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J Korean Soc Ther Radiol. 1997 Mar;15(1):71-78. Korean. Original Article.
Kim MH , Yi BY , Nha SK , Choi EK , Kim JH , Chang H .
Department of Radiation Oncology, Asan Medical Center, College of Medicine, University of Ulsan, Seoul, Korea.
Department of Physics, University of Ulsan, Kyung Nam, Korea.
Abstract

PURPOSE: To collect beam data for dynamic wedge fields using conventional measurement tools without the multi-detector system, such as the linear diode detectors or ionization chambers. MATERIALS AND METHODS: The accelerator CL 2100 C/D has two photon energies of 6MV and 15MV with dynamic wedge angles of 15o, 30o, 45o and 60o. Wedge transmission factors, percentage depth doses(PDD's) and dose profiles were measured. The measurements for wedge transmission factors are performed for field sizes ranging from 4x4cm2 to 20x20cm2 in 1-2cm steps. Various rectangular field sizes are also measured for each photon energy of 6MV and 15MV, with the combination of each dynamic wedge angle of 15o, 30o, 45o and 60o. These factors are compared to the calculated wedge factors using STT(Segmented Treatment Table) value. PDD's are measured with the film and the chamber in water phantom for fixed square field. Converting parameters for film data to chamber data could be obtained from this procedure. The PDD's for dynamic wedged fields could be obtained from film dosimetry by using the converting parameters without using ionization chamber. Dose profiles are obtained from interpolation and STT weighted superposition of data through selected asymmetric static field measurement using ionization chamber. RESULTS: The measured values of wedge transmission factors show good agreement to the calculated values. The wedge factors of rectangular fields for constant Y-field were equal to those of square fields. The differences between open fields' PDDs and those from dynamic fields are insignificant. Dose profiles from superposition method showed acceptable range of accuracy(maximum 2% error) when we compare to those from film dosimetry. CONCLUSION: The results from this superposition method showed that commissionning of dynamic wedge could be done with conventional dosimetric tools such as point detector system and film dosimetry winthin maximum 2% error range of accuracy.

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