Varian's dynamic targeting IGRT technology aims at tumors
Varian's On-Board Imager device was the focus of a symposium the company sponsored during which four prominent medical physicists discussed the technology's abilities to help improve radiation therapy by increasing the accuracy of tumor targeting and adapting to tumor motion, according to a release.

The symposium was part of the annual meeting of the American Association of Physicists in Medicine (AAPM). Over 350 medical physicists attended the event.
Each of the presenters discussed clinical and research experience using a medical linear accelerator equipped with the On-Board Imager device to deliver dynamic image-guided radiation therapy (IGRT), Varian said.

Presenter Peter Cossmann, PhD, and his colleagues at the Hirslanden Klinik in Aarau, Switzerland, treated fifteen lung cancer patients, using the On-Board Imager device to track the tumor motion due to respiration.

"Thoracic tumors do not always move in predictable ways as a patient breathes," said. Cossmann. "By using the On-Board Imager in fluoroscopic mode, we're actually looking into the patient just prior to treatment, and we can see how the tumor is moving relative to the planned treatment field."

In treating these lung cancer patients, Cossmann's team used Varian's respiratory gating technology to turn the radiation beam on and off as the tumor moved in and out of range, delivering the dose only at a particular point in the patient's respiratory cycle. They used the On-Board imager to verify that the tumor was in the required position whenever the beam was turned on.
Similarly, Tinsu Pan, PhD of the M.D. Anderson Cancer Center in Houston, discussed how his team is investigating ways that the three-dimensional cone-beam CT images can be correlated with the respiratory cycle.

"To effectively treat a moving target, we need to know if tumor motion has changed between simulation and treatment," said Pan. "The On-Board Imager gives us that capability. Without this tool, we really could not do this."

Rather than using respiratory gating, Fang-Fang Yin, PhD, and his colleagues at the Duke University Medical Center, have developed a procedure for using a 'breathhold technique' that requires patients to hold their breath for a short period of time during imaging and treatment.

Yin is also conducting experiments to see whether cone-beam CT images of soft tissue structures-including tumors-can be used to verify a patient's position.

"We have been taking both radiographic and cone-beam CT images several times a week when setting up patients for treatment," Yin said. "We're analyzing the data to see if the 3D image enables us to improve our positioning and targeting accuracy. We looked at head and neck and prostate cancer cases, and found that the cone-beam CT images help us to identify patient positioning errors in order to treat more accurately."

In addition, Yin and his colleagues at Duke have been working with the On-Board Imager device on what he calls 'cone-beam digital tomosynthesis' (CBDT) -- a faster alternative to cone beam CT imaging for generating 3D images that show soft-tissue contrast.

Finally, Timothy Fox, PhD, of Emory University presented observations after a full year's experience using the On-Board imager in radiographic mode to fine-tune patients' positions prior to treatment.

"We have treated over 257 patients and delivered more than 2800 fractions using the On-Board Imager since June 2004," Fox said, adding that these have included brain, head and neck, prostate, breast, and gastro-intestinal cancer cases, as well as some pediatric cases. Over that period Fox and his team reduced the time it takes to complete the patient positioning from 7.5 minutes to under four minutes.

The process at Emory involves taking two X-ray images just prior to treatment, and using system software to match them with treatment planning images. The software then calculates how much the patient needs to be moved, to align the tumor properly with the beam. According to Fox, images showed that many patients required positioning shifts. Fox is now analyzing his accumulated data about patient positioning shifts to see if he can find systematic reasons for the displacements.