RADIOBIOLOGICAL MODELS OF TISSUE-RESPONSE TO RADIATION IN TREATMENT PLANNING SYSTEMS

Aims: To present several biological concepts and models of tissue resp onse to fractionated radiotherapy. To describe practical implementatio n of these models in three-dimensional treatment planning systems. Met hods: Models of cell survival, Equivalent Uniform Dose (EUD) and Tumor Control Probability (TCP) are discussed. These models are based on th e target-cell hypothesis which assumes that response of organs and tis sues to radiation therapy can be explained and mathematically describe d in terms of survival of the specific target-cells. Results: Several formulae for deriving and calculating EUD and TCP for a given three-di mensional dose distribution are presented and discussed. Conclusions: Biological models of tissue response to radiation, when used wisely, h ave a potential to be useful in radiation therapy treatment planning, The models can advance our understanding of the underlying biological mechanisms, and may help in designing new and better treatment strateg ies. They should be particularly useful in modern conformal radiothera py where treatment strategy for each patient can be individualized and optimized according to patient characteristics and available technolo gy of delivering sophisticated treatment plans.