AMSTERDAM, THE NETHERLANDS. Tamoxifen therapy has long been used in the treatment of metastatic breast cancer and in the prevention of recurrence of breast cancer. It is known that tamoxifen increases the risk of developing endometrial cancer, but so far it has been assumed that this "side effect" was of relatively little importance. Now researchers at the Netherlands Cancer Institute question this assumption. Their study involved 299 women who had developed endometrial cancer subsequent to being diagnosed with breast cancer and 860 matched controls who had been diagnosed with breast cancer, but had not developed endometrial cancer.
The researchers found that 36.1 per cent of the women with endometrial cancer had used tamoxifen as
compared to 28.5 per cent in the control group. This translates into a 50 per cent greater risk among
tamoxifen users. The risk increased with duration of use and was almost seven times higher for women who
had taken tamoxifen for at least five years when compared to the controls. The severity of the cancer and
the presence of sarcomas were also much higher among long-term users. The three-year endometrial-
cancer-specific survival was 76 per cent for long-term tamoxifen users versus 94 per cent for non-users.
The researchers calculate that 20 excess cases of endometrial cancer would develop in 1000 tamoxifen
users followed up for 10 years.
They conclude that the benefits of tamoxifen therapy still outweigh the risks in the treatment of metastatic
breast cancer and in prevention of contralateral cancer. However, they seriously question widespread use
of tamoxifen as a preventive agent against breast cancer in healthy women.