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KMID : 1164220040160010001
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Journal of Korean Society for Radiotherapeutic Technology
2004 Volume.16 No. 1 p.1 ~ p.9
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Study on the beam properties of small field sizes
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Cho Jeong-Hee
Lee Sang-Kyu
Ahn Seung-Kwon
Park Jae-Il
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Abstract
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Purpose : The aim of this study is to investigate the properties of small field size and to measure the penumbra and central axis depth dose varying to the jaw setting and off axis distance for indicate this data to small field sizes radiation therapy.
Material and methods : The percentage depth dose, beam profile and central axis output dose was measured by farmer type ion chamber and pinpoint chamber using Primart linac with 6MV energy. Beam quality and penumbra variations according to the central axis shift, from center to every 2cm outside increment, and field size, from to was investigated and compared with that of the standard geometrical condition’s results
Results : The differences of measured values between two ion chamber was about at 10cm depth with field sizes but as field size increased this differences was diminished gradually. Measured data from various off axis distance with the different asymmetric collimations are not changed significantly but as size decreased the dose variation was increased and at field size dose difference among off axis distance was as much as , and as shallower the measured depth the central axis dose variations among the OAD was increased, penumbra was not changed noticeably depending on off axis distance but the percentage of penumbra from its initial field sizes was strongly dependant on field sizes and penumbra occupation rates of its own field sizes ranging from at to at field size.
Conclusion : For imrt treatment, there are several numbers of different gentry angles with beams of nonuniform fluences are required and several complex factors involved. Among them the characteristics of beam output varying to the geometrical setting and design of collimators are of important to attaining a good treatment results. As mentioned in results the differences of measured values are changed significantly depends on ion chamber volume, depths and field size. For providing quality radiation treatment, especially at small field size, those factor’s should have considering deliberately.
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KEYWORD
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small field size, off axis distance, penumbra, beam profile
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