• The Journal of Korean Society for Radiation Therapy
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• v.13 no.1
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• pp.47-50
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• 2001

Purpose : 체내에서 optimal한 dose distribution을 얻기 위해 도입된 IMRT를 시행할 때 사용하는 MLC는 기계특성상 장기적인 check와 교정을 요한다. 또한 MLC는 static하지 않고 dynamic하기 때문에 leaf position의 실제 위치가 매우 중요하다. MLC QA를 위해 QA 소요시간 및 노력이 많이 MLC position check 요구되는 불편이 있어, 삼성서울 병원에서 필름을 이용한 MLC position check device를 제작하여 간편하게 사용하고 그 우수성에 대해 결과를 보고하고자 한다. Materials and Method : MLC position을 check하기위해 사각형 의 device(acryl:$40{\times}40{\times}5cm$)를 제작하였다. position check device의 정확도를 위해 표면에 real scale 표시용으로 Pb marker를 2cm 간격으로 부착하였다. MLC QA film은 마주보는 (opposite) MLC leaf의 간격이 4mm로 set up한 상태에서 double exposure하여 marker를 이용하여 오차를 분석하였다. MLC position check device의 효율성을 평가하기위해 6MV photon으로 10번 반복 실험을 실시하여 MLC leaf의 평균오차를 조사하고 조사에 걸린 시간을 비교하여 유용성을 평가하였다. Results : 실험결과 MLC leaf의 평균오차를 조사하고 조사에 걸린 시간을 비교하여 유용성을 평가하였다. 범위는 1.45mm였고, 최소 오차는 0.34mm로 나타났다. 1회 조사에 걸린 시간은 평균적으로 약 30분 정도 소요되었다. MLC position check device의 효율성을 평가한 결과오차의 반복성은 관찰되지 않았다. Conclusion : MLC leaf position의 정기적인 check와 교정을 위해 본원에서는 주1회 정기 position error를 check하고 있으며, 평균 월1회 교정을 한다. 본 실험을 통해 MLC position check device를 사용하여 leaf 오차가 2mm이하의 만족할 만한 결과를 얻었다. 또한 MLC position check device를 사용하면 짧은 시간에 모든 leaf의 position error를 쉽게 측정할 수 있고 한 장의 필름으로 모든 결과를 평가할 수 있어 경제성 및 업무의 효율성도 높일 수 있었다.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.74-82
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• 2002

In Intensity Modulated Radiotherapy (IMRT), radiation is delivered in a multiple of Multileaf Collimator (MLC) subfields. A subfield with a small leaf-to-leaf opening is highly sensitive to a leaf-positional error. We introduce a method of identifying and rejecting IMRT plans that are highly sensitive to a systematic MLC gap error (sensitivity to possible random leaf-positional errors is not addressed here). There are two sources of a systematic MLC gap error: Centerline Mechanical Offset (CMO) and, in the case of a rounded end MLC, Radiation Field Offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error ΔRFO that results in all leaf-to-leaf gaps that are either too large or too small by (2ㆍΔRFO), whereas assuming that CMO is zero introduces systematic error ΔCMO that results in all gaps that are too large by ΔCMO = CMO. We introduce a concept of the Average Leaf Pair Opening (ALPO) that can be calculated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error of Δ$\chi$ and show that it is inversely proportional to ALPO; explicitly it is equal to, (equation omitted) in which $\varepsilon$ is generally of the order of 1 mm and Δx=2ㆍΔRFO+CMO. This analytic relationship is verified with independent numerical calculations.

• The Journal of Korean Society for Radiation Therapy
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• v.26 no.2
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• pp.305-312
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• 2014

