Portal-image IMRT verification

ABSTRACT

The aim of the present work was to develop and utilizing the use of an amorphous silicon electronic portal imaging device [EPID] in IMRT dosimetric verification. Neither pre-irradiation nor extra build-up materials was needed for EPID dosimetry applications. Accurate absolute dosimetry [output] obtained using the EPID up to 250 cGy with mean deviation of 0.39% and 0.62% while the maximum observed deviation was 0.7% and 1.9% at 250 MUs for 6 and IS MV photon energy respectively. Beam-by-beam fluence profiles obtained from portal images were used in air [i.e. without phantom presence] The EPID estimated the relative dose up to 1.5% accuracy. The in-air absolute dose [i.e. number of MUs] of an arbitrary clinical breast [aperture-based] IMRT test fields were extracted, the difference rose to 1.7% in the most severe tested field. One disadvantage of beam-by-beam verification is that the cumulative effect of dose errors from all beams is not quantified, however, it allows the potential origin of dose errors to be isolated more easily. The EPID could estimate the dose at each segment with average accuracy of around 1% for central axis positions and up to 1.7% for off-axis positions in the tested fields. The absolute dose verification plus the fluence map verification of IMRT fields may represent a sufficient procedure to examine the step-and shoot-IMRT treatment. Multi-leaf-collimator [MLC] related QA also tested using EPED. The mean difference between EPID effective penumbra results and both ion chamber and film measurements was 0.06 and 0.01 cm respectively. These results could justify the use of EPID in dosimetric applications including aperture-based IMRT verification, and quality control programs