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KMID : 0362919960140010077
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Journal of the Korean Society for Therapeutic Radiology and Oncology
1996 Volume.14 No. 1 p.77 ~ p.85
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Dose Distribution and Characterization for Radiation Fields of Multileaf Collimator System
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Chu Sung-Sil
Kim Gwi-Eon
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Abstract
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Purpose: Multileaf collimator(MLC) is very suitable tool for conformal radiotherapy and commissioning measurements for a multileaf collimator installed on a dual energy accelerator with 6 and 10MV photons are required. For modeling the collimator with treament planning software, detailed dosimetric characterization of the multileaf collimator including the penumbra width, leaf transmission between leaf leakage andlocalization of the leaf ends and sidesis an essential requirement.
Methods and Materials: Measurement of characteristic data of the MLC with 26 pair block leaves installed on CLINAC 2100C linear accelerator was performed. Low sensitive radiographic film(X-omatV) was used for the penumbra measurement and separate experiments using radiographic film and thermoluminescent dosimeters were performed to verify the dose distribution, Measured films were analized with a photodensitometer of WP700i scanner.
Results: For 6 & 10 MV x-ray energies, approximately 2.0% of photons incident on the multileaf collimator were transmitted and an additional 0.5% leakage occurs between the leaves. Localizing the physical end of the leaves showed less than 1mm deviation from the 50% decrement line and this difference is attributed to the curved shaped end on the leaves. One side of a single leaf corresponded to the 50% decrement line, but the opposite face was aligned with a lower value. this difference is due to the tongue and groove used to decrease between leaf leakage.Alignment of the leaves to form a straight edge resulted larger penumbra at far position from isocenter as compare with divergent alloy blocks. When the MLC edge is stepped by sloping field, the isodose lines follow the leaf pattern and produce scalloping isodose curves in tissue. The effective penumbra by 45 degree stepped MLC is about 10mm at 10cm depth for 6MV x-ray. The difference of effective penumbra in deep tissue between MLC and divergent alloy blocks is small (5mm)
Conclusion: Using the characteristic data of MLC, the MLC has the clinlical acceptability and suitability for 3-D conformal radiotherapy except small field size.
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KEYWORD
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Multileaf collimator, Penumbra, Field shaping, Conformal therapy
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