Purpose : The purpose of this study is to analyze the mechanical and leaf speed accuracy of the dynamic multileaf collimator (DMLC) and determine the appropriate period of quality assurance (QA). Materials and Methods : The quality assurance of the DMLC equipped with Millennium 120 leaves has been performed total 92 times from January 2012 to June 2014. The the accuracy of leaf position and isocenter coincidence for MLC were checked using the graph paper and Gafchromic EBT film, respectively. The stability of leaf speed was verified using a test file requiring the leaves to reach maximum leaf speed during the gantry rotation. At the end of every leaf speed QA, dynamic dynalog files created by MLC controller were analyzed using dynalog file viewer software. This file concludes the information about the planned versus actual position for all leaves and provides error RMS (root-mean square) for individual leaf deviations and error histogram for all leaf deviations. In this study, the data obtained from the leaf speed QA were used to screen the performance degradation of leaf speed and determine the need for motor replacement. Results : The leaf position accuracy and isocenteric coincidence of MLC was observed within a tolerance range recommanded from TG-142 reports. Total number of motor replacement were 56 motors over whole QA period. For all motors replaced from QA, gradually increased patterns of error RMS values were much more than suddenly increased patterns of error RMS values. Average error RMS values of gradually and suddenly increased patterns were 0.298 cm and 0.273 cm, respectively. However, The average error RMS values were within 0.35 cm recommended by the vendor, motors were replaced according to the criteria of no counts with misplacement > 1 cm. On average, motor replacement for gradually increased patterns of error RMS values 22 days. 28 motors were replaced regardless of the leaf speed QA. Conclusion : This study performed the periodic MLC QA for analyzing the mechanical and leaf speed accuracy of the dynamic multileaf collimator (DMLC). The leaf position accuracy and isocenteric coincidence showed whthin of MLC evaluation is observed within the tolerance value recommanded by TG-142 report. Based on the result obtained from leaf speed QA, we have concluded that QA protocol of leaf speed for DMLC was performed at least bimonthly in order to screen the performance of leaf speed. The periodic QA protocol can help to ensure for delivering accurate IMRT treatment to patients maintaining the performance of leaf speed.

• The Journal of Korean Society for Radiation Therapy
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• v.28 no.1
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• pp.47-55
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• 2016

To evaluate the position accuracy of the MLC. This study analyzed the variations of the dosimetric leaf gap(DLG) and MLC transmission factor to reflect the location of the MLC leaves according to the dose rate variation for dynamic IMRT. We used the 6 MV and 10 MV X-ray beams from linear accelerator with a Millennium 120 MLC system. We measured the variation of DLG and MLC transmission factor at depth of 10 cm for the water phantom by varying the dose rate to 200, 300, 400, 500 and 600 MU/min using the CC13 and FC-65G chambers. For 6 MV X-ray beam, a result of measuring based on a dose rate 400 MU/min by varying the dose rate to 200, 300, 400, 500 and 600 MU/min of the difference rate was respectively -2.59, -1.89, 0.00, -0.58, -2.89%. For 10 MV X-ray beam, the difference rate was respectively ?2.52, -1.69, 0.00, +1.28, -1.98%. The difference rate of MLC transmission factor was in the range of about ${\pm}1%$ of the measured values at the two types of energy and all of the dose rates. This study evaluated the variation of DLG and MLC transmission factor for the dose rate variation for dynamic IMRT. The difference of the MLC transmission factor according to the dose rate variation is negligible, but, the difference of the DLG was found to be large. Therefore, when randomly changing the dose rate dynamic IMRT, it may significantly affect the dose delivered to the tumor. Unless you change the dose rate during dynamic IMRT, it is thought that is to be the more accurate radiation therapy.

• Progress in Medical Physics
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• v.26 no.4
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• pp.223-228
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• 2015

Double-focused micro-Multileaf Collimator (${\mu}MLC$) is able to create radiation fields having sharper dose gradients at the field edges than common MLC. Therefore, ${\mu}MLC$ has been used for the stereotactic radiosurgery (SRS) and Stereotactic Radiotherapy (SRT). We evaluated the dosimetric characteristics of a doublefocused Dynamic-${\mu}MLC$ (DMLC) attached to the Elekta Synergy linear accelerator. For this study, the dosimetric parameters including, Percent Depth Dose (PDD), Leaf leakage and penumbra, have been measured by using of the radiochromic films (GafChromic EBT2), EDGE diode detector and three-dimensional water phantom. All datas were measured on 6 MV x-ray. As a result, The DMLC shows transmission below to 1% and because of double-focused construction of the DMLC, the penumbras of fields with DMLC are independent from the field sizes. In this paper, the resulting dosimetric evaluations proved the applicability of the DMLC attached to the Elekta Synergy linear accelerator.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.138-140
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• 2002

