IBIS-I: five years of tamoxifen keeps preventing breast cancer for at least 20 years
In women at increased risk, 5 years of tamoxifen reduced breast cancer by 29% over a median 16 years of follow-up, with the benefit continuing long after the pills stopped, but no reduction in breast cancer deaths.
IBIS-I randomised 7,154 women aged 35-70 at increased risk of breast cancer to tamoxifen 20 mg daily or placebo for 5 years. This long-term analysis had a median follow-up of 16 years.
Breast cancer (invasive plus ductal carcinoma in situ) occurred in 251 tamoxifen-arm and 350 placebo-arm women (HR 0.71). The reduction was similar in the first 10 years and after 10 years, showing a carry-over effect. Oestrogen-receptor-positive invasive cancer fell by a third; ER-negative cancer was unaffected. There was no difference in breast cancer mortality. Endometrial cancer and thromboembolic events were increased mainly during active treatment.
With the earlier NSABP P-1 trial (49% reduction at 5 years), IBIS-I is the basis for guideline recommendations of tamoxifen for risk reduction.
- Breast cancer HR 0.71 (95% CI 0.60-0.83) over median 16 years; 251 vs 350 cases
- ER-positive invasive breast cancer HR 0.66; no effect on ER-negative disease
- Benefit continued beyond 10 years after randomisation (HR 0.69 for years 10 and later)
- No significant difference in breast cancer deaths (31 vs 26) or all-cause mortality
- Endometrial cancer was increased during the 5 treatment years but not afterwards
For women at raised risk, a 5-year course of tamoxifen offers long-lasting protection against the commonest kind of breast cancer. Uptake is low because of side effects and fear of rare serious harms; low-dose tamoxifen (TAM-01) is now being tested to improve the balance. It has not been shown to save lives.
- No mortality benefit despite fewer cancers, partly because ER-positive cancers are highly treatable
- Only about 10% of eligible women in most countries accept preventive tamoxifen
- Increased endometrial cancer and venous thromboembolism during treatment, concentrated in postmenopausal women
- Risk assessment relied on family history criteria of the 1990s rather than modern risk models
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