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Breast Cancer Stem Cells Seem to Survive Radiation Therapy

Discovery may help scientists develop drug therapies to better combat tumors

TUESDAY, Dec. 19, 2006 (HealthDay News) -- Breast cancer stem cells, a type of cell that scientists have recently discovered is difficult to kill, may be especially resistant to radiation therapy, a new study suggests.

In fact, the radiation can even increase the growth of these stubborn stem cells, report researchers from the University of California, Los Angeles, David Geffen School of Medicine.

"This population of stem cells is more radiation-resistant than are non-stem cells," said Dr. Frank Pajonk, an assistant adjunct professor of radiation oncology at UCLA and corresponding author on the study. "We are the first to report this."

Radiation treatment involves exposure to high-energy rays or particles that destroy cancerous cells. It is often recommended after surgery for breast cancer, according to the American Cancer Society.

Pajonk and his colleagues exposed breast cancer stem cells and "normal" breast cancer cells to either single or multiple doses of radiation. More of the stem cells, also called cancer-initiating cells, lived through the radiation than did the other breast cancer cells.

One good example, according to Pajonk: While 46 percent of the stem cells survived treatment with 2 Gray of radiation (a dose typically used for breast cancer treatment), only 20 percent of normal breast cancer cells did.

Then, the team simulated clinical treatment that is interrupted -- a challenge that Pajonk and other health-care providers face when patients don't make all their scheduled appointments due to fatigue, inconvenience or other factors. Pajonk and his colleagues suspect this reduces the effectiveness of radiation, and the study suggests they are correct.

When Pajonk's team exposed the cells to a higher dose of 3 Gray, every day for five days, then stopped the treatment before what would be considered a full round, the proportion of stem cells actually increased.

Pajonk's team speculated that this may happen because the radiation activates a signaling pathway that gives the stem cells the messages to self-renew.

How is it that these cells are resistant to radiation? "They may have something like a natural radiation protectant inside of them that prevents the radiation-induced DNA damage that normally kills the breast cancer cells," Pajonk said.

The findings are published in the Dec. 20 edition of the Journal of the National Cancer Institute.

The new study sends a clear message to cancer researchers, said Dr. Maximilian Diehn, a resident in radiation oncology and postdoctoral fellow at the Stanford University School of Medicine. He co-authored an editorial accompanying the study results. "The main take-home point is that this gives more evidence that we should be studying cancer stem cells more," he said. "Those cells have properties different than the rest of the tumor."

Eventually, he said, scientists may be able to develop new drugs that would overcome this resistance to radiation, he said.

The concept of cancer stem cells is fairly new, said Diehn and Pajonk. For five years or so, it has been increasingly the topic of discussion in breast cancer research, as well as prostate cancer, melanoma and other types of tumors.

A better understanding of cancer stem cells could go a long way toward treatment success, Diehn said. "Often, less than one percent of cancer cells in a tumor are actually cells critical for keeping the tumor alive and potentially spreading the cancer. This is the cancer stem cell," he said.

The new research should not discourage women from getting radiation therapy if it is recommended, Diehn and Pajonk agreed. "Radiation treatment is still one of the best treatments available for women with breast cancer," Diehn said. It's also important, he said, to follow the treatment schedule as recommended and not to have gaps in treatment because that could make the stem cells proliferate.

In another new study, Dutch researchers found that comparing current mammograms to previous ones is valuable and can reduce referrals for lesions that turn out not to be cancerous. The researchers asked 12 experienced radiologists to read 160 mammograms twice; in one case, they had previous mammograms to refer to and, in the other, they did not.

When they had access to the previous mammograms, their detection performance improved. Having the prior mammogram to look at reduced referrals by 44 percent for suspicious areas that turned out not to be cancerous.

The findings are published in the January issue of the journal Radiology.

More information

To learn more about radiation treatment for breast cancer, visit the American Cancer Society.

SOURCES: Maximilian Diehn, M.D., Ph.D., resident in radiation oncology and postdoctoral fellow, Stanford University School of Medicine, Stanford, Calif.; Frank Pajonk, M.D., Ph.D., assistant adjunct professor of radiation oncology, University of California, Los Angeles, David Geffen School of Medicine; Dec. 20, 2006, Journal of the National Cancer Institute; January 2007, Radiology
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