Medical Physics Team at UNM Cancer Center Expands Treatment Choices

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Newswise Phoebe T. Ward, Jr. was the first patient to receive stereotactic radiosurgery at the University of New Mexico Cancer Center. The procedure killed the tumor in Ms. Wards brain by hitting it with x-rays. It took no more than 15 minutes but preparation for the procedure took months.

Its a single treatment and theres no way to remove the radiation, says Eder Calderon. So we have physicists doing checks. We have the radiation therapists doing checks. We also include the radiation oncologist in some of those checks. Mr. Calderon is a medical physicist in the Department of Radiation Oncology at the UNM Cancer Center. He skillfully controls the ionizing radiation delivered to a person with cancer to noninvasively kill tumors, including brain tumors. He and the UNM Cancer Center medical physics team develop procedures to expand how they use the existing radiation equipment, giving people with cancer more options. Stereotactic radiosurgery is one of the new procedures theyve put in place; hippocampal-sparing radiation is another. Ms. Ward has received both.

One precise dose Radiosurgery and radiation therapy kill cancer cells with radiation, but they require vastly different procedures. People receiving radiation therapy get many small doses over several weeks. The radiation targets the tumor but some goes into the nearby healthy tissue. Normal, healthy cells can repair themselves more quickly than cancerous ones, so cancer cells around the tumor also die but the healthy tissue recovers. Using the same idea, some radiation treatments target large volumes of the body to kill lurking cancer cells that are too small to see on scans.

Radiosurgery is far more precise. We make it very conformal around the target, says Mr. Calderon. Radiosurgery targets the tumor but covers as little of the surrounding tissues as possible. This precision is necessary because radiation oncologists give the treatment in a single dose. The radiation must kill the tumor cells but must spare the healthy cells around them.

Both procedures require the medical physics team to integrate two sets of systems. One set of systems controls the radiation. The other set of systems controls how the radiation reaches its target, the cancer tumor.

Controlling radiation by filtering it Lenses that can bend visible light, such as eyeglass lenses, are inexpensive compared with lenses that bend x-rays. X-ray lenses are more costly and difficult to produce because x-rays pass through most objects without deflecting much. So instead of trying to bend x-rays, radiation equipment filters them.

The x-ray source inside radiation machines gives off x-rays in all directions. To control the direction of the radiation, heavy shielding around the source blocks the x-rays in all directions except one. A tungsten cone fits onto the shielding to filter the x-rays further so that only a single, directed beam of x-rays escapes the machine.

The shape of the cone and the hole in the middle of it affect how sharp or diffuse the resulting beam of x-rays is. For stereotactic radiosurgery, Mr. Calderon explains were going to have very, very sharp gradients in the edges of that hole. The medical physics team at UNM Cancer Center can make the gradient fit within millimeters. The x-ray beam for radiation therapy can be more diffuse its gradient is larger so its more like the edges of a beam from a flashlight.

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Medical Physics Team at UNM Cancer Center Expands Treatment Choices