الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and image-guided radiotherapy (IGRT) are the most advanced techniques in radiotherapy, which use irregular fields–using multileaf collimators (MLC) in a linear accelerator. The accuracy of these techniques depends on the dosimetric characteristics of the multileaf collimators. The dosimetric characteristics include dose rates, percentage depth doses, surface dose, dose in the build-up region, penumbra, and width of 50% dose levels. There is an option for optimizing the jaws to the irregular MLC field to reduce the scattered radiation and intra- and inter-leaf radiation leakage beyond the field. In this study, 80 leaf MLC system has been taken to compare and differentiate their characteristics with 6 MV and 10 MV photon beams. The MLC system in Varian linear accelerator is used as a tertiary collimator, that is, below the collimator jaws. When both ‘X’ and ‘Y’ jaws are optimized to the MLC field, the surface dose decreases by reducing the intra- and inter-leaf leakage radiation. The dosimetric characteristics were measured by three field-defining methods; (1) ”Jaw only”, (2) ”MLC only”, and (3) ”MLC + Jaw”,. It was found that its characteristics were quite similar to those of the standard collimator (jaws) system and penumbra. Dose rate for 6-MV and 10-MV photon beams was higher for ‘MLC only’ field than that for the other two field-defining methods. The PDD comparison shows that the surface dose and dose in the build- up region were more for ‘MLC only’ fields. Beam profile analysis shows that the flatness and symmetry for both the systems were within 2.6%; the ‘width of 50% dose level’ and penumbra were slightly higher for ‘MLC only’ fields in both energies.The results of this study suggest that standard collimator jaws should be optimized to the irregular MLC field (i.e., MLC+Jaw) to minimize the surface dose, dose rate, penumbra, and dose in the build-up region.