I’m guessing that phones are ringing off the hook at oncologists’ offices today as breast cancer patients hear the news that taking tamoxifen for 10 years, rather than the usual five, will lower their risk of recurrence and ultimately dying from breast cancer. That news comes from a long-awaited clinical trial published in the journal Lancet involving nearly 13,000 breast cancer patients of all ages who had “estrogen-receptor positive’’ cancer, the most common type that’s treated with estrogen-blocking tamoxifen.

The European researchers found that those who were randomly given tamoxifen for 10 years had a 12 percent likelihood of dying from breast cancer over the 15-year study compared to a 15 percent risk for those who were given placebos after their first five years on tamoxifen.


About 3 percent of the women who took 10 years of tamoxifen, however, developed endometrial cancer — a known problem caused by the drug — which was about double the rate of those who took tamoxifen for fewer years. Endometrial cancer is rarely life threatening and is usually treated with surgery.

“It’s going to be practice changing, in my view, immediately for pre-menopausal women with breast cancer,’’ said Dr. Eric Winer, chief of women’s cancers at the Dana-Farber Cancer Institute who was attending the San Antonio cancer conference where the results were presented on Wednesday. Those who have the most common type of “estrogen-receptor positive’’ breast cancer will probably be advised to continue taking tamoxifen for an additional five years, he said.

On an individual level, however, breast cancer patients will need to weigh their options carefully. Endometrial cancer is a real risk, but it’s usually more common in older women over age 60. Patients should also realize that “the benefit isn’t huge, it’s modest,’’ Winer said.

And some women simply can’t tolerate tamoxifen and are eager to get off of the drug as quickly as possible. They might have hot flashes, moodiness, and vaginal dryness making sex very painful. “About 10 to 15 percent of women don’t like being on it,’’ Winer added, “and unfortunately, younger women seem to have more of these side effects.’’


Those with small, non-aggressive tumors with no spread to nearby lymph nodes might consider only taking tamoxifen for five years if they’re plagued by side effects since their risk of recurrence is very small. Those with larger, more aggressive tumors, however, might feel more compelled to stay on the drug for a decade.

For women whose breast cancer was diagnosed after menopause, the picture gets even more complicated. They’re usually given newer drugs called aromatase inhibitors (Arimidex, Femara, Aromasin) in addition to or instead of tamoxifen for a total of five years of treatment. Dana Farber patients typically get two years of tamoxifen followed by five years of aromatase inhibitors.

“I’ve long believed that these extra years of treatment would help,’’ said Winer. That’s because with estrogen-receptor positive cancers, half of all recurrences happen beyond five years of diagnosis.

The new study found that 21 percent of those taking tamoxifen for 10 years had a recurrence during the study compared to 25 percent taking tamoxifen for the shorter time period. The biggest differences in recurrence rates were seen between 10 and 15 years after the cancer diagnosis.

“That was confusing to some of the oncologists at the meeting,’’ Winer said, but it could have to do with tamoxifen’s cancer-preventing benefits lasting for up to five years after women stop taking the drug.

Post-menopausal breast cancer patients could be given the option to take estrogen-blocking drugs for longer, but oncologists might be left in a quandary about which drugs to give and for how long. “Should these patients be given 10 years of treatment with an aromatase inhibitor? Should they have 5 years of an aromatase inhibitor followed by 5 years of tamoxifen? Would more than 10 years of tamoxifen be even better than 10 years? No data exist to support any of these options,’’ wrote Dr. Trevor Powles, an oncologist at the Cancer Centre London in England, in an editorial that accompanied the study.


Many doctors may simply choose to wait for more studies before changing their treatment protocols.