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Normal tissue not spared in new forms of breast cancer radiotherapy

October 31, 2007

JACKSONVILLE, Fla. − A five day course of radiotherapy to treat breast cancer may, in some cases, expose as much lung and heart tissue to potentially toxic radiation as does the standard six weeks of treatment, say researchers at Mayo Clinic Jacksonville.

That because the short treatment, known as partial breast irradiation, focus radiation to a small sector of the breast through multiple beams, these beams can pass through the breast to the heart and lungs that lie behind, researchers found. Radiating the entire breast over weeks, as is standard practice, can expose much of the heart and lungs to long periods of lower dose radiation, they say.

These findings, presented at the annual meeting of the American Society for Therapeutic Radiation and Oncology (ASTRO), suggest that both methods carry risks and benefits that may be equivalent, says the study's lead investigator, Laura Vallow, M.D.

"This tells us that the standard course of therapy isn't that bad in terms of its exposure to normal tissue, but also that, sometimes, partial breast irradiation may not spare as much normal tissue as we hope," Vallow says.

Oncologists are currently testing the two modes of radiotherapy − whole breast irradiation (WBI) and 3-D conformal partial breast irradiation (PBI) − in a large federally-supported clinical trial that is enrolling thousands of women across the country who have been diagnosed with early stage breast cancer and have undergone lumpectomy.

The hope is that a short course of radiotherapy will be as effective as the much longer course, and that this could lead to increased use of breast conservation over mastectomy, Vallow says. "Many women may be opting for a mastectomy instead of a lumpectomy in order to avoid weeks and weeks of radiation treatment," she says. "If the results of both methods are equivalent, then perhaps some of these women will choose less drastic surgery."

In two studies being presented at ASTRO, Vallow, along with Ashley Gale, M.S., Anudh Jain, M.D., and a team of physicists at Mayo Clinic Jacksonville, used radiological parameters that were prepared for patients in the clinical trial and employed computers to assess how much of normal heart and lung tissue would be exposed if either WBI or PBI were used.

"We suspect there are fundamental differences in the amount of exposure to radiation a patient has using these techniques, but no one has ever looked at how much normal tissue is spared," she says. "We are interested in the finer points of treatment planning, with the ultimate goal of making treatment more tolerable with less radiation exposure."

In the first study, the researchers analyzed radiation plans for 25 patients enrolled in the clinical trial and they calculated what radiation to the lung would be if WBI or PBI was used. They found that PBI exposes a slightly larger volume of the lung to low doses of radiation, but also exposes a smaller volume of lung to high doses than WBI does.

"Patients are getting more exposure overall to their lungs with partial breast irradiation but less lung tissue is irradiated to higher doses compared to whole breast irradiation," Vallow says.

For 14 of the same patients who had a lumpectomy in their left breast, investigators calculated radiation to normal heart tissue. They found that PBI and WBI delivered about the same amount of radiation to the heart of patients whose tumor was located in the middle of the breast. In patients whose tumors were closer to the armpit, PBI did not affect normal heart tissue, Vallow says.

"This study shows how far we have really come in the delivery of radiotherapy," she says. "We are looking for nuances of tissue exposure."

Mayo Clinic