ABSTRACT
When the patient is set up for each fraction, the anatomy may be di¢erent from that used for the initial treatment planning. Typically, the deviations that are most harmful are the so-called systematic deviations, which are also relatively easier to account for by an either o¬ine or online correction strategy. Ÿe random components of the deviations, although less harmful than the systematic deviations, are generally di›cult to be fully accounted for and require an online correction strategy. One of the advantages of online correction strategies over o¬ine methods is that online strategies can correct for both systematic and random variations. In addition, o¬ine corrections may not be applicable to a course of therapy with a small number of treatment fractions, such as hypofractionated or stereotactic treatment regimens.