The spinal cord dose is the one of the limiting factor for the radiation treatment of the head & neck (H&N) or the thorax region. Due to the fact that the cord is the elongated shaped structure, it is not an easy task to maintain the cord dose within the clinically acceptable dose range. To overcome this problem, the spinal cord partial block technique (PBT) with the dynamic Multi-Leaf Collimator (dMLC) has been developed. Three dimension (3D) conformal beam directions, which minimize the coverage of the normal organs such as the lung and the parotid gland, were chosen. The PBT field shape for each field was designed to shield the spinal cord with the dMLC. The transmission factors were determined by the forward calculation method. The plan comparisons between the conventional 3D conformal therapy plan and the PTB plan were performed to evaluate the validity of this technique. The conformity index (CI) and the dose volume histogram (DVH) were used as the plan comparison indices. A series of quality assurance (QA) was performed to guarantee the reliable treatment. The QA consisted of the film dosimetry for the verification of the dose distribution and the point measurements. The PBT plan always generated better results than the conventional 3D conformal plan. The PBT was proved to be useful for the H&N and thorax region.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.305-308
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• 2002

More complex radiotherapy techniques using multi leaf collimation(MLC) such as intensity-modulated radiation therapy(IMRT) has been increasing the significance of verification of leaf position and motion. Due to the reliability and robustness, quality assurance(QA) of MLC is usually performed with portal films. However, the advantage of ease of use and capability of providing digital data of electronic portal imaging devices(EPIDs) have attracted many attentions as alternatives of films for routine quality assurance in spite of the concerns about their clinical feasibility, efficacy, and the cost to benefit ratio. In our work, the method of routine QA of MLC using electronic portal imaging(EPI) was developed. The verification of availability of EPI images for routine QA was performed by comparison with those of the portal films which were simultaneously obtained when radiation was delivered and known prescription input to MLC controller. Specially designed test patterns of dynamic MLC were applied to image acquisition. Quantitative off-line analysis using edge detection algorithm enhanced the verification procedure in addition to on-line qualitative visual assessment. In conclusion, the EPI is available enough for routine QA with the accuracy of portal films.

• Progress in Medical Physics
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• v.26 no.3
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• pp.153-159
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• 2015

This study is to evaluate thedosiemtric leaf gap (DLG) at different depths for dynamic intensity-modulated radiation therapy (IMRT) in order to evaluate the absolute dose and dose distribution according to the different positions of tumors and compare the measured and planned the multileaf collimator (MLC) transmission factor (T.F.) and DLG values. We used the 6 MV and 15 MV photon beam from linear accelerator with a Millenium 120 MLC system. After the import the DICOM RT files, we measured the absolute dose at different depths (2 cm, 5 cm, 10 cm, and 15 cm) to calculate the MLC T. F. and DLG. For 6 MV photon beam, the measured both MLC T. F. and DLG were increased with the increase the measured depths. When applying to treatment planning systemas fixed transmission factor with its value measured under the reference condition at depth of 5 cm, although the difference fixed and varied transmission factor is not significant, the dosiemtric effect could be presented according to the depth that the tumor is placed. Therefore, we are planning to investigate the treatment planning system whichthe T. F. and DLG factor according to at the different depths can be applied in the patient-specific treatment plan.

• Journal of the Korean Society of Radiology
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• v.16 no.7
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• pp.863-870
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• 2022

DLG (Dosimetric Leaf Gap) and transmission factor are important parameters of MLC modeling in treatment planning system. In this study, DLG and transmission factor of HD-MLC were measured using detector with different measuring volumes, and the accuracy of the treatment plans was evaluated according to the DLG values. DLG was measured using the dynamic sweeping gap method with Semiflux3D and MicroDiamond detectors. Then, 10 radiation treatment plans were generated to optimize the DLG value and compared with the measurement results. Photon energies 6, 8, 10 MV, the DLG measured by Semiflux3D were 0.76, 0.83, and 0.85 mm, and DLG measured by MicroDiamond were 0.78, 0.86, and 0.9 mm. All plans were measured by portal dosimetry and analyzed using Gamma Evaluation. In the 6 MV photon beams, the average gamma passing rate were 94.3% and 98.4% for DLG 0.78 mm and 1.15 mm. In the 10 MV photon beam, the average gamma passing rate were 91.2% and 97.6% for DLG 0.9 mm and 1.25 mm. HD-MLC needs accurate modeling in the treatment planning system. DLG could be used measured data using small volume detector. However, for better radiation therapy, DLG should be optimized at the commissioning stage of LINAC.

• The Journal of Korean Society for Radiation Therapy
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• v.14 no.1
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• pp.49-57
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• 2002