Investigation of Gravitational Sag Effect on Dosimetric Leaf Gap (DLG) at Various Gantry Angles in Radiation Therapy Medical Accelerator
Dasti Uli Florida Damanik, Akbar Azzi, Amar Ma^ruf Irfan Muhamadi, Aloysius Mario Yudi Putranto

Faculty of Mathematics and Natural Science, Universitas Indonesia
Department of Radiotherapy, MRCCC Siloam Hospital Semanggi

Abstract

The accuracy of Multi-Leaf Collimator (MLC) modeling in commercial Treatment Planning Systems (TPS) is critical for high-precision radiotherapy techniques such as IMRT. The standard guidelines of the Dosimetric Leaf Gap (DLG) measurements was only at gantry angle 0 degree, yet gantry rotation during treatment induces gravitational sag effects that may alter effective leaf positions and compromise dosimetric accuracy. This study investigates DLG variations across gantry angles of 0, 90, 180, and 270 degree through direct experimental measurements on a Varian Clinac iX 6 MV linear accelerator with Millennium 120-leaf MLC at MRCCC Siloam Hospitals Semanggi. Measurements were performed using synchronous sweeping gap (gap widths: 5, 10, 20, 30 mm) and asynchronous sweeping gap methods (20 mm main gap, offset s = 0, 2, 5, 10, 15, 20 mm) at all four gantry angles using an IBA FC65-G ion chamber in a solid water phantom at 10 cm depth and 90 cm SSD. Ion chamber readings were corrected using kTP, kpol, and ks correction factors following the IAEA TRS-398 protocol. Synchronous sweeping gap results indicate DLG varied across gantry angles, with the highest value observed at G90 degree and the lowest at G270 degree. Asynchronous sweeping gap measurements demonstrate a a reduction of 8.5-9.8% in the corrected electrometer reading ratio relative to the open field reference across all gantry angles as leaf offset increases from s = 0 to 20 mm, confirming the tongue-and-groove effect of the Millennium 120-leaf MLC. These findings indicate that gravitational effects on the MLC mechanism influence DLG values and should be considered in clinical MLC calibration protocols to improve IMRT treatment planning accuracy.

Keywords: Dosimetric Leaf Gap, Multi-Leaf Collimator, IMRT, Varian Clinac iX.

Topic: Medical Physics and Biophysics