The aim of our study was to compare the image and dosimetric quality of two different imaging systems. The first one is a fluoroscopic electronic portal imaging device (first generation), while the second is based on an amorphous silicon flat-panel array (second generation). The parameters describing image quality include spatial resolution [modulation transfer function (MTF)], noise [noise power spectrum (NPS)], and signal-to-noise transfer [detective quantum efficiency (DQE)]. The dosimetric measurements were compared with ionization chamber as well as with film measurements. The response of the flat-panel imager and the fluoroscopic-optical device was determined performing a two-step Monte Carlo simulation. All measurements were performed in a 6 MV linear accelerator photon beam. The resolution (MTF) of the fluoroscopic device (f 1/2 = 0.3 mm(-1)) is larger than of the amorphous silicon based system (f 1/2 = 0.21 mm(-1)), which is due to the missing backscattered photons and the smaller pixel size. The noise measurements (NPS) show the correlation of neighboring pixels of the amorphous silicon electronic portal imaging device, whereas the NPS of the fluoroscopic system is frequency independent. At zero spatial frequency the DQE of the flat-panel imager has a value of 0.008 (0.8%). Due to the minor frequency dependency this device may be almost x-ray quantum limited. Monte Carlo simulations verified these characteristics. For the fluoroscopic imaging system the DQE at low frequencies is about 0.0008 (0.08%) and degrades with higher frequencies. Dose measurements with the flat-panel imager revealed that images can only be directly converted to portal dose images, if scatter can be neglected. Thus objects distant to the detector (e.g., inhomogeneous dose distribution generated by a modificator) can be verified dosimetrically, while objects close to a detector (e.g., a patient) cannot be verified directly and must be scatter corrected prior to verification. This is justified by the response of the flat-panel imaging device revealing a strong dependency at low energies.

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The aim of this study was to investigate if microMOSFETs are suitable for the dosimetry and quality assurance of beta sources. The microMOSFET dosimeters have been tested for their angular dependence in a 6 MeV electron beam. The dose rate dependence was measured with an iridium-192 afterloading source. By varying the source-to-surface distance (SSD) in a 12 MeV electron beam the dose rate dependence in an electron beam was also investigated. To measure a depth dose curve the dose rate at 2, 5, 8 and 12 mm distance from the beta source train axis was determined with the OPTIDOS and the microMOSFET detector. A comparison between the two detector types shows that the microMOSFET is suitable for quality assurance of beta sources for endovascular brachytherapy (EVBT). The homogeneity of the source is checked by measurements at five points (for the 60 mm source at 10, 20, 30, 40 and 50 mm) along the source train. The microMOSFET was then used to evaluate the influence of a common stent type (single layer stainless steel) on the dose distribution in water. The stent led to a dose inhomogeneity of +/-8.5%. Additionally the percentage depth dose curves with and without a stent were compared. The depth dose curves show good agreement which means that the stent does not change the beta spectrum significantly.

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