A new study suggests long-term use of tamoxifen is linked to increased risk of developing an aggressive type of breast cancer, but experts say the drug's proven track record in reducing risk of both recurrence of the disease and death outweigh potential negative effects.
Scientists at the Fred Hutchinson Cancer Research Center in Seattle looked at 728 women who had been diagnosed with breast cancer. Those women were compared to 367 others diagnosed with both a first and a second breast cancer.
A second breast cancer can mean a tumour occurring in the opposite breast or when two separate tumours develop in the same breast.
The main finding from the study, published online yesterday in the journal Cancer Research, was that tamoxifen lowered the risk of any second breast cancer overall by about half, says lead author Dr. Christopher Li.
While long-term use of the drug, for five or more years, reduced the more common type of breast cancer by about 60 per cent, it also increased the risk of a more aggressive subtype of the disease by more than 400 per cent, Li says.
Tamoxifen is an estrogen-blocking drug. Li says such drugs were specifically designed to block the estrogen receptor to prevent the growth of breast cancer cells. But not all breast cancers have the estrogen receptor (ER), Li says. Those that don't tend to be more aggressive and to spread more quickly.
"For the estrogen receptor-positive cancer, we have targeted therapy that again has been proven to reduce mortality," says Li. "That's one of the reasons ER-negative cancers are more worrisome because we don't have a targeted treatment for them."
Tamoxifen lowers breast cancer patients' risk of dying of the disease, and has also been shown to lower a woman's risk of developing a recurrent breast cancer and a second breast cancer, he noted. But use of tamoxifen also comes with risk of stroke, as well as the risk of endometrial cancer, he adds.
"So here we're finding that we're adding potentially another risk to the risk-benefit equation," Li says. "We're finding that there is this increased risk in this more aggressive subtype of second breast cancer. However, we also overall found using tamoxifen did lower the risk of any type of second breast cancer overall."
"For that reason, we don't really think that this study changes the overall risk-benefit equation because the benefits for most women who are eligible to use this treatment are going to still outweigh the risks."
Dr. Kathleen Pritchard with Sunnybrook Odette Cancer Centre in Toronto agrees with Li about the risk-benefit factor.
"I think the benefits of taking tamoxifen for someone who's already had a breast cancer and is trying to prevent recurrence of that breast cancer way outweigh the downside of what's shown in a study like this," says Pritchard.