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KMID : 1164220040160020033
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Journal of Korean Society for Radiotherapeutic Technology
2004 Volume.16 No. 2 p.33 ~ p.41
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A Comparison of coincidence between the Light field & the Radiation field using film and BIS
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Seok Jin-Yong
Jung Yun-Ju
Choi Byeong-Don
Park Jin-Hong
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Abstract
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Purpose : Film has been the primary tool in coincidence testing between the light field and the radiation field, which constitutes the quality assurance list of a linear accelerator. But there is a great chance of errors being different among the observer when using film. Thus this study set out to use the BIS(Beam Image System) in addition to film in comparing and evaluating coincidence results between the two fields and in searching for the improvement measures.
Materials & Methods : Photon beam of 6 and 15MV was exposed to film and the BIS using a linear accelerator. The light and radiation fields were each . The gantry angle was when using film and when using the BIS. The devices adopted to test coincidence between the two fields were a ruler and film scanner when using film. With the BIS, the width of the scanned light and radiation fields was measured for errors with setting the X and Y axis.
Results : The visual measurements of the observer with film resulted that the radiation field was bigger than the light field and that their maximum error was 1.9mm. The results were the same with the measurements using the film scanner except for the average error, which was less than 1.9mm. On the contrary, the measurements using the BIS showed that the light field was bigger than the radiation field at the gantry angle of . The maximum error was 0.96mm, and the error range was $<{\pm}2mm$ both in the X and Y axis. The average error of , Y was the smallest in the order of the visual film measurements, film scanner measurements, and BIS measurements
Conclusion : This requires a careful measurement for accurate quality assurance since errors are much different according to each observer that tests coincidence between visual fields with film. And an observer needs to use another image device or develop a measuring device of his own if it seems necessary for accurate measurements.
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