[R] Cumulative dose in survival models

[klaus at kaae-consult.dk]

Dear List,
 
I want to fit a CPH survival model with a time-dependent cumulative exposure variable. More specifically, I have exposure measurements for all cohort members at discrete times (yearly measurements) during follow-up.
 
I suppose that when estimating the model parameters the most correct would be to have the cumulative exposure for everybody at risk at each death time, as I cannot assume the cumulative exposure to be constant between the yearly measurements . Thus, I need to calculate (interpolate) cumulative exposure for everybody at risk at each death time and pass this to the estimation routine.
 
Does anyone know how to do this in R? Any help would be much appreciated.
 
Klaus Andersen