Comparative Analysis of MLC Dosimetric Parameters for Varian LINACs: Unique 6X, Clinac iX and TrueBeam at AECH NORI

Purpose: The dosimetric leaf gap (DLG) and transmission factor are two important systematic parameters used to model the rounded MLC leaf end effect when commissioning an Eclipse treatment planning system (TPS). This study investigates the influence of gantry and collimator angles on the dosimetric leaf gap (DLG) and leaf transmission factor (LTF) in three Varian LINAC equipped with rounded end Millennium 120 MLCs. This research also compared the DLG and LTF values for the three LINAC systems. While Varian guidelines recommend DLG measurements at zero degrees for both gantry and collimator angles, this research aims to address the knowledge gap by assessing DLG and LTF variations at different gantry and collimator angles. Materials and Method: Measurements were performed on the Varian TrueBeam, Clinac iX and Unique 6X LINACs equipped with a Millennium 120 MLC. The beams utilized in this study had energies of 6 MV, 15 MV, 6 FFF. LTF and DLG were determined using PTW Farmer, Semiflex and Pinpoint ionization chambers in water phantoms at gantry angles 0° and 90°. For each gantry angle, measurements were also taken at two collimator angles 0° and 90°. For eight different gantry angle measurements Sun Nuclear stereo phantom (stereoPHAN) was used along with the semiflex chamber. The same methodology was further extended to the Octavius 4D phantom and portal dosimetry to verify the consistency of the results. Results: Varian specifies that the transmission through its MLCs should not exceed 2% while the DLG is expected to remain below 2mm [8]. The analysis of DLG and LTF values across different Varian linear accelerators demonstrated consistent trends among the investigated chamber types. Chamber wise analysis indicated overall means of 1.30 ± 0.14 mm (Farmer), 1.32 ± 0.16 mm (Semiflex), and 1.36 ± 0.09 mm (PinPoint) for DLG, with corresponding LTF values of 1.35 ± 0.08, 1.41 ± 0.10, and 1.45 ± 0.13. The slightly higher spread observed for Semiflex and PinPoint suggests greater variability compared with Farmer, though all chambers provided clinically acceptable consistency. Conclusion: This work evaluated dosimetric leaf gap and transmission for three Varian LINACs (TrueBeam, Clinac iX, and Unique 6X) using different ionization chambers, with verification through Octavius 4D phantom and portal dosimetry. TrueBeam yielded the lowest mean DLG (1.22 mm), Clinac iX the highest (1.46 mm), and Unique 6X intermediate (1.41 mm), while transmission factors remained within 1.3% to 1.5% across all machines. Chamber related differences wereminor, with PinPoint measuring slightly higher than Farmer and Semiflex. Overall, the results demonstrate that both machine design and chamber type influence measurements, though the variations remain within clinically acceptable limits for treatment planning accuracy.