Summary of findings 2. Prognostic biomarker ‐ prevention studies.

Women receiving endocrine therapy: mammographic density reduction versus no mammographic density reduction
Patient or population: women receiving endocrine therapy (selective oestrogen receptor modulator or aromatase inhibitor) Setting: prevention of breast cancer Intervention: mammographic density reduction Comparison: no mammographic density reduction
Outcomes	Impact	№ of participants (studies)	Certainty of the evidence (GRADE)
Incidence of invasive breast cancer and DCIS ‐ tamoxifen	OR 0.32 (95% CI 0.14 to 0.72) for per cent density reduction ≥ 10% compared with 0% density change (visual assessment in 5% bands from mammograms 12 to 18 months apart; Cuzick 2011a)	51 cases/456 controls (1 study)	⊕⊕⊝⊝ LOW 1
CI: confidence interval; DCIS: ductal carcinoma in situ; OR: odds ratio
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: We are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: Our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: We have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

¹Certainty of evidence was initially graded moderate because data for this outcome were from a randomised controlled trial. We downgraded the certainty of the evidence from moderate to low (GRADE downgrading factor ‐ risk of bias): cutpoint for the mammographic density biomarker was not determined a priori, but was based on the observed study data.