Estimation of the adjusted cause‐specific cumulative probability using flexible regression models for the cause‐specific hazards

In competing risks setting, we account for death according to a specific cause and the quantities of interest are usually the cause‐specific hazards (CSHs) and the cause‐specific cumulative probabilities. A cause‐specific cumulative probability can be obtained with a combination of the CSHs or via the subdistribution hazard. Here, we modeled the CSH with flexible hazard‐based regression models using B‐splines for the baseline hazard and time‐dependent (TD) effects. We derived the variance of the cause‐specific cumulative probabilities at the population level using the multivariate delta method and showed how we could easily quantify the impact of a covariate on the cumulative probability scale using covariate‐adjusted cause‐specific cumulative probabilities and their difference. We conducted a simulation study to evaluate the performance of this approach in its ability to estimate the cumulative probabilities using different functions for the cause‐specific log baseline hazard and with or without a TD effect. In the scenario with TD effect, we tested both well‐specified and misspecified models. We showed that the flexible regression models perform nearly as well as the nonparametric method, if we allow enough flexibility for the baseline hazards. Moreover, neglecting the TD effect hardly affects the cumulative probabilities estimates of the whole population but impacts them in the various subgroups. We illustrated our approach using data from people diagnosed with monoclonal gammopathy of undetermined significance and provided the R‐code to derive those quantities, as an extension of the R‐package mexhaz.