Motion effects in (intensity modulated) radiation therapy: a review

During a course of fractionated radiation therapy and between the fractions the tissues of the human body may move relative to some reference location in which the radiation therapy was planned. This has been known for over a century and simple 'coping mechanisms' (margins) have been used to approximately compensate. Since the introduction of highly accurate conformal radiation therapy and intensity-modulated radiation therapy (IMRT) attention has focused strongly in the last few years on understanding and compensating more appropriately for these motions. Thus, unlike most of the reviews in this special 50th anniversary issue which look back over decades of development, this one looks back at most within just the past decade and reviews the current situation. There is still much more work to be done and many of the techniques reviewed are themselves not yet implemented widely in the clinic.