The purpose of radiation therapy is tumor control. In view of clonogen proliferation it makes sense to deliver the necessary dose in as short a time as possible,and as well-targeted as possible. The limiting factor in tumor resistant is normal tissue reaction: Normal tissue reaction must not exceed """"""""tolerance"""""""" level. The development and application of high technology particularly in computers and computer based and assisted imaging has stimulated great progress in tumor localization and treatment planning, and even in the technology of delivery and its quality assurance, i.e., the spatial aspects of the issue. Decision-making as to the amount of dose and its distribution over time (by fractionation or protraction) is still essentially empiricle, however. The present project continues the development and exploration of a theoretical description of time-dose response of tumors as well as normal tissues. Our work has concentrated on extensions of the Linear-Quadratic model. A paper [1] was devoted to establishing the relationship between the alpha,beta model, including repair of sublethal damage for conventional and (accelerated) hyperfractionation and the alpha,beta model for continuous low dose rate. The study of implications of high dose rate versus low dose rate brachytherapy therapy is ongoing. An interactive computer program and graphical tools to help clinical guidance in the search for high dose rates compatible with constant tumor control as well as no more severe toxicity has reached a useful stage. Changed priorities related to changes in the medical staff have temporarily slowed-down the developments.
