Invasive breast cancer risk not modified by dose, delivery routes of oestrogen therapy
In women who have undergone hysterectomy and use hormone therapy, the risk of invasive breast cancer does not differ significantly when directly comparing different doses, formulations and routes of delivery, according to a study.
Researchers looked at 26,525 postmenopausal women with a hysterectomy, aged 50−79 years at study entry, from the Women's Health Initiative (WHI) Observational Study. Mean duration of hormone therapy use was 18.5 years for conventional-dose conjugated equine oestrogen (CEE), 17.4 years for low-dose CEE, 19.3 years for high-dose CEE, 14.7 years for oral oestradiol and 14.0 years for transdermal oestradiol.
Over an average follow-up of 8.2 years, the absolute risk of invasive breast cancer was 43 per 10,000 person-years in women with hysterectomy using CEE alone. Cox proportional hazards models showed that the risk did not significantly differ between low-dose CEE (<0.625 mg) users and conventional-dose CEE (0.625 mg) users (hazard ratio [HR] 0.99; 95 percent CI, 0.65−1.48).
Compared with conventional-dose CEE, transdermal oestrogen was associated with a lower risk of invasive breast cancer (HR, 0.75; 0.47−1.19) while oral oestradiol was associated with a higher risk (HR, 1.20; 0.84−1.39), although the differences did not reach statistical significance.
The finding of a lower breast cancer risk in the current WHI oestrogen-alone trial may extend to lower doses of CEE, as well as transdermal route of delivery, researchers said. However, further research is needed to confirm these hypotheses.
Future investigations should be focused on different doses of CEE, with comparative analyses performed between transdermal and oral oestradiol, researchers added.