This report originally appeared in the July 2011 issue of DOTmed Business News

By Omar Dawood

Radiation therapy goes back more than 100 years. And today, it’s a critical part of tumor control. In fact, about half of all cancer patients receive some type of radiation as part of their treatment. In the mid-20th century, the next evolution — radiosurgery — arrived and is now used to treat tumors non-invasively throughout the body.

Whether we’re talking about conventional radiation therapy or radiosurgery, the goal is the same: provide effective tumor control while minimizing injury to normal, healthy tissue.

One of the critical challenges to achieving this goal is accounting for tumor movement. Tumors or organs surrounding a tumor site can shift during treatment due to respiration, bowel gas, filling of the bladder or other patient movement—this is especially true for tumors in the lung and prostate. As a result, clinicians traditionally have needed to expand treatment margins to include not only the tumor, but also significant normal tissue, opening the door for unacceptable toxicity and injury to healthy cells.



Conventional radiation therapy, for example, was developed to treat widespread, diffused disease and even today is most often employed for palliative purposes or in conjunction with surgery. Equipment employs wide fields of radiation that encompass both the tumor and a large amount of normal tissue. To account for movement, pre-treatment imaging is often used to determine the location of the tumor and potential variation due to motion. Given that patients and tumors are not static during treatment and tumor motion is not tracked in real time during the delivery of conventional radiation therapy, this process is inaccurate at best.

Radiosurgery, on the other hand, was developed to ablate tumors (not just slow their progress) and works by delivering a large, very precise dose of radiation via hundreds of uniquely angled non-coplanar beams that all intersect at the tumor. By ensuring high levels of accuracy, radiosurgery allows clinicians to reduce treatment margins, maximally exclude normal tissue from the treatment field and complete treatment in just one to five sessions.

Despite the advances of radiosurgery, motion management has historically been a challenge. Going back to the 1960s when the first applications of radiosurgery were for intracranial targets, motion was managed by bolting the patient’s skull to a rigid metal frame and securing it to the treatment table. The discomfort and restrictions of frames meant that treatment was usually completed in one day or the patient was hospitalized. Some radiosurgery systems still require a frame and are limited to treating tumors only in the head and upper cervical spine.

Even systems designed to treat extracranial targets, including many gantry-based devices, need to account for motion in order to achieve desired levels of accuracy. Some systems employ motion compensation techniques to minimize exposure of healthy tissue. For example, when treating lung tumors, techniques might include respiratory gating, breath holding and abdominal frames. All of these require patients—many with already compromised lung function—to perform difficult breathing maneuvers or be placed in uncomfortable positions. In addition, they are all based on assumptions about tumor location and the patient’s breathing pattern—it’s an imperfect science and ripe with the potential for inaccuracy.

Likewise, when treating prostate cancer, some gantry-based radiosurgery systems rely on pre-treatment CT scans to initially set up the patient and guide treatment planning. The problem is that pre-treatment scans represent a snapshot in time; because patients are not static throughout the treatment and the systems are unable to deliver radiation fast enough to overcome movement, this approach is not always accurate and healthy tissue still is subject to injury.

Building on many of the motion management challenges inherent in gantry-based systems, some advanced radiosurgery systems have now made not only tumor tracking, but also automatic radiation beam correction during treatment, a reality.

For example, there are now systems that leverage sophisticated and continual image guidance software to track lung tumors during respiration. These devices recognize even the slightest shifts that might occur and automatically correct for intra-fraction tumor motion in real time and throughout treatment. In short, these systems are able to “synchronize” radiation beam delivery to lung tumor motion, allowing clinicians to significantly reduce treatment margins and achieve maximal sparing of critical structures without the need for gating, breath holding or frames.

While movement in the lung tends to follow the rhythmic pattern of respiration, the prostate presents a unique challenge in terms of motion management. As a result of gas or fluid in the bowels, the prostate is subject to random and excessive target motion that can make it difficult to achieve high levels of accuracy. Conventional radiation therapy systems, in particular, are unable to respond automatically to unexpected prostate motion during treatment.

The ideal way to ensure accuracy during prostate treatments and spare critical structures is not just to see the motion, but also the ability to act on what is seen. For example, some systems now offer intelligent and adaptive technology capable of tracking and correcting for prostate movement. This allows clinicians to reduce treatment margins, include less normal tissue in the treatment area, and minimize radiation exposure to the rectum, urethra and neurovascular bundles. The benefits include preservation of erectile function and acute and chronic urethral and rectal toxicity rates comparable to conventional radiation therapy and surgery.

Dr. Omar Dawood is vice president of global medical affairs at Accuray Incorporated of Sunnyvale, California., and an adjunct clinical instructor at the Georgetown University School of Medicine.