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BioMed Research International
Volume 2013 (2013), Article ID 461801, 8 pages
Research Article

A Light-Field-Based Method to Adjust On-Axis Rounded Leaf End MLC Position to Predict Off-Axis MLC Penumbra Region Dosimetric Performance in a Radiation Therapy Planning System

1Department of Radiation Oncology, E-Da Hospital, Kaohsiung 824, Taiwan
2Department of Medical Imaging and Radiological Science, Central Taiwan University of Science and Technology, Taichung 411, Taiwan
3Medical Physics & Informatics Laboratory, Department of Electronics Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Kaohsiung 807, Taiwan
4Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 824, Taiwan

Received 28 June 2013; Accepted 27 August 2013

Academic Editor: Ching Chong Jack Yang

Copyright © 2013 Jia-Ming Wu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Purpose. An analytical and experimental study of split shape dose calculation correction by adjusting the position of the on-axis round leaf end position is presented. We use on-axis corrected results to predict off-axis penumbra region dosimetric performance in an intensity-modulated radiation therapy treatment planning system. Materials and Methods. The precise light-field edge position () was derived from the on-axis 50% dose position created by using the nominal light field for geometric and mathematical manipulation. Leaf position () could be derived from by defining in the treatment planning system for monitor unit calculation. On-axis offset (correction) could be obtained from the position corresponding to 50% of the central axis dose minus the position. The off-axis 50% dose position can then be derived from the on-axis 50% dose position. Results. The monitor unit calculation of the split shape using the on-axis rounded leaf end MLC penumbra region could provide an under-or overdose of 7.5% per millimeter without an offset correction. When using the on-axis rounded leaf end offset correction to predict the off-axis dose, the difference between the off- and on-axis 50% dose position is within ±1.5 mm. Conclusions. It is possible to achieve a dose calculation within 0.5% error for an adjusted MLC leaf edge location in the treatment planning system with careful measurement and an accurate on-axis offset correction. Dose calculations located at an off-axis spilt shape region should be used carefully due to noncorrectable errors which were found to be up to 10%.