Li 2013.

Study characteristics
Methods	Retrospective cohort study. Non‐randomised endocrine therapy allocation. Initial breast cancer diagnosis 1993 to 1995, follow‐up until 31 December 2008. Follow‐up from initial diagnosis (median 14.2 years). Prognostic biomarker. Treatment setting.
Participants	Sweden. 974 women. Age 50 to 74 years at first diagnosis (median age 62 to 63 years). Postmenopausal women only. All invasive at first diagnosis.
Comparisons	Tamoxifen 20 mg daily (n = 231), tamoxifen 40 mg daily (n = 123), tamoxifen 20 + 40 mg daily (n = 108), tamoxifen "other" dose daily (n = 12), median 60 months of treatment. Fully automated area‐based method measuring absolute dense area. Relative density reduction more than 20% vs stable density (≤ 9% increase to ≤ 10% reduction) at 6‐ to 36‐month follow‐up mammogram.
	Outcome: breast cancer mortality (time to death caused by breast cancer).
Notes
Item	Authors' judgement	Support for judgement
Study participation	Unclear	Source population for the study was all postmenopausal women diagnosed with invasive breast cancer aged 50 to 74 years. This is a different source population to the review target population, but includes relevant subsets. The study source population excludes premenopausal women from the review target population, and includes some women with hormone‐receptor negative disease. 3979 women were eligible, and 3345 of 3979 approached to join the study (84%) completed the study questionnaire a mean 4 months after diagnosis (SD 1.4 months), which indicates a low risk of bias in study participation of those with early breast cancer death (due to time to administer questionnaire). 527 women were excluded due to not matching the target population (such as premenopausal and not first cancer). 1844 women were excluded from the study sample (mainly due to lack of mammograms (1603 women)). A large proportion of the potential sample was thus excluded. There was no comparison between other characteristics of included/excluded participants in study sample.
Study attrition	Yes	Follow‐up information from population registry
Prognostic factor measurement	Unclear	Valid and reliable density measure according to review protocol criteria. The cutpoints chosen for density change were said to be chosen "a priori", but no justification was provided for using them; they have not been reported elsewhere.
Outcome measurement	Yes	Outcome ascertained from national population registry. Clear definitions of start of follow‐up and reasons for censoring were provided.
Study confounding	Yes	Adequate adjustment: time interval between baseline and follow‐up mammograms, age at baseline mammogram, ever HRT use, BMI at interview, time since menopause at baseline mammogram, oestrogen receptor status, tumour size, number of metastatic nodes, grade, radiotherapy treatment, chemotherapy treatment, and change in absolute non‐dense area. Analysis for tamoxifen‐treated groups was additionally adjusted for length of tamoxifen treatment.
Statistical analysis and reporting	Yes	Sufficient presentation of data to assess adequacy. Conceptual framework for adjustment was to include prognostic factors and/or those associated with density.