Errors and uncertainties in external radiation therapy. A system analysis with a cell kinetic model

The relative importance of accurate knowledge and control of various biologic and physical factors in external radiation therapy was estimated with a simple cell kinetic treatment response model for a simulated tumour therapy situation. A set of parameter values was chosen for a reference case so that the calculated probability of zero surviving tumour cells was about 0.5. The parameters were varied, one at a time, over a range that was considered representative with respect to uncertainty in dose calculation, dose delivery and the description of the pertinent patient data. Also parameters in the cell kinetic model were varied. The calculations demonstrated a strong influence on the number of surviving cells from even moderate changes in the parameters which affect the tumour dose. Predictions with regard to curability are heavily dependent on the reliability of the biologic parameters in the model.