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Summary
Background
The optimal ways of using aromatase inhibitors or tamoxifen as endocrine treatment for early breast cancer remains uncertain.

Methods
We undertook meta-analyses of individual data on 31 920 postmenopausal women with oestrogen-receptor-positive early breast cancer in the randomised trials of 5 years of aromatase inhibitor versus 5 years of tamoxifen; of 5 years of aromatase inhibitor versus 2–3 years of tamoxifen then aromatase inhibitor to year 5; and of 2–3 years of tamoxifen then aromatase inhibitor to year 5 versus 5 years of tamoxifen. Primary outcomes were any recurrence of breast cancer, breast cancer mortality, death without recurrence, and all-cause mortality. Intention-to-treat log-rank analyses, stratified by age, nodal status, and trial, yielded aromatase inhibitor versus tamoxifen first-event rate ratios (RRs).

Findings
In the comparison of 5 years of aromatase inhibitor versus 5 years of tamoxifen, recurrence RRs favoured aromatase inhibitors significantly during years 0–1 (RR 0·64, 95% CI 0·52–0·78) and 2–4 (RR 0·80, 0·68–0·93), and non-significantly thereafter. 10-year breast cancer mortality was lower with aromatase inhibitors than tamoxifen (12·1% vs 14·2%; RR 0·85, 0·75–0·96; 2p=0·009). In the comparison of 5 years of aromatase inhibitor versus 2–3 years of tamoxifen then aromatase inhibitor to year 5, recurrence RRs favoured aromatase inhibitors significantly during years 0–1 (RR 0·74, 0·62–0·89) but not while both groups received aromatase inhibitors during years 2–4, or thereafter; overall in these trials, there were fewer recurrences with 5 years of aromatase inhibitors than with tamoxifen then aromatase inhibitors (RR 0·90, 0·81–0·99; 2p=0·045), though the breast cancer mortality reduction was not significant (RR 0·89, 0·78–1·03; 2p=0·11). In the comparison of 2–3 years of tamoxifen then aromatase inhibitor to year 5 versus 5 years of tamoxifen, recurrence RRs favoured aromatase inhibitors significantly during years 2–4 (RR 0·56, 0·46–0·67) but not subsequently, and 10-year breast cancer mortality was lower with switching to aromatase inhibitors than with remaining on tamoxifen (8·7% vs 10·1%; 2p=0·015). Aggregating all three types of comparison, recurrence RRs favoured aromatase inhibitors during periods when treatments differed (RR 0·70, 0·64–0·77), but not significantly thereafter (RR 0·93, 0·86–1·01; 2p=0·08). Breast cancer mortality was reduced both while treatments differed (RR 0·79, 0·67–0·92), and subsequently (RR 0·89, 0·81–0·99), and for all periods combined (RR 0·86, 0·80–0·94; 2p=0·0005). All-cause mortality was also reduced (RR 0·88, 0·82–0·94; 2p=0·0003). RRs differed little by age, body-mass index, stage, grade, progesterone receptor status, or HER2 status. There were fewer endometrial cancers with aromatase inhibitors than tamoxifen (10-year incidence 0·4% vs 1·2%; RR 0·33, 0·21–0·51) but more bone fractures (5-year risk 8·2% vs 5·5%; RR 1·42, 1·28–1·57); non-breast-cancer mortality was similar.

Interpretation
Aromatase inhibitors reduce recurrence rates by about 30% (proportionately) compared with tamoxifen while treatments differ, but not thereafter. 5 years of an aromatase inhibitor reduces 10-year breast cancer mortality rates by about 15% compared with 5 years of tamoxifen, hence by about 40% (proportionately) compared with no endocrine treatment.

Funding
Cancer Research UK, Medical Research Council.

click for full text: http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(15)61074-1/abstract

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