ADDITIONAL STORY TIPS FOR THE 2005 AAPM ANNUAL MEETING

College Park, MD--July 8, 2005—The following are some additional story tips for the the 47th Annual Meeting of the American Association of Physicists in Medicine (AAPM), which will take place July 24-28, 2005 in Seattle, WA at the Washington State Convention & Trade Center. The general press release for the meeting can be found at http://www.aapm.org/meetings/05AM/VirtualPressRoom/

I. RADIATION DOSE BUILDS UP SHARPLY AS X-RAYS PASS THROUGH SKIN, NEW MEASUREMENTS SUGGEST

A McGill University-NIST collaboration (contact Slobodan Devic, devic@medphys.mcgill.ca) has produced evidence that the radiation dose delivered to the skin by x-rays increases more sharply with depth than previously assumed. The results will help doctors to avoid damage to the skin that is caused by radiation en route to a tumor inside the body. Accurately determining skin dose is particularly crucial nowadays with the advent of complex radiation therapy procedures that aim sophisticated patterns of radiation from many different angles to deliver as much dose to a tumor while avoiding healthy tissue.

The results will also help doctors more fundamentally understand how dose varies as radiation passes through different layers of the skin. The thicknesses of different skin layers vary widely throughout the body. For example, the top layer of the skin, known as the epidermis, has a thickness of 1.5 mm at the sole of the foot, but only 50 microns, or 0.05 mm, at the eyelid.

What causes radiation dose to increase as it passes through the skin? The answer is that x-rays, made of high-energy photons, do not deposit their energy directly in the body. Instead, x-ray photons first have to interact with matter to create energetic charged particles (mostly electrons) in living tissue. These charged particles then deposit energy in the body and deliver radiation dose. As a result, radiation dose intensifies as the x-rays penetrate deeper into the skin.

Measuring the radiation dose delivered to special radiochromic film inside a solid water "phantom" standing in for the mostly water human tissue, researchers found that from a depth of 10 microns to 1 millimeter in the skin, the percent depth dose increased sharply from 14 % to 43 %, where the percentage relates the dose at a given depth to the dose deposited at the depth of maximum dose deposition, being by definition a 100% value. In other words, the measuremen ts suggest that a 10-micron depth of skin would receive 14 units of radiation, while skin at 1 mm depth receives 43 units, which is a more than threefold increase in terms of absolute radiation dose. The researchers measured this increase for a 6-MV x-ray beam passing through a 100-square-centimeter opening, a typical energy and beam size for radiation therapy, though results might differ slightly for other x-ray energies and beam sizes.

Devic and collaborators hope that this new measurement will enable radiation oncologists to make better estimates of the skin dose and avoid skin injuries during cancer therapy, especially during complex multi-beam procedures such as intensity-modulated radiation therapy (IMRT). (Paper MO-D-T-617-8, Monday, 2:54 PM.)

II. NEW ENERGY-BASED DOSIMETRY APPROACH MAY LEAD TO MORE ACCURATE DOSES TO TUMORS

Larry DeWerd of the University of Wisconsin-Madison (ladewerd@wisc.edu) is working to develop a new, more direct way of doing dosimetry, the science of measuring radiation dose. A more direct approach promises higher accuracy in delivering the desired radiation dose to a tumor while minimizing damage to healthy surrounding tissue.

DeWerd is particularly interested in a new way of measuring doses for brachytherapy, the popular technique of inserting radioactive sources into a part of the body, such as the prostate, to deliver radiation for killing tumors. The prostate treatment has been popular by using dozens or even hundreds of tiny seeds, each about 5mm x 1 mm diameter. In the past the radiation dose from these seeds have been measured in air and the method used has been to convert the air measurement to a dose in tissue. This indirect method involves a step of converting the air dose into the prostate dose, and large uncertainties can result from the conversion process. The dose in tissue is basically energy absorbed in a kilogram of tissue (or water).

Using sensitive, state-of-the-art low-temperature heat-measurement devices, DeWerd's approach is to measure the energy emitted by the sources directly with the use of a device called a calorimeter. Also, the spectrum of radiation, the range of energies emitted by the multiple radioactive substances in the seeds, changes with depth as it is absorbed in tissue. Previous work has determined how energy is absorbed at different depths of tissue.

Combining all of the above information would be a new way of dosimetry using an energy-based approach. Still in the experimental phase, this approach promises to be a more direct way to standardize radiation sources and thus determine the radiation dose in the tumor and surrounding tissue with potentially higher accuracy. Small errors in the estimated radiation dose from each brachytherapy seed can add up for the dozens of seeds that are typically used, and the new approach offers the prospect of sharpening the precision of the total dose delivered to a tumor. (Paper MO-E-T-618-7, Monday, 5:12 PM)

III. BETTER 3D IMAGES USING FM RADIO TECHNIQUE

Taking a cue from FM radio broadcasts, researchers have found a way to eliminate troublesome image "artifacts" from a potentially revolutionary new 3D technique called "cone beam imaging." Compared to conventional CT scans, cone-beam imaging promises lower radiation doses and more precise imaging of moving objects such as the heart or a catheter traveling through an artery. Already available commercially, the new scanners obtain a 3D image of a patient in one fell swoop of a cone-shaped x-ray beam, thereby capturing the entire region of interest in a single pass. Conventional CT scans, on the other hand, must rotate a fan-shaped x-ray beam several times around a patient and piece together the images to line up the picture fragments properly. In addition, cone-beam machines make more efficient use of the x-rays that pass through a patient. Conventional CT scanners must use a collimator, a narrow opening blocking the x-rays that do not travel in a straight line from the x-ray source to detector. The x rays that change direction while in the patient are considered "scattered" photons and produce visual artifacts that make it more difficult to see the desired image. The collimator removes most of the scattered photons, but also requires higher x-ray doses to build up an acceptable image. Cone-beam imagers, by their very nature of their wider beams, cannot use a collimator, so they waste fewer photons, but they are susceptible to even worse image artifacts. Now, Siemens researchers (Jonathan Maltz, jonathan.maltz@siemens.com) have devised a solution to this problem. They use a technique called "frequency modulation" (FM) to separate the good x-rays from the scattered ones. FM is the same type of modulation used in radio broadcasting that enables broadcasters to broadcast many different stations without interfering with one another. A special grating made of a heavy metal such as tungsten shifts the frequency of incoming x-ray photons by different amounts. Desirable x rays traveling in a straight line from source to detector are shifted (modulated) by a different frequency than the undesirable ones that have been scattered within the patient. By "tuning" to the frequency of the desirable x-rays, the researchers are able to view the 3D image without the scattered photons. Demonstrated through computer simulations and validated by geometrical proofs, this solution will be applicable to all types of cone-beam systems. (Paper Tu-D-I-611-7, Cone Beam CT, 1:30 PM)