| Περίληψη: | The purpose of this study is to determine and analyze a series of
measurements for the Dosimetric Leaf Gap (DLG) value using different set-ups as well as to compare the measured with the configured treatment planning system (TPS) values and evaluate its clinical significance.
Materials and Methods: All measurements were conducted on three medical linear
accelerators in two different institutions. Two Varian VitalBeamTM linear accelerators, namely VB1 (6 MV, 10 MV, 6 MV FFF) and VB2 (6 MV, 10 MV) both equipped with the 120 Millennium MLC and a Varian EDGETM (6 MV, 6 MV FFF, 10 MV, 10 MV FFF) linac equipped with the 120 HD Millennium MLC, were used. Measurements were performed with a Farmer type ionization chamber using the sweeping gap technique and with GAFCHROMIC EBT3 films using the cross-field dose width technique. Measurements were carried out for all energies, using a PMMA slab phantom, a water tank and a plastic solid water phantom at SSD 90 and 100 cm, at 4.2 and 8.4 cm depths, corresponding to 5 and 10 cm water equivalent depths. All ion chamber readings were converted into dose according to the TRS-398 protocol. The determined DLG values from the measuring processes were compared with the configured in the TPS DLG values. During the validation process an optimal range of DLGmeas values was derived, being appropriate for different VMAT treatment plans in different anatomical regions. Plans were retrospectively calculated with the DLGmeas values and compared with the existing treatment plans calculated with the already configured DLG values with respect to Gamma Passing Rate (GPR %) and Dose Volume Histograms (DVHs), so as to determine an optimal DLG value for each linac.
Results: The DLGmeas values showed variations with depth and SSD, for all energies, for all three linacs with both measurement techniques. The differences ranged from 0.01-0.6 mm for the different set-ups, energies and the three studied linacs with the sweeping gap tecnique. The cross-field dose technique also showed small differences between DLGmeas values for the three linacs, for all energies and studied set-ups. Most DLGmeas values determined with the sweeping gap technique and SSD=100 cm compared with the TPS already configured values showed percentage differences lower than 10 %. That was not the case for the cross-field dose width technique for any set up configuration and studied linac. The measured Gamma Passing Rate, GPR (%) results for the H&N treatment plans calculated with the DLGmeas values of 1.48 and 3.40 mm showed slightly lower values compared to the ones calculated with the already DLG configured values in the TPS whereas the same plans calculated with 2.85 mm showed GPRs almost equal to the ones derived with the DLG values in the TPS. All GPR (%)
values measured with DLGmeas selected range were above the action limit of 90 %. This was exactly the case for the prostate treatment plans. In a significant majority of the studied treatment plans which were calculated with the DLGmeas range values, it was observed that the dose coverage (V95) for the PTV exceeded 95 % and the dose constraint for OARs didn’t exceed the corresponding limits.
Conclusion: An optimal range of DLG values should be carefully selected for fine
tuning of the parameter as closer to the already configured value in the TPS. The
described methodology is effective and time efficient in measuring DLG and may be easily adopted by medical physicists to have their own institutional values.
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