Optimization of radiation therapy II: the critical-voxel model

The Complication Factor (CF) is an objective function recently introduced for use in the optimization of radiation therapy X treatment planning. Unlike earlier objective functions based upon physical/geometrical criteria, such as tumor dose uniformity, minimal integral-dose, etc., the CF stems from a simple biological/probabilistic model of radiation damage in living organisms. The CF defines the integral-response of an organ as that fraction of it rendered non-functional by irradiation; this parameter is significant if the net amount of damage to the organ is of importance but details of its spatial distribution are not as, for example, might be nearly the case with liver. This approach does not work, however, if complications in any one individual volume-element are critical, as with spinal cord or tumor recurrence. Several authors have addressed the latter problem, and we find that the probabilistic argument common to their methods fits comfortably within the CF framework. Drawing attention to the distinct differences between the integral-response and critical-voxel cases hopefully will be of value in the further development of biological modelling, for application in radiotherapy and elsewhere.