The Study Of Patient Dose Verification Using The Electronic Portal Imaging Device

The Electronic Portal Imaging Device is a new kind of imaging devices which obtain images from the output direction of beam using the detector matrix. And it has been routinely used for patient set-up and field verification. The images are also given the dose distribution at the plane of EPID, so they can be used for dose verification during treatment. Basically, there are two approaches to EPID dosimetry. In the"forward approach", the measured portal image is compared with a predicted dose or photon fluence at the plane of the EPID, which is calculated within the treatment planning system (TPS) or an independent algorithm. In the"backward approach", the portal images are used to reconstruct the dose within the patient or phantom.Here by using the latter back-projection algorithm, the actual dose verification inside a phantom or patient would be done from the portal images. The dose-response characteristics of an amorphous silicon-based electronic portal imaging device (a-Si EPID) under the clinical conditions would be investigated and corrected. Two dimensional dose distributions inside the midplane of a phantom or patient are reconstructed from the portal images and CT images. And those would be compared with the calculated results from TPS. By measuring an appropriate build-up layer, the correction of the a-Si EPID dose response could be achieved. By developing the back-projection algorithm, two dimensional dose distributions inside a phantom or patient are reconstructed from portal images. The MATLAB language would be used for the software development about the back-projection dose algorithm. The algorithm model includes the primary and scattere dose at the position of the EPID and within the phantom. The back-projection dose model and software would be verified by using the comparison between the calculated and measured dose in the phantom-chamber system and patient plan. Under the clinical conditions, the variation in the a-Si EPID response could be reduced within±1% by the corresponding correction. In the model using the scatter correction kernels, the absolute dose in the central axis of the field was under 2%, and the off-axial profiles of one field inside the midplane of a phantom were kept well in agreement. The clinical results showed that the comparison between the reconstructed and calculated doses was satifactory.The a-Si EPID could be used for dose verification of patient treatment planning by correcting the portal images. The developed back-projection model and the application software based on MATLAB in the study have showed the good practicability during the measurement within the phantom and patient plan. They would meet the accuracy of clinical requirement.