Development of calibration procedure of an electronic portal imaging device a-Si1000 used for IMRT and VMAT

摘要

In recent years Electronic Portal Imaging Devices (EPIDs) have become indispensable tools for the patient verification set-up in radiotherapy. While EPIDs are primarily used for the assessment of patient positioning and quality control of the accelerator parameters, they also represent an attractive method for the dose verification. Therefore, for their use in dosimetry, it is necessary to investigate their physical characteristics and perform their calibration. The aim of the present study is the development of a calibration procedure and investigation of the stability and the linearity of the signal of an amorphous silicon detector, the a-Si 1000 for Varian systems. The calibration is carried out on the central axis of the beam in the center of the image. To test the signal stability, the detector is placed in the central axis, at a fixed distance of the source with an open field 10 × 10 cm~2 and irradiated at regular time intervals, using the 6MV photon beam. The linearity of the signal with the dose and with the dose rate is investigated by varying the dose between 1 cGy to 900 cGy for each dose rate 100, 200, 300, 400 and 500 UM/min. The EPID is calibrated in terms of absolute dose in two steps, the first one is the image calibration, and the second one is the dose calibration. The image calibration is carried out to correct the raw image involving the measurement of a Dark Field (DF) image acquired with no radiation to measure electronic noise, and a Flood Field (FF) image acquired with a maximum open field to determine pixel sensitivities. The dose calibration is carried out with the 0.6 cm~3 TM 30013 Farmer ionization chamber (PTW Freiburg Germany) placed in the water phantom at the depth d_(max)=1.5 cm. Once calibrate, the dose measured at the center of the detector placed in the center axis of the beam is confronted with the dose measured with the 0.3 cm~3 TM 31013 Semiflex ionization chamber (PTW Freiburg Germany) placed in the center of a cylindrical phantom dedicated for the verification of treatment plans in IMRT and VMAT techniques. Five treatment plans are verified. The stability results reveal a deviation about 0.37% after the first 4 minutes of irradiation and about 0.10% after 30 minutes of irradiation. The linearity investigation exhibited a correlation factor that exceeds 0.99 which allows us to conclude that the detector has a good linearity of the signal with the dose and with the dose rate. The comparison between the dose measured in the central of the detector and that measured using the Semiflex ionization chamber placed in the center of a cylindrical phantom show that the deviation doesn't exceed 3%.