4-dimensional computed tomography imaging and treatment planning.

In the era of conformal therapy and intensity-modulated therapy, there is an increased desire to raise tumor dose to facilitate improved survival and decrease normal tissue dose to reduce treatment-related complications. Setup accuracy and internal motion limit our ability to reduce margins. Internal motion has both interfraction and intrafraction components, although only the intrafraction component will be addressed here. Intrafraction motion is significant for lung, liver, and pancreatic radiotherapy and to a lesser extent breast and prostate radiotherapy. A method to explicitly account for intrafraction motion is to temporally adjust the treatment beam based on the tumor position with time such that the motion of the radiation beam is synchronized with the tumor motion. This addition of time into the 3-dimensional treatment process is termed 4-dimensional (4D) radiotherapy. Four-dimensional radiotherapy may allow safe clinical target volume-planning target volume margin reduction to achieve the goals of raised tumor dose and decreased normal tissue dose. This article discusses methodology for 4D CT imaging and 4D treatment planning, with some comments on 4D radiation delivery.