• The Journal of Korean Society for Radiation Therapy
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• v.29 no.2
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• pp.83-91
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• 2017

Objectives: The purpose of this study was to evaluate the error of the leaf position accuracy of the MLC due to the gravity effect according to the gantry angle by using picket fence test using EPID and GafChromic EBT3 film. Materials and Methods: A 5 cm solid phantom was placed on the table and the SAD was set to 100 cm. The EBT3 film was placed exactly over the solid phantom and covered a 1.5 cm solid phantom and the picket fence test was performed. The EPID was measured under the same conditions as the EBT3 film at SID 100 cm. The gantry angles were measured at $0^{\circ}$, $90^{\circ}$, $180^{\circ}$ and $270^{\circ}$ in order to evaluate the position of the MLC according to the gantry angle. For the geometric evaluation of the MLC, the leaf position accuracy of the MLC was analyzed using the analysis program. Results: In case of EPID, when the gantry angle was changed to $0^{\circ}$, $90^{\circ}$, $180^{\circ}$, $270^{\circ}$, the difference of the position errors of the leaves was 0.18 mm, 0.31 mm, 0.20 mm, 0.26 mm on the average and the maximum values of the errors were respectively 0.44 mm, 0.54 mm, 0.34 mm, 0.44 mm. In case of EBT3 film, when the gantry angle was changed to $0^{\circ}$, $90^{\circ}$, $180^{\circ}$, $270^{\circ}$, the difference of the position errors of the leaves was 0.19 mm, 0.21 mm, 0.19 mm, 0.31 mm on the average and the maximum values of the errors were respectively 0.35 mm, 0.45 mm, 0.36 mm, 0.48 mm. Conclusion: In this study, we analyzed the position error of the leaf of the MLC according to the gantry angle, and confirmed the position error of the leaf by gravity effect. As a result of comparing the leaf position accuracy using EPID and EBT3 film according to the variation of gantry angle, a larger error occurred in the error analysis method using EPID than that of EBT3 film. Therefore, in the case of IMRT based on MLC, as well as verification of accurate dosimetry should be conducted, it is considered that the quality control and verification for the precise operation of the MLC will be needed. and it is necessary to compare and verify the method of analysis.

• Progress in Medical Physics
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• v.35 no.2
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• pp.45-51
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• 2024

Purpose: Accurate operation of the multi-leaf collimator (MLC), a key technology in intensity modulated radiation therapy (IMRT), is essential for safe and optimal radiation treatment. The Halcyon^TM linear accelerator has a collimator with low leakage and radiation transmission, making it suitable for IMRT. The limitations of the existing Halcyon^TM MLC quality assurance (QA) method were supplemented with a mathematical method, and the results were analyzed. Methods: Electric portal imaging device (EPID) images obtained by performing the MLC QA plan on the Halcyon^TM was analyzed using Python. The picket fence tests were performed and compared using the maximum pixel value and mathematical methods. Dose rate, gantry speed, and leaf speed variation plan were performed for dose transmission comparison. Results: For the maximum pixel value, the minimum distance between leaf junctions was 13.86 mm, and the maximum was 16.06 mm. However, for the mathematical method, the minimum and maximum were 14.54 mm and 15.68 mm, respectively. This suggests that setting the peak value to the highest value may cause an error in interpretation due to the limitations of the pixels of the EPID image. Performing QA on the remaining items confirmed that the measured values were within 3% of tolerance. Conclusions: The presented analysis method applied to the MLC QA can derive more reasonable and valid values than existing methods, which will help with MLC monitoring by reducing errors in excessive interpretation.

• Progress in Medical Physics
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• v.19 no.3
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• pp.164-171
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• 2008

One of the most important task in commissioning intensity modulated radiotherapy (IMRT) into a clinic is the characterization of dosimetry performance under small monitor unit delivery conditions. In this study, method of evaluating dose monitor linearity, beam flatness and symmetry, and MLC positioning accuracy using a diode array is investigated. Siemens Primus linear accelerator (LA) with 6 and 10 MV x-rays was used to deliver radiation and the characteristics were measured using a multi array diodes. Monitor unit stabilities were measured for both x-ray energies. The dose linearity errors for the 6 MV x-ray were 2.1, 3.4, 6.9, 8.6, and 15.4 % when 20 MU, 10 MU, 5 MU, 4 MU, and 2 MU was delivered, respectively. Greater errors were observed for 10 MV x-rays with a maximum of 22% when 2 MU was delivered. These errors were corrected by adjusting D1_C0 values and reduced to less than 2% in all cases. The beam flatness and symmetry were appropriate without any correction. The picket fence test performed using diode array and film measurement showed similar results. The use of diode array is a convenient method in characterizing beam stability, symmetry and flatness, and positioning accuracy of MLC for IMRT commissioning. In addition, adjustment of D1-C0 value must be performed when a Siemens LA is used for IMRT because factory value usually gives unacceptable beam stability error when the MU/segment is smaller than 20.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.2
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• pp.185-198
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• 2016

In this work, experimental errors from an impact hammer test were investigated to develop a statistic-based technique for updating a finite element model. Digital signal processing was analyzed by using theoretical models and experiments when errors occurred during the experimental procedure. First, the duration time and peak level of the excitation signal, the stiffness and position of elastic springs connecting the specimen as well as the support, position and mass of the accelerometer were considered as error factors during the experiment. Then the picket fence effect, leakage, and exponential window function were considered as candidate error factors during the digital signal processing. Finally, methods to reduce errors are suggested.