The battle against respiration-induced organ motion in external beam radiotherapy.

The latter 2 decades of the last century have witnessed significant improvements in external beam radiotherapy (EBRT), moved primarily by the advances in imaging modalities and computer-based treatment planning. These advancements lead to introducing the addition of a fourth-dimension, time, to the three-dimensional geometry in EBRT. The new era in EBRT presents challenges and opportunities to compensate for the effect of respiratory-induced target motion and improve treatment output.