EPID in vivo Dosimetry

Introduction: The most modern radiotherapy devices are equipped with an Electronic Portal Imaging Device (EPID) system which is located on opposite side of the machine’s head. EPID system is often used to setting up the position verification during or between radiotherapy sessions.
Material and Methods: Various types of dosimeters have been used to setting up position verification over the years. According to progress in rapid and high-resolution imaging techniques, EPID as a special type of in vivo dosimetry is used nowadays. Amorphous silicon (A-si) which is applied in radiotherapy centers is the most extensive type of EPID. This EPID is a two-dimensional matrix of pixels. Each pixel acts as a transistor and is connected to a photodiode. Photodiode generates electrical current. An x-ray converter is a copper plate connected to a phosphor plate is needed at the top of the matrix. One of the disadvantages of the former dosimeters was the lack of display of 2D and 3D dose distribution. This property exists in EPIDs. One of the most important features of the dose distribution in IMRT is the focus of high concentration of dose to the target volume and protection of organs at risk (OAR) volumes. This feature provides the delivery of higher doses to target than Three- dimensional conformal radiotherapy (3DCRT). Therefore, small deviation of the designed set-up position in this technique results lower doses to the target volume and significant doses to OARs. 3D display along with proper dosimetric features such as high spatial resolution and digital images produced by EPIDs makes their application in the IMRT technique very prominent. In many studies, the dosimetric results of the EPID have been validated, and several studies have revealed that Utilization of Monte Carlo simulation. Algorithms are proposed to reconstruct the amount of received doses from the EPID images data, based on the EPID pixels in each center. Multiple factors and kernels in algorithms designing are applied for effects such as scatter radiations and off-axis ratios. In some studies, verification of dose in the EPID surface and back it to a certain depth of a definite patient have been done to reconstruct the patient received dose. A number of studies have found that, dose prediction is also done by adding EPID to planning CT scan.
Results: Recent clinical evidence suggests that EPID images should be taken more than once because of patient-positioning errors in each session and patient morphological changes during the course of treatment.
Conclusion: EPIDs can be used in dosimetric purposes. Electronic portal imaging systems are a special type of in vivo dosimeter, Even these imaging systems can provide 2D and 3D dosimetric information from the actual or patient received dose in addition to proper dosimetric properties.