Abstract. Emilsson L, Lindahl B, K€ oster M, Lambe M, Ludvigsson JF (V ardcentralen V€ armlands Nys€ ater,

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Importance Cancer patients who use statins appear to have a substantially better survival than non-users in observational studies. However, this inverse association between statin use and mortality in cancer patients may be due to selection bias and immortal time bias. Objective We used observational data to emulate a randomized trial of statin initiation that is free of selection bias and immortal time bias. Design We used data on 17,372 cancer patients from the SEER-Medicare 2007-2009 database with complete follow-up until 2011. Individuals were duplicated, with each replicate assigned to either the strategy “statin initiation within 6 months of diagnosis” or “no statin initiation”. Replicates were censored when they stopped following their assigned strategy, and the potential selection bias was adjusted for via inverse-probability weighting. Hazard ratios (HR), cumulative incidences, and risk differences were calculated for all-cause mortality and cancer-specific mortality. We then compared our estimates with those obtained using the same analytic approaches as in previous observational studies. Setting The SEER-Medicare data is a linkage between 17 American cancer registries and claims files from Medicare and Medicaid in 12 states. Participants We included individuals with a new diagnosis of colorectal, breast, prostate, and bladder cancer who had not been prescribed statins for at least 6 months before cancer. Exposure Statin initiation within 6 months from cancer diagnosis. Main outcome measure Cancer-specific and all-cause mortality. Results The adjusted HR (95% CI) comparing statin initiation vs. no initiation was 1.00 (0.88-1.15) for cancer-specific mortality and 1.07 (0.93-1.21) for overall mortality. Cumulative incidence curves for each group were almost overlapping. On the contrary, the methods used by prior studies resulted in a strong inverse association between statin use and mortality. Conclusion and relevance After using methods that are not susceptible to selection bias from prevalent users and to immortal time bias, initiation of statins within 6 months of cancer diagnosis did not appear to improve 3-year cancer-specific or overall survival.