Klinik für Strahlenheilkunde

In a linear accelerator, electrons—negatively charged particles—are accelerated using high-frequency radiation. The accelerated electrons can be used directly for radiation therapy. Since electrons do not penetrate very deeply into the tissue, they are used only for superficial tumors, such as skin cancers.

For the irradiation of deeper tumors, photon radiation is necessary. It is generated when accelerated electrons strike a metal—the so-called target, which is usually made of tungsten—and are decelerated. The higher the energy of the photon radiation, the greater its penetration depth into the tissue.

After passing through the target, the radiation beam passes through a primary collimator, which defines the maximum extent of the radiation field, and a filter, which ensures the homogeneity of the beam. For irradiation, however, the individual shape of the radiation field must be precisely adapted to the contour of the tumor. Field shaping, i.e., the shaping of the desired field, is achieved using a multi-leaf collimator (also known as a multi-lamella collimator). The radiation head of the irradiation device contains up to 160 lamellae, each one to ten millimeters wide, which capture the tumor contour during treatment planning. The position data for each lamella is stored and transmitted to the accelerator for each irradiation session, ensuring that it is adjusted with millimeter precision to the patient’s individual anatomy.

Basic imaging for radiation therapy planning is performed using a CT scanner. The patient positioning table and the positioning and immobilization devices used for each patient are identical to those used with the linear accelerator. As part of the preparation process, once the patient has been positioned, several cross-sectional images of the area of the body to be irradiated are acquired. The physician then marks the region to be treated on these cross-sectional images.