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Showing 2 results for Transmitted Dosimetry
M. Mohammadi, E. Bezak, Volume 2, Issue 4 (3-2005)
Abstract
ABSTRACT Background: for treatment verification in radiation therapy. Several physical characteristics of Scanning Liquid filled Ionization Chamber EPID (SLIC-EPID) including: extra build-up layer, reproducibility and uniformity, and noise level were investigated. Electronic Portal Imaging Devices (EPIDs) have found an outstanding positionMaterials and Methods: equilibrium, 1-30 mm white water materials (RW3) were placed on the EPID cover and the variation of pixel values were investigated. To assess the short term reproducibility, a series of 10 consecutive Electronic Portal Images (EPIs) were acquired. The variation of pixel values were then determined in irradiated field using MATLAB software. For long term reproducibility, the described above experiment was then repeated seven times. To determine the noise level in EPID images, 10 consecutive flood images were acquired. The measurement was repeated after two days during a fortnight. To determine the extra build-up layer to reach the electronicResults: No significant variation was observed between the maximum thickness of build-up layer required for the central axis and peripheral points. For reproducibility measurements, no systematic variation was observed between mean, maximum and minimum acquired pixel values. Both the long-term and short term reproducibility was found to be less than 1%. The noise level was generally less than 1% and this can be referred as an acceptable dose level 5 mm of RW3 material was found to increase the pixel values to the maximum possible..Conclusion: can be used for dosimetry. However, for a particular linac energy and EPID image acquisition mode, the extra build-up layer thickness must be known for the EPID to be used for dosimetric purposes The physical characteristics, measured in this work, suggest that the SLIC-EPID. Iran. J. Radiat. Res., 2005 2 (4): 175-183
M. Mohammadi, E. Bezak, Volume 3, Issue 1 (6-2005)
Abstract
ABSTRACT Background: 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.
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