July 25, 2001 When it comes to planning the best treatment for cancer patients, sometimes bigger is better. At this week's meeting of the American Association of Physicists in Medicine (AAPM) in Salt Lake City, researchers will discuss a new "large-bore" x-ray imaging device that allows patients to have their CT scan taken in the actual treatment position. Before now, many patients with breast cancer, lung cancer, and Hodgkin's lymphoma were not able to extend their arms into treatment position because of limitation of the 'bore-size" of the CT scanner. This technology offers the possibility of planning a patient's radiation treatment more effectively.
In cancer radiation treatment, an important step is the planning process, in which physicians determine such things as where to send cancer-killing radiation, which areas to avoid, and how much dose to deliver.
To accomplish this task, physicians often use radiation-planning computers that use patient data taken from computed tomography (CT) scanners. Also known as CAT scanners, these devices take x-ray pictures of a cancer region from a variety of angles to build a 3D image of the patient's tumor. To get scanned, the patient lies on a table in treatment position. The table then slides so that the body region of interest is inside a large vertical ring or "bore" around which the x-ray camera and detection equipment rotate.
However, conventional bores have a diameter of only 70 cm (about 28 inches). This size makes it difficult for some patients to be placed in treatment position. For example, a breast cancer patient must lift an arm at almost right angles to her back, so that the x-ray camera can target the breast region and avoid other sensitive areas which do not contain the cancer. A patient with Hodgkin's lymphoma must often lift both arms for imaging of the lymph nodes. The larger bore diameter allows patients to receive their CT scan in treatment position, which in many cases includes special immobilization casts that keep the body region of interest steady for imaging purposes.
At the upcoming AAPM meeting, which has 2,600 attendees, medical physicists will discuss the performance of a new bore scanner with a larger, 85-cm diameter opening, which was installed at the Mallinckrodt Institute of Radiology in December 2000.
Why wasn't this technology available before? Larger imaging devices are actually much harder to make, says medical physicist Sasa Mutic, an assistant professor of radiology at the Mallinckrodt Institute of Radiology, who is co-author on the AAPM meeting paper describing the new CT scanner. In the larger-bore CT scanner, x-rays have to travel longer distances to the patient's body, and this degrades the quality of the image. The new system's manufacturer and designer, Marconi Medical Systems of Cleveland, Ohio, solved this problem by making a specially designed spiral CT scanner. In this design, the x-ray tube rotates continuously around the patient in a helix pattern while the detectors are stationary. The design greatly increases the quality of the images that are obtained.
In addition, the new device allows for a larger "scanner field of view" (SFOV) compared to traditional scanners. SFOV is the size of the largest object that can be seen on a CT scanner. Larger SFOV allows for full imaging of cancer regions in large and obese patients, who sometimes have trouble entering the smaller openings. It also enables oncologists to get a more accurate view of a patient's body dimensions which helps them to calculate more accurately the appropriate radiation for cancer treatment.
The cost of the large-bore CT scanner is about $900,000, comparable to other systems used for the same purpose. It was first installed at Mallinckrodt, but now it is also being used in several other radiation centers across the U.S. and in Europe.
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