WEDNESDAY, March 23, 2005 (HealthDay News) -- Women who take tamoxifen to guard against the return of breast cancer face a six-fold increase in the risk for uterine sarcoma, a rare malignancy of the muscles and supporting tissue of the uterus.
An Israeli team of researchers reported the finding Wednesday at the Annual Meeting on Women's Cancer in Miami Beach, Fla.
"There is an increased risk for uterine sarcoma even after you stop taking the tamoxifen," said study author Dr. Ofer Lavie, head of the division of gynecologic oncology at Carmel Medical Center in Haifa, Israel.
While there have been case reports of uterine sarcoma associated with tamoxifen use, this report is believed to be the first large study to document the risk, Lavie said.
The study helps give a more complete picture of the risks of taking tamoxifen, Lavie said. It is routinely prescribed for five years for women who have had breast cancer to prevent recurrence, although scientists have known for years it boosts the risk of getting endometrial cancer.
Still, the benefit of tamoxifen is believed to outweigh the risk; endometrial cancer is less common than breast cancer and not usually fatal, experts said.
About 40,000 new cases of cancer of the body of the uterus will be diagnosed in the United States in 2005, according to the American Cancer Society. More than 95 percent will be endometrial cancers; about 4 percent, or 1,600, will be uterine sarcomas.
In the new study, Lavie's team evaluated the records of 1,508 Israeli women diagnosed with breast cancer. Among those women, 874 used tamoxifen. Among the tamoxifen users, 17, or 1.9 percent, developed endometrial cancer compared to only four, or 0.6 percent, of those who didn't use tamoxifen.
Four tamoxifen users, or 0.5 percent, developed uterine sarcoma, while the group that did not take tamoxifen reported no uterine sarcomas, making the risk six-fold higher for those who used tamoxifen, the researchers said.
The uterine sarcomas appeared after a median use of tamoxifen of nearly seven years, the researchers found, compared to just under six years for the endometrial cancers.
But another expert cautions that the study isn't the last word.
"This is an excellent study, and appropriately interpreted," said Emily White, a researcher at the Fred Hutchinson Cancer Research Center in Seattle. "It confirms that tamoxifen use increases the risk of endometrial cancer, and is highly suggestive that it also increases the risk of a rare form of uterine cancer, uterine sarcoma. However, because uterine sarcoma is so rare, it [the study] did not have the power to answer the question about the relationship between tamoxifen and uterine sarcoma with certainty."
Until more research is in, White said, women who have been diagnosed with breast cancer should know that tamoxifen reduces the risk of recurrence and of a breast cancer in the opposite breast. "The benefits outweigh the risk," she said.
Women should also know that newer drugs with similar actions, called aromatase inhibitors, are being studied and may have fewer adverse effects. "Women should follow the advice of the physicians" who weigh their individual benefits and risks, White said.
Women on tamoxifen should be monitored carefully, Lavie said. And they shouldn't take it more than the recommended five years, he said. "That's because its efficiency declines after five years, not due to the risk of cancer," he added.
To learn more about uterine sarcoma, visit the American Cancer Society.