Abstract
Radiotherapy is generally a local treatment. The radiation oncologist marks image-based the area to be irradiated (targetvolume) as well as areas which should be spared (organs at risk). Three important volumes differ in marking of the targetvolume: gross tumor volume, clinical target volume and planning target volume. Gross tumor volume (GTV) means thevolume where tumor is traceable in CT, MRI, ultrasonic sound, PET or clinical inspection and palpation. Clinical targetvolume (CTV) involves the GTV and the microscopic tumor expansion: peritumoral regions, lymph nodes, perineural orperivascular tumor infiltration, etc. The Planning Target Volume includes the CTV as well as the possible mobility of thetumor and the imprecision in positioning of patients at the linear accelerator. With an exact positioning of the patient at theirradiation device, PTV margin can be considerably reduced. That leads to a reduction of exposure in the healthy tissue andto a dose weighting in the tumor area.
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