I have been asked an interesting modelling question by a clinician. The outcome he is trying to predict whether a particular type of cancer will occur. One of the covariates is an indicator variable if the patient previously had another kind of cancer. Other covariates are cancer diameter and margin size of this prior cancer. The margin size is how many millimetres the surgeon cut around the cancer when it was removed. For patients that have not had the prior cancer, these values are obviously missing.
I believe that making diameter be 0 millimetres makes sense, but there can be no sensible value for margin size. What model formula should I use? What theory should I learn?