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

 ABSTRACTBackground: Although Electronic Portal imaging Devices (EPIDs) 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 EPID, SLIC-EPID, and the variation of transmitted dose with the shift of inhomogeneity inside of phantom was also evaluated were investigated.Materials and Methods: The SLIC-EPID pixel values were converted to the dose values using ionization chamber calibration and KODAK Extended Dose Range films (EDR2 films). The variation of EPID 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 SLIC-EPID, a 0.5 cm of thickness can be detected with a good reliability. The relative error of EPID 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 inplane and crossplane 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 SLIC-EPID can be used as a reliable two-dimensional dosimeter.