Radiation therapy treatment plans are not delivered to a patient in a single session, but rather as a sequence of daily treatment fractions over an extended period of time (usually on the order of 4-8 weeks). Changes in patient geometry during the course of treatment introduce two different sources of uncertainty caused by changes in patient geometry that can compromise the quality of a treatment plan. First, interfraction motion uncertainty refers to changes that take place between treatment fractions, caused by the fact that (i) patients cannot be perfectly repositioned at the accelerator for each fraction, and (ii) structures can move or change between fractions. Second, intrafraction motion uncertainty refers to changes that take place during a fraction, for example, due to breathing. We study the consequences for radiation therapy treatment planning of these two sources of uncertainty and conclude that they should be incorporated into the optimization model in different ways. We discuss different treatment planning paradigms, their tractability, and their requirements in terms of both models of patient uncertainty and imaging.
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