Portal dose measurements with electronic portal imaging devices

Summary form only given. Measuring portal dose with an electronic portal imaging device in external beam radiotherapy can be used to perform routine dosimetric quality control checks on linear accelerators and to verify treatments (in vivo dosimetry). For both the fluoroscopic CCD camera based systems and the systems based on an array of liquid-filled ionization chambers accurate methods have been developed to derive portal dose images (PDIs) from measured portal images. The measured portal dose (i) can be compared with a corresponding predicted PDI (using the patient's planning CT) or (ii) used to derive the dose in a point or in the midplane of the patient to be compared with the dose calculated by the treatment planning system. The developed methods allow an independent check of the absolute dose delivered to the patient. First clinical evaluation showed that deviations between the actual and planned dose delivery could be detected, which were related to changes between the actual patient anatomy and the anatomy according to the planning CT. Another application is verification of dynamic multileaf collimation. Camera based systems are especially suitable for this task, due to their capability to measure simultaneously in all points of the field and their high data acquisition rate.