Journal List > Prog Med Phys > v.26(3) > 1098475

Chang, Kwak, Cho, Jeong, Bae, Yoon, and Lee: Evaluation of Dosimetric Leaf Gap (DLG) at Different Depths for Dynamic IMRT

Abstract

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.

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Fig. 1.
Start (a) and end (b) positions of the 10 mm sliding slit movementfor dosimetric leaf separation file.
pmp-26-153f1.tif
Fig. 2.
Schematic diagram for calculating of DLG under various depths and source-surface distances.
pmp-26-153f2.tif
Fig. 3.
An example of linear extrapolation for derivation of a DLG at different depths (2 cm, 5 cm, 10 cm, 15 cm) for 6 MV photon beam.
pmp-26-153f3.tif
Fig. 4.
Measured and planned transmission factor (a) and dosimetric leaf gap (b) at different depths for 6 MV and 15 MV photon beam.
pmp-26-153f4.tif
Table 1.
Comparison of measured and planned transmission factor at different depths for 6 MV and 15 MV photon beam.
Depth (cm)   6 MV     15 MV  
Measurement (%) RTP (%) Difference (%) Measurement (%) RTP (%) Difference (%)
2 1.42 1.40 −1.41 1.63 1.64 0.61
5 1.43 1.46 2.10 1.62 1.63 0.62
10 1.48 1.47 −0.68 1.63 1.63
15 1.54 1.55 0.65 1.63 1.65 1.23
Table 2.
Comparison of measured and planned dosimetric leaf gap (DLG) at different depths for 6 MV and 15 MV photon beam.
Depth (cm)   6 MV     15 MV  
Measurement (cm) RTP (cm) Difference (%) Measurement (cm) RTP (cm) Difference (%)
2 0.1101 0.1146 4.09 0.1279 0.1299 2.53
5 0.1125 0.1186 5.42 0.1320 0.1339 0.90
10 0.1160 0.1096 −5.52 0.1313 0.1302 −0.69
15 0.1194 0.1017 −14.82 0.1333 0.1287 −3.38
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