In-vivo dosimetry with a semiconductive detector-panel for IMRT and VMAT radiotherapy techniques: At the LKH Feldkirch, the curative or palliative treatment of malign tumors is carried out with photon radiation. These photons are produced by a linear accelerator (Elekta, Stockholm, Sweden) and the tumor within the patient is irradiated from outside (external radiotherapy). In order to achieve local tumour control and spare healthy tissue, a treatment plan needs to be calculated for each patient individually. Previous to patient treatment, the calculated plan has to be verified. There are a number of different possibilities in order to measure the dose of a treatment plan. An ionization chamber (PTW 0.125cm3) is used as a reference. In order to analyze dose planes, measurements are performed with radiochromic films (EBT3 film, Ashland, Covington, USA). For in vivo dosimetry, the semiconductor detector (electronic portal imaging device, EPID) is placed behind the patient and receives the radiation transmitted through the patient or phantom. This results in a transmission image of the irradiated plan, similar to X-ray images. This image is then used for back projecting the dose within the patient or the phantom at a certain point. The dosimetric back projection is performed by EPIgray (Dosisoft, Cachan, France). In order to use the EPID panel and the respective software for clinical application, acceptance tests including several verification measurements are required. This master thesis compares and analyzes EPIgray results to previously mentioned measuring devices. The dosimetric comparison and juxtaposition is carried out for conventional fields (quadratic, wedge, asymmetric and irregular radiation fields), as well as IMRT and VMAT plans. The conventional fields are irradiated onto water equivalent material and measured with the ionization chamber, EBT3 film and EPID panel. Additionally, the EPIgray system is tested with an inhomogeneous lung equivalent. Six actual plans were calculated for IMRT and VMAT plans, respectively. Measurements were performed with the ionization chamber and the EPID panel using the body phantom (IBA dosimetry, Schwarzenbruck, Germany). The evaluation of EPID dosimetry shows excellent agreement with the ionization chamber measurement for conventional fields and real intensity modulated treatment plans. The deviation of the conventional fields was 1.21% 0.25%, concerning the irregular and asymmetric fields the deviation was 0.96% 0.15% and the deviation of the wedge-shaped fields was 1.64% 0.73%. Concerning the 12 IMRT and VMAT plans, the deviation between ionization chamber measurements and the back projection of EPIgray was -2.38% 1.54% and 0.04% 2.56%.