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Fractionation in the context of radiation therapy is used to describe the division of radiation dose into several smaller doses. Typically fractionation is used in reference to photon therapy; however, more recently other modalities of radiation have involved the use of fractionation. Historically, radiation was given in one large bolus, but this process has several disadvantages which preclude its use today (citation). A large dose of radiation has the requirement of being given accurately, as otherwise major normal tissue toxicity can occur, as well as undertreatment of the tumor. In this way, fractionation of the radiation dose can provide more lee-way regarding accuracy, as the fractions of radiation are typically separated by time which allows normal tissue to repair itself, and lower doses are given each fraction. The total dose to the tumor will be the same as it would for a large single dose; it is the total dose that is important in outcomes (citation).

Hyperfractionation and Hypofractionation refer to the relative number of fractions with respect to a single day. In hyperfractionation, multiple doses are given in a single day, though typically the dose number is two per day. In hypofractionation, the number of fractions are sought to be minimized; again, this can pose a problem if the doses are not given accurately. However, with the advent of IMRT and IGRT, the localization of the tumor and accurate dose can be given. This is the basis of SBRT and its related modalities.