Dosimetry with a fluoroscopic electronic portal imaging device

echniques for dosimetric verification of radiotherapy treatments using a CCD camera based fluoroscopic electronic portal imaging device (EPID) are described. The dosimetric characteristics of the EPID were investigated and a method was developed to derive portal dose images (PDIs) from measured EPID images. EPID and ionization chamber measurements agreed to within 1% (1 s). Subsequently, an algorithm was developed to predict these PDIs using the planning CT data of the patient and the irradiation geometry as determined in the treatment planning process. Furthermore, a method was developed to derive the on-axis patient dose from an EPID measurement, which was then compared with the intended dose. The method allows the discrimination of errors that are due to changes in patient anatomy and errors due to a deviating cGy/MU-value. For 115 prostate cancer patients the differences between the average on-axis measured portal dose and the predicted portal dose for the three open beams were small: -0.3±2.3% (1 s). However, large (up to 15%) off-axis differences between measured and predicted PDIs were found, which were caused by frequently occurring gas pockets inside the rectum of the patients during treatment or during acquisition of the planning CT scan. The detected gas pockets did sometimes extend into the tumor volume area as outlined in the CT scan, implying internal organ motion. Finally, methods were developed for pretreatment verification of intensity modulated fields produced with compensators or dynamic multileaf collimation (DMLC). EPID measurements of dose profiles generated with DMLC agreed within 2% (1 s) with predictions and ionization chamber measurements.