The physical characteristics of a SLIC-EPID for transmitted dosimetry

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 position Materials 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 electronic Results: 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