الفهرس 
introductionOnly 14 pages are availabe for public view
 |  | from | 127 |  |  |
| | | from | 127 | | |
Abstract
The success of radiation therapy depends critically on the accuracy of patient alignment in treatment position day after day. Electronic portal imaging devices (EPIDs) are widely used to monitor patient position during daily radiotherapy sessions. Several on-line and off-line verification protocols have been developed for this purpose. Therefore, patient positioning is verified before treatment delivery. EPID enables acquiring an image of the exit radiation from the patient, immediately before or during the treatment delivery. Portal images are usually taken to verify patient set-up and positioning prior to radiation therapy treatment. Historically these images were taken using radiographic films. In fact, EPID is better than film imaging with respect to acquisition speed and the potential to use computer aided analysis. Because of these reasons, electronic portal imaging has become an important tool in radiation therapy. Recently the role of EPIDs has been expanded beyond patient imaging to become a useful tool for radiotherapy dosimetry. EPID was also found to be objective , efficient and feasible for performing some linac quality assurance (QA) tests .The ability of the Amorphous Silicon (aSi) EPID to acquire a large two-dimensional (2D) array of digitized x-ray data in real time is extremely attractive for dosimetric measurements. Therefore, this study was carried out at Assiut Military Center of Radiotherapy to investigates the potential use of EPID in radiotherapy. The EPID’s performance for linearity with MU and dose rate was verified and it was found to be proportional over the entire measured range. The EPID response increases with the increasing of dose and dose rate. During two weeks, short term repeatability was found to be excellent. The average percentage for relative error was found to be 1.4 %. Results of reproducibility and uniformity confirmed the stability of short term as well as on long term scales of EPID. The minimum long-term uniformity obtained from mean pixel values was 0.33 % for 10×10 cm2and minimum long-term uniformity observed was 0.29% for 20×20 cm². The flatness results were good, the maximum flatness value was 2.7 % for field size 10×10 cm² and 1.8 % for field size 18×18 cm² which are within tolerance (less than 3%). The standard deviation for The flatness of the EPID was found to be 0.82 % for field size 10×10 cm² and for field size 18×18 cm², the standard deviation was found to be 0.6 %. Another use of EPID for coincidence of light field versus radiation field was also carried out and the relative error was found to be within 1%. The radiation isocenter of the linac was verified. This is done with collimator rotation about a sphere of 1 mm radius. We found that the radius is 0.2 mm which is within tolerance ≤1 mm. The results show that the aS500 EPID has the potential to be used as a relative dosimeter ; making it a very easy and efficient tool for daily QA. The profiles acquired using EPID deviated in shape and magnitude by up to 16% from the ion chamber profiles. Some potential applications of EPID to perform QA of linac beam properties, its ability to perform optical and mechanical linac QA have been explored. The EPID’s capability to give constant output, flatness , wedge profile and wedge factors with high level of accuracy and reproducibility was demonstrated. The use of EPID for linac QA could be simplified by improving the available software analysis tools thus making it more efficient. QA tests that were done are considered as an indirect measure of dosimetric properties of the linac. QA tests could be made more efficiently, than by current methods, using an electronic portal imaging device (EPID). By studying the flatness and field size response. EPID response was compared with chamber dose with the increasing of field size. Flatness measured by using the two opposite equidistant points from the central axis.