A new continuous-time statistical model for modelling the effect of radiotherapy treatment is proposed. In contrast with classical models, it takes in account heterogeneity of cell damage and repair. In this paper, a bi-scale continuous-time model is derived to describe tumor and normal tissue lifespans. We show that the cell lifespan can be described by a continuous-time Markov model. The tumor control probabilities and normal tissue complication probabilities are expressed. The model has been implemented into Matlab and numerical simulations have emphasised the effects of the model parameters on the model output.