Intensity-modulated radiotherapy - technology and clinical applications

Intensity-modulated radiotherapy (IMRT) is one of the most important developments in radiooncology of the last years. As an extension of 3D conformal radiotherapy, it provides the possibility of delivering a high radiation dose to the tumor tissue, protecting radiosensible critical organs nearby or even surrounded by the target. This is realized by the overlaying of beams from different directions, which are not homogeneous like in conventional radiotherapy, but inhomogeneous themselves. The sum of the beams from all directions forms finally the desired complex-shaped dose distribution. Because choosing the right intensity modulation of each beam is a nontrivial problem, the best treatment plan could only hardly be found in a trial-and-error process, but has to be computer- optimized. Therefore, IMRT has always to be spoken together with the term of 'inverse planning'. This describes the approach of telling the computerized planning system the desired radiation doses to the target and the allowed maximum doses to organs at risk (OAR), then letting the system compute the optimal modulated beams, called 'fluence profiles', by an iterative algorithm. Because the ideal dose distribution, giving 100% dose to the tumor and 0% dose to the healthy tissue, is never realizable, inverse planning has to be understood as an optimization problem. The present article shows the technical bases of IMRT and inverse planning and then describes important clinical applications.