ABSTRACT
The description of frontline tumor control probability (TCP) and normal tissue complication probability (NTCP) modeling will be the subject of Volume II of this textbook. As a bridge between the basic radiobiology of Volume I and the “macroscopic” modeling of clinical outcomes in Volume II, this chapter will describe why the basic biological parameters are important for modeling radiotherapy response. Local control is thought to occur when all the stem cells in the tumor are killed by the radiation treatment. And since the microdosimetric distributions of inactivating energy depositions are random (stochastic), Poisson statistics are again employed to calculate the proportion of the various tumor types in which every stem cell is inactivated (i.e., the TCP) as a function of dose. For these TCP values to predict clinical response accurately, the values of intrinsic radiosensitivity of the stem cells and their total number in the tumor (i.e., the cells that must be inactivated) should be known. These parameters are not usually determined for individual tumors but reasonable estimates are available for populations of like tumors in different patients. Intertumor heterogeneity should be taken into account when modeling the radiation response of groups of human cancers. This heterogeneity arises from distributions in clonogen intrinsic radiosensitivity, oxygenation status, growth fraction and possibly other factors. In addition, host factors such as immune response can also play a role. TCP models are important for determining the effectiveness of novel fractionation schemes, the effect of departures from homogeneous irradiation, variations in initial clonogen number and other factors.
