The evaluation of minimum detectable phantom thickness change using a scanning liquid filled ion chamber EPID dose response

Although electronic portal imaging devices have originally developed for positioning verification, they can also be used for dosimetric purposes. In the current work, the dose response of minimum detectable thickness of a Scanning liquid filled ion chamber electronic portal imaging devices, Salic-Electronic portal imaging devices, and the variation of transmitted dose with the shift of inhomogeneity inside of phantom were investigated. Materials and methods: the Salic-electronic portal imaging devices pixel values were converted to the dose values using ionization chamber calibration and Kodak extended dose range films. The variation of electronic portal imaging devices dose values with phantom thickness was investigated. In order to find the rate of dose deposited per centimetre of phantom, several reference points were defined and the variation of dose delivered to the points in the vicinity of reference points was investigated. Two cm thick foam layer, as air gap, was shifted in the beam direction to evaluate the variation of transmitted dose with the shift of inhomogeneity position inside of phantom. Results: an exponential decrease of the transmitted dose values was observed with the increase of the thickness of attenuators. The maximum and minimum rate of dose deposited per unit of phantom thickness was found to be 5.45% /cm and 3.78% /cm, respectively. Due to the reproducibility and noise level of Salic-electronic portal imaging devices, a 0.5 cm of thickness can be detected with a good reliability. The relative error of Electronic portal imaging devices dose values increases with an increase of phantom thickness for both data sets. The relative error did not exceed 0.7%. No significant variation in transmitted dose in plane and cross plane profiles were found with the shift of inhomogeneity in the beam direction. Conclusion : The minimum detectable thickness is an important factor to evaluate an imager for dosimetric purposes. The Salic-electronic portal imaging devices can be used as a reliable two-dimensional dosimeter