ABSTRACT
Major developments in radiotherapy fractionation have taken place during the past four decades and continue to do so, and these have grown directly out of increasing understanding in clinical radiation biology. The relationships between equieffective total dose and dose per fraction for late-responding tissues, early responding tissues and tumours provide the basic information required to biologically optimise radiotherapy according to the dose per fraction, number of fractions and tumour type, site and treatment plan. A linear-quadratic framework can be used to describe this relationship between total isoeffective dose and the dose per fraction in fractionated radiotherapy. The linear-quadratic (LQ) model thus creates a robust quantitative environment for considering the balance between early reactions, late reactions and effect on the target tumour as dose per fraction and total dose are changed. This has been one of the most important advances in radiobiology applied to therapy.
