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," he 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.


"Cone-beam CT images are created when we continually generate X-ray images while rotating the On-Board Imager 360 degrees around the patient," Yin said. "With CBDT, we use software to reconstruct a three-dimensional image from a smaller set of images-we need only a 40 degree rotation. The image is not quite as robust as we get with a full 360-degree cone-beam CT, but it is sufficient for identifying subtle anatomical features and localizing the tumor, and it decreases the amount of dose that the patient receives." This promising application of the On-Board Imager device has the potential to make 3D imaging safer, faster and more practical for daily IGRT treatments.

Radiographic Imaging at Emory University

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," he 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.

"The On-Board Imager is helping us quantify patient set-up variances, and we have an opportunity to analyze the data to see if there are commonalities among patients for whom there have been large positioning shifts. Were they all set up using the same type of immobilization devices? Did they lose a considerable amount of weight? Prior to the advent of IGRT, we didn't know the extent of our set-up variances. The On-Board Imager is generating a wealth of information that can help us understand exactly what is going on.

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