This clinical practice guideline is based on available evidence from six randomised trials assessing three different ways of incorporating aromatase inhibitors in adjuvant endocrine therapy for hormone receptor-positive early breast cancer in post-menopausal women:
- starting adjuvant endocrine therapy with an aromatase inhibitor compared with starting with tamoxifen (ATAC, BIG-98)
- switching to an aromatase inhibitor after 2–3 years of tamoxifen compared with continuing on tamoxifen, both for total of 5 years of adjuvant endocrine therapy (IES, ITA, ABCSG/ARNO)
- extending adjuvant endocrine therapy by adding 5 years of an aromatase inhibitor after an initial 5 years of tamoxifen (MA.17). (see Table 1 for details)
Adjuvant endocrine therapy with an aromatase inhibitor significantly improved disease-free survival in post-menopausal women with hormone receptor-positive early breast cancer compared with tamoxifen or placebo in all six available randomised trials. Improvements in overall survival have not been demonstrated but follow-up to date is relatively short. Most women had few side effects on either aromatase inhibitors or tamoxifen and, where reported, quality of life was comparable for aromatase inhibitors and tamoxifen. The long-term effects of aromatase inhibitors are not yet known. While some trials have examined the use of aromatase inhibitors for 5 years, the optimal duration of aromatase inhibitor therapy and the optimal sequence in which different adjuvant endocrine therapies should be administered for the management of early breast cancer is not yet established.