Prog Med Phys.  2016 Sep;27(3):111-116. 10.14316/pmp.2016.27.3.111.

Clinical Implications of High Definition Multileaf Collimator (HDMLC) Dosimetric Leaf Gap (DLG) Variations

• Chang KH
• Ji Y
• Kwak J
• Kim SW
• Jeong C
• Cho B
• Park JH
• Yoon SM
• Ahn SD
• Lee SW

• KMID: 2376540
• DOI: http://doi.org/10.14316/pmp.2016.27.3.111

Abstract

This study is to evaluate the dosimetric impact of dosimetric leaf gap (DLG) and transmission factor (TF) at different measurement depths and field sizes for high definition multileaf collimator (HD MLC). Consequently, its clinical implication on dose calculation of treatment planning system was also investigated for pancreas stereotactic body radiation therapy (SBRT). The TF and DLG were measured at various depths (5, 8, 10, 12, and 15 cm) and field sizes (6×6, 8×8, and 10×10 cm²) for various energies (6 MV, 6 MV FFF, 10 MV, 10 MV flattening filter free [FFF], and 15 MV). Fifteen pancreatic SBRT cases were enrolled in the study. For each case, the dose distribution was recomputed using a reconfigured beam model of which TF and DLG was the closest to the patient geometry, and then compared to the original plan using the results of dose-volume histograms (DVH). For 10 MV FFF photon beam, its maximum difference between 2 cm and 15 cm was within 0.9% and it is increased by 0.05% from 6×6 cm² to 10×10 cm² for depth of 15 cm. For 10 MV FFF photon beam, the difference in DLG between the depth of 5 cm and 15 cm is within 0.005 cm for all field sizes and its maximum difference between field size of 6×6 cm² and 10×10 cm² is 0.0025 cm at depth of 8 cm. TF and DLG values were dependent on the depth and field size. However, the dosimetric difference between the original and recomputed doses were found to be within an acceptable range (<0.5%). In conclusion, current beam modeling using single TF and DLG values is enough for accurate dose calculation.

Keyword

HDMLC; Dosimetric leaf gap (DLG); MLC transmission factor; Stereotactic body radiation therapy (SBRT); Pancreas

MeSH Terms

Humans
Pancreas

Figure

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  Fig. 1 Measurement of tumor size and depth.

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  Fig. 2 Measured transmission factor (a) and dosimetric leaf gap (b) at a field size of 10×10 cm2 at different depths (2, 5, 10, and 15 cm) and with a photon beam energy of 6 MV, 6 MV FFF, 10 MV, 10 MV FFF, and 15 MV.

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  Fig. 3 Measured transmission factor (a) and dosimetric leaf gap (b) with a 10 MV FFF photon beam at different field sizes (6×6, 8×8, and 10×10 cm2) and different depths (5, 10, and 15 cm).

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  Fig. 4 Dose volume histogram (DVH) of original plan (▲) and modified plans (■) for depths 5 cm (a), 8 cm (b), and 10 cm (c).

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