Choice of imputation method for missing metastatic status affected estimates of metastatic prostate cancer incidence

To study how handling missing data on M stage in a clinical cancer register affects estimates of incidence of metastatic prostate cancer. Estimates of age-standardized incidence of metastatic prostate cancer were obtained by the use of data in a population-based clinical cancer register in Sweden and using four methods for imputation of missing M stage. Adjusted survival was used to compare men with known and imputed M stage. The proportion of men with missing M stage was high (66%) and varied according to the risk group and over calendar time. The estimated incidence of metastatic disease varied depending on imputation method, with all methods indicating a decreasing incidence over time. A combination of deterministic imputation (DI) and multiple imputation (MI) produced adjusted survival curves for men with imputed M stage that best resembled the survival for men with known M stage. Plausible estimates of incidence of metastatic prostate cancer in clinical cancer registers can be obtained by the use of a combination of DI of missing M stage and MI. •Choice of imputation method for missing M stage affects the estimated incidence.•Setting missing all M stage to M0 results in biased estimate of incidence.•Multiple imputation can be used to gain insights about this bias.