A control circuit provides an opportunity via a user interface for a user to specify at least one custom DVH estimation model training feature. The control circuit then combines a predetermined set of DVH estimation model training features with a user-specified customer DVH estimation model training feature to provide a combined feature set. The control circuit uses the combined feature set to train a knowledge-based DVH estimation model which is then used to provide a DVH estimation for use when developing/optimizing a radiation treatment plan. That resultant radiation treatment plan then controls a radiation-administration platform to provide a therapeutic radiation dose to a patient.