Intensity-Modulated Radiation Therapy: Quality Assurance with the Mutual Information Index (MI)

Radiation therapy is an important method of treating neoplastic diseases with ionizing radiation. For this treatment electromagnetic or corpuscular (electrons, protons, neutrons) radiation is used which causes ionization and excitation in the energy absorbing tissue [1]. The conventional radiation therapy applies beams with a homogeneous dose distribution or uses wedges to optimize the dose to the cancerous volume and prevents high doses in organs of risk (e.g. rectum, eye). The individual treatment plan is developed by the physicist who sets the beams in a treatment planning system which calculates the dose distribution. With the objective of ideal adjustment to the cancerous volume it is advantageous to use the ``inverse treatment planning'' and the intensity-modulated radiation therapy (IMRT) especially for concave volumes. Therefore, the dose distribution is designated by the physicist and the individual treatment plan is developed including the number and direction of beams and the multi-leaf collimators' position. Due to the fact that the leaf collimator conforms the dose to the tumor volume IMRT protects the adjacent tissue as good as possible. Before an application of IMRT starts a quality assurance is important. It is especially necessary to check the accuracy of the system including the data of the planning software.