ABSTRACT
Low dose-rate irradiation is the ultimate form of fractionation, equivalent to multiple infinitely small fractions being given without radiation-free intervals, and thereby damage induction and repair take place at the same time. In clinical radiotherapy, continuous low dose rate is used in brachytherapy either by permanent or temporary implantation of radioactive sources (e.g. I-125, Pd-103) into tumours. By utilising remote afterloading of medium or high dose-rate sources, notably Ir-192, various combinations of dose rate and fractionation can also be chosen, such as pulsed dose-rate and high dose-rate brachytherapy. With external beam treatments using intensity modulated radiotherapy or other techniques using interruptions like breathholding, the dose-rate effect may also have some impact as the longer treatment times per session which can be needed in more complex plans, may lead to a reduction in effectiveness. Increasing dose rates by using flattening-filter-free linacs or other technologies also raises the possibility of a change in biological effectiveness for these treatments.
