Dynamic Targeting Section: The Drive for Precision

Hundreds of hospitals around the country have adopted intensity modulated radiation therapy (IMRT) and 3D conformal radiation therapy (3D-CRT). These institutions have discovered that implementing IMRT brings a number of benefits. IMRT and 3D-CRT allow clinicians to more completely treat tumors; IMRT, for example, allows oncologists to actually paint the tumor with radiation and shapes the beam to avoid surrounding tissue. Clinicians can escalate the radiation dose hitting the tumor, and, at the same time, minimize the dose to surrounding healthy tissue and critical structures. For patients, IMRT can mean fewer complications and improved local tumor control. But researchers and clinicians realize that although IMRT can form and deliver an exquisitely accurate dose, if the target is not exactly localized the treatment cannot achieve its full effect. Also, the ability to compensate for changes in tumor shape or position has become more critical. George T.Y. Chen, Ph.D., professor at Harvard Medical School and director, radiation biophysics at Massachusetts General Hospital (Boston), explains, "The advent of new technologies such as IMRT has spurred a renewed interest in understanding how much organs move as a function of time."

Varian Medical Systems (Palo Alto, Calif.) has developed a clinical process that serves as a natural extension of IMRT and 3D-CRT. This suite of new products and technologies is collectively referred to as Dynamic Targeting. Dynamic Targeting is aimed at more precise localization of the target and at controlling for motion of the target caused by breathing during treatment and on a day-to-day basis due to organ movement. Some Dynamic Targeting tools such as RPM-Respiratory Gating and the Exact Couch with Indexed Immobilization are currently on the market and benefiting patients, while the product development "pipeline" is full of more products to come.

A Primer on Organ Motion
Research has shown that many tumor sites are not stationary within the body; they move around both due to respiratory motion and daily physiological changes. Organ motion is, unfortunately, an all-too-complex problem that has sometimes undermined the effectiveness of radiation therapy. Radhe Mohan, Ph.D., professor and director of radiation physics at M.D. Anderson Cancer Center (Houston), explains, "The basic problem is that we design and deliver treatment based on a static CT scan. In doing so, we make an assumption that the tumor’s shape, position and size remains the same over 40 days or more. But we know that is not the case. We end up opening the radiation beams pretty wide so that the tumor remains targeted. This exposes a significant fraction of normal tissue to radiation, which, in turn, limits our ability to deliver a high enough dose to kill all of the cancer."