Prog Med Phys.  2015 Sep;26(3):168-177. 10.14316/pmp.2015.26.3.168.

Assessment for the Utility of Treatment Plan QA System according to Dosimetric Leaf Gap in Multileaf Collimator

• Lee SS
• Choi SH
• Min CK
• Kim WC
• Ji YH
• Park S
• Jung H
• Kim MS
• Yoo HJ
• Kim KB

• KMID: 2069480
• DOI: http://doi.org/10.14316/pmp.2015.26.3.168

Abstract

For evaluating the treatment planning accurately, the quality assurance for treatment planning is recommended when patients were treated with IMRT which is complex and delicate. To realize this purpose, treatment plan quality assurance software can be used to verify the delivered dose accurately before and after of treatment. The purpose of this study is to evaluate the accuracy of treatment plan quality assurance software for each IMRT plan according to MLC DLG (dosimetric leaf gap). Novalis Tx with a built-in HD120 MLC was used in this study to acquire the MLC dynalog file be imported in MobiusFx. To establish IMRT plan, Eclipse RTP system was used and target and organ structures (multi-target, mock prostate, mock head/neck, C-shape case) were contoured in I'mRT phantom. To verify the difference of dose distribution according to DLG, MLC dynalog files were imported to MobiusFx software and changed the DLG (0.5, 0.7, 1.0, 1.3, 1.6 mm) values in MobiusFx. For evaluation dose, dose distribution was evaluated by using 3D gamma index for the gamma criteria 3% and distance to agreement 3 mm, and the point dose was acquired by using the CC13 ionization chamber in isocenter of I'mRT phantom. In the result for point dose, the mock head/neck and multi-target had difference about 4% and 3% in DLG 0.5 and 0.7 mm respectively, and the other DLGs had difference less than 3%. The gamma index passing-rate of mock head/neck were below 81% for PTV and cord, and multi-target were below 30% for center and superior target in DLGs 0.5, 0.7 mm, however, inferior target of multi-target case and parotid of mock head/neck case had 100.0% passing rate in all DLGs. The point dose of mock prostate showed difference below 3.0% in all DLGs, however, the passing rate of PTV were below 95% in 0.5, 0.7 mm DLGs, and the other DLGs were above 98%. The rectum and bladder had 100.0% passing rate in all DLGs. As the difference of point dose in C-shape were 3~9% except for 1.3 mm DLG, the passing rate of PTV in 1.0 1.3 mm were 96.7, 93.0% respectively. However, passing rate of the other DLGs were below 86% and core was 100.0% passing rate in all DLGs. In this study, we verified that the accuracy of treatment planning QA system can be affected by DLG values. For precise quality assurance for treatment technique using the MLC motion like IMRT and VMAT, we should use appropriate DLG value in linear accelerator and RTP system.

Keyword

Multileaf Collimator; IMRT; AAPM TG-119 report; Dosimetric leaf gap; MobiusFx

MeSH Terms

Humans
Particle Accelerators
Prostate
Rectum
Urinary Bladder

Figure

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  Fig. 1. The contoured structures in I'mRT phantom image for (a) multi-target, (b) prostate, (c) head and neck and (d) C-shape case provided by AAPM TG-119 report.

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  Fig. 2. The linear extrapolation for a DLG in corrected reading versus gap. The graph of DLG is for photon energy 6 MV with IBA CC13 ion chamber.

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  Fig. 3. The figure illustrates gamma map between Eclipse RTP system and MobiusFx according to DLG.

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  Fig. 4. The plot (a-e) illustrate the dose volume histograms (DVH) for multi-target case according to DLG.

Reference

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