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Longer Tamoxifen Use Helps Breast Cancer Survival

Testing Longer Courses of Tamoxifen

The study compared nearly 7,000 women with early-stage, estrogen-sensitive breast cancers. Early-stage cancers were considered to be any cases in which all detected disease could be completely removed.

About half the women in the study stopped taking tamoxifen after the recommended five years, while the other half took the drug for 10 years.

Over the next 10 years, 1,328 women in the study saw their cancer recur. There were 617 recurrences in the group that took tamoxifen for 10 years compared to 711 among women who took the drug for five years. Overall, women who stayed on tamoxifen cut their risk of having cancer come back from 25.1% to 21.4%

There were also fewer deaths from breast cancer in the group that stayed on tamoxifen -- 12.2% in the 10-year group compared to 15% in the 5-year group.

“It’s a small extra gain. I don’t want to oversell it. It’s small, but it’s real,” says researcher Richard Peto, a professor of medical statistics and epidemiology at the University of Oxford in the U.K.

“I suspect it will affect the treatment of hundreds of thousands of women worldwide, and it will avoid a few thousands deaths each year worldwide,” he says.

Benefits Greatest in the Second Decade After Diagnosis

The biggest differences between the two groups were seen in the second decade after diagnosis.

For example, other studies have found that women who take tamoxifen for five years can cut their risk of dying of breast cancer by about 30% in the decade that spans 10 to 20 years post-diagnosis, compared to not taking the drug at all.

The new study suggests that women who continue to take tamoxifen for another five years may reduce that risk even further, by an additional 30%.

Researchers estimate that women with early estrogen-sensitive tumors who take tamoxifen for a full 10 years would have about half the risk of dying of breast cancer in the second 10 years after diagnosis compared to women who don’t take the drug.

“You’re holding down the disease that’s scattered around the body and this produces benefit years and years after the treatment is finished,” Peto says.

This isn’t the first study to test the effects of longer courses of tamoxifen. Previous studies had mixed results. Some concluded that the additional risks of the drug didn’t outweigh its benefits, but those studies were much smaller and had limited power to see differences between their study groups.

Peto notes that they did see slightly more cases of uterine cancer and blood clots in women who doubled their time on tamoxifen -- 1.6% of women in the five-year group were diagnosed with uterine cancer compared to 3.1% in the 10-year group.

There were about twice as many cases of pulmonary embolism, cases where a dangerous blood clot makes its way to the lungs -- 21 in the five-year group as opposed to 41 in the 10-year group. There was no significant increase in stroke risk.

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