Modern radiotherapy practices utilize information from various imaging modalities, acquired prior to initiating administration of the therapy, to plan the treatment. The imaging provides information about the location of the target volume and other anatomical features so that a treatment plan can be developed that provides an optimal dose distribution to have the best chance of achieving the intended effect of treatment while minimizing side effects.
However, difficulties arise when trying to administer the radiation, since target volumes/critical structures are constantly moving within the body. For example, in parts of the body moving with respiration, the target volumes/critical structures may change position or shape during the radiation beam delivery throughout any given fraction. Furthermore, a course of therapy may extend over many days, during which the target volume/patient may shrink or grow and/or move. Hence, the exact location of the target volume/critical structures may change between the time of treatment planning imaging and the actual administration of a treatment.