The most common toxicity is rectal toxicity, which is currently on the order of about 1% to 2% for colorectal cancer patients receiving treatment, says Dr. Parda. This rate has plummeted in recent years from previous toxicity ranges of 20% to 30%.

"Improved dosing and targeting of radiation has increased tumor control probability and decreased normal tissue complication probability - that is what we call the therapeutic ratio," says Dr. Parda. "This has resulted in improved cure rates, organ preservation, functional outcome, cosmetic outcome, quality of life and palliation."


Adaptive radiation therapy

The next big thing in linear accelerator therapy is adaptive radiation therapy (ART), which is dynamic adaptation of treatment as the tumor changes during the course of therapy.

"Adaptive radiation therapy is a function of adapting the radiation dose on a regular basis," says Dr. Parda. "Since the tumor that you're treating changes in size and shape over time, the question is whether you need to modify the radiation on day 10 compared to day one."

Another foreseeable advancement in the technology is real-time image guidance. It may be the ultimate vision, but we're not quite there yet.

"As it stands, the cone-beam CT scans are taken right before the treatments," says Dr. Parda. "The whole reason that we're interested in MV CT is that we are using the actual treatment beam to generate the image. There currently is no real-time imaging solution where you are making real-time adjustments in position and dose, but that is essentially the incremental direction of adaptive radiation therapy."

The hierarchy of radiation therapy systems

The linear accelerator is just one of many modalities in external beam radiation therapy and surgery. The Gamma Knife, manufactured by Elekta, is dedicated to intracranial radiosurgery. Although it is still a very viable technology, the advancements in linear accelerators are somewhat overshadowing that modality, says Dr. Parda.

"We have better manipulation of variables associated with dosing and radiation now and that makes linacs more precise and individualized for delivering radiation," Dr. Parda explains. "That's why most major radiation centers are moving away from Gamma Knife technology and toward the linear accelerator based technology."

Other more experimental modalities, like proton therapy systems, can cost a radiation oncology facility $100 million or more, but because the entrance and exit dose is far less, there is a great advantage for pediatric applications. Even more astounding are carbon ion systems, which are debuting at $300 million, says Dr. Parda.

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