology. Before each daily IMRT treatment, the B.A.T. ® acquires an image of the prostate and surrounding anatomy using an ultrasound probe placed on the skin surface. This is a painless non-invasive procedure which takes only a few minutes prior to each treatment. The goal is to detect any slight change in position of the prostate gland that may result from movement or variations in filling of the bladder or rectum. An alternative way to localize the prostate (or many other tumor types) daily is via the use of gold marker seeds placed within the prostate gland or tumor prior to treatment. These are called fiducial markers, and are visualized via x-rays on the actual treatment table just prior to turning the real beam on. Finally, some machines have the capability to obtain a quick CT scan of the target area with you in the treatment position each day.

Another technique you might hear about is respiratory gating. As noted above, tumors can move around in your body to some degree, especially with breathing (especially tumors within the lung and abdomen). Our IGRT techniques allow us to see these tumors moving, and with the latest accelerator technology we can account for that movement--we track the position of the tumor throughout the breathing cycle, and only turn the beam on when the tumor is centered within the treatment area. This so-called gating allows us to make the treatment volume (and hence normal tissue exposure) even smaller, thus protecting more normal organs from the effects of the treatment. Gating often lengthens the overall time you are on the treatment couch by 5-10 minutes, but we feel is worth the extra time in selected situations. In very special situations, we can use a special linear accelerator called a Cyberknife to treat very small well-defined tumors. Rather than using respiratory gating, this machine actually moves to follow the tumor as you breathe!

Rest assured, your FROG doctor will carefully review your particular situation, and choose the best tool from this arsenal to attack your tumor.