AAPM: Computer Simulations Lower Radiation Dose In Breast Cancer

ORLANDO, Aug. 1 -- A mixture of radiation beams -- sharply focused by a computer modeling system -- can reduce the dose needed to treat breast tumors, researchers reported here.

The so-called "hypofractionated" approach relies on using both electron beams and photon beams, with the exact combination tailored to an individual patient using high-speed statistical analysis, Jinsheng Li, Ph.D., of the Fox Chase Cancer Center in Philadelphia reported at the American Association of Physicists in Medicine meeting,

In a cohort study of 78 patients, Dr. Li said in an interview, the method allowed radiation oncologists to use a four-week treatment period, delivering 2.25 Gray of radiation to the whole breast in each of 20 fractions.

But because the mixed beam approach allows the radiation to be tightly focused, he said, the tumor bed gets an additional 0.55 Gy/day of electron radiation.

Typically, radiation oncologists deliver 2.0 Gy to the whole breast in each of 30 fractions over six weeks, he said, "so the total dose is a little bit less, but the biological effect is the same."

While it is too early to predict survival or disease-free survival for the patients in the study, Dr. Li said, grade II skin complications were significantly reduced as a result of the new method, which he and Chang-Ming Ma, Ph.D., also of Fox Chase, discussed.

Physicians have been turning to so-called mixed-beam radiation therapy (MBRT) to treat shallow tumors, such as those in the breast, because beams of electrons and photons have different but complementary characteristics. Electrons can only penetrate to shallow depths, while photons can penetrate more deeply.

But the exact shape of the radiation field that's needed varies depending on the size and shape of the tumor, the presence or absence of other tissue types, and even the patient's breathing, he said. That's why he and Dr. Ma turned to fast computers using so-called Monte Carlo simulations.

The computers can simulate billions of trips of each beam to the unique landscape of each tumor. The statistics from these billions of trials are used to determine the best beam properties and mixtures, he said. "This used to take a whole day or more, but now we can do it in several minutes," he said.

For shallow tumor treatment, Dr. Li said, "MBRT can provide improved target dose conformity and uniformity, adequate skin coverage or avoidance, and reductions in the dose to the adjacent normal organs and critical structures."