Figure 2.

Concept of inverse planning of intensity modulated radiotherapy (IMRT). In the past, physicians performed target contouring and determined numbers and angles of the beams, and then the machine performed radiotherapy (“forward planning”). The core of IMRT technique is the socalled “inverse planning,” in which the physician sets ideal dose of targets and normal organs, and relative importance for the tumor and surrounding organs in advance. Then, the machine computes and determines numbers and angles of the beams.