Planning target volume

 When the patient moves or internal organs change in size and shape during a fraction of treatment or between fractions (intra- or inter-fractionally), the position of the CTV may also move. Therefore, to ensure a homogeneous dose to the CTV throughout a fractionated course of irradiation, margins must be added around the CTV. These allow for physiological organ motion (internal margin) and variations in patient positioning and alignment of treatment beams (set-up margin), creating a geometric planning target volume. The planning target volume (PTV) is used in treatment planning to select appropriate beams to ensure that the prescribed dose is actually delivered to the CTV.




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