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Study Confirms Tamoxifen Raises Endometrial Cancer Risk

Risk heightened by length of time on the drug

WEDNESDAY, March 2, 2005 (HealthDay News) -- Doctors often prescribe the drug tamoxifen to breast cancer patients over the long term, to help reduce the chances of cancer recurrence.

But a new study confirms tamoxifen can also boost the risk of endometrial cancer in both pre-menopausal and postmenopausal women, with that risk increasing the longer a woman takes the drug.

However, Debbie Saslow, director for breast and gynecologic cancer control for the American Cancer Society, stressed that doctors have long understood that tamoxifen carries this caveat.

The new study isn't going to change how doctors do things, she predicted, but "it helps people make more informed choices."

The findings appear in the March 2 issue of the Journal of the National Cancer Institute.

In the study, British researchers from the Institute of Cancer Research compared treatment information on 813 patients who had endometrial cancer after their diagnosis of breast cancer, to that of 1,067 breast cancer patients who didn't develop an endometrial malignancy.

Led by Anthony Swerdlow for the British Tamoxifen Second Cancer Study Group, the investigators found that tamoxifen treatment was associated with 2.4 times the risk of endometrial cancer compared to no treatment.

They also found that the risk of endometrial malignancy increased with duration of treatment and did not diminish within five years of treatment cessation. Both pre-menopausal and postmenopausal women experienced a similar risk, the study found.

Tamoxifen has been used to treat breast cancer patients for more than 25 years, and "we've known for quite some time that one of the most serious side effects of tamoxifen is the risk of endometrial cancer," Saslow noted.

The point of the new study, she said, was to obtain more details, such as identifying women at greater risk of getting endometrial cancers, and to elucidate factors that might increase or decrease the risk.

According to the researchers, the findings suggest that heightened risks for endometrial cancer make treatment with tamoxifen beyond the standard five years "questionable."

"Almost all women with estrogen receptor-positive breast cancer are prescribed tamoxifen for five years," Saslow said. In this type of breast cancer, there is an overabundance of receptors on the cancer cells to which estrogen binds, fueling cancer cell growth. Tamoxifen interferes with the activity of estrogen, preventing that growth.

It's important for women to keep in perspective that endometrial cancers are much less common than breast cancer and are usually not fatal, Saslow said.

"There is usually early warning, abnormal [uterine] bleeding," she said. All women should alert their doctors as soon as possible if they have abnormal uterine bleeding, she said.

This year, about 40,000 new cases of endometrial cancer will be diagnosed in the United States, according to the American Cancer Society, and more than 7,300 deaths are expected. In comparison, more than 211,000 cases of invasive breast cancer will be diagnosed in 2005, with more than 40,000 deaths.

Even with the increased risk of endometrial cancer, the British researchers conclude tamoxifen's benefits far outweigh its risks when used for five years or less.

And while the question was not addressed in the study, Saslow said tamoxifen decreases the risk of getting breast cancer in the opposite breast "by a large amount, about 50 percent. It's one of the reasons the death rates from breast cancer have been going down."

While the findings shouldn't alarm patients, they may want to ask more questions about their therapy, Saslow said. For example, women with breast cancer might talk to their doctors about a new class of drugs called aromatase inhibitors, which are being used increasingly in women with breast cancer. They also stop the body from making estrogen, but in a different way than tamoxifen, Saslow explained. More importantly, they haven't been found to boost endometrial cancer risk, she said.

According to Saslow, the key point to remember, however, is that "the risks of dying from breast cancer if you don't take tamoxifen are greater than if you do take it."

More information

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

SOURCES: Debbie Saslow, Ph.D., director, breast and gynecologic cancer control, American Cancer Society, Atlanta; March 2, 2005, Journal of the National Cancer Institute
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