Study of 6 MV Photon Beam Dose Profiles and Investigation of Jaw Motion Effects on the Beam Dose Profiles and the Dose Delivered in a Water Phantom

Abstract

This study investigated the medical linear accelerator jaw motion effects on the delivered dose. The focus was to examinate the photon beam dose profiles as a function of the jaw motion and depending on the depth in the water phantom along the central beam axis and the irradiation field sizes. The measurements of dose profiles were performed for a 6 MV photon beam in a water phantom, the field sizes of 3×3 cm², 4×4 cm², 6×6 cm², 8×8 cm², 10×10 cm², 12×12 cm², 15×15 cm², 20×20 cm², 25×25 cm², 30×30 cm², and 35×35 cm² at depth of 1.5 cm, 5 cm, 10 cm, 20 cm, and 30 cm on the central beam axis, and source-to-surface distance (SSD) of 100 cm for Varian Clinac 2100 linear accelerator. The purpose of this study was to investigate the jaw motion effects on the dose profiles and subsequently on the delivered dose depending on the field size and depth in the water phantom and also the beam dose profiles’ symmetry about the central beam axis. We proceeded to calculate two parameters: the normalized width and the difference of the width between the left and right width of dose profiles as a function of the field size and depth along the central beam axis. We have shown that the motion of the jaws to define the irradiation field size have impacts on the beam dose profiles and its symmetry. the spread of the width difference was in a wave motion along the central beam axis and within a range of off-axis distance from -2 mm to 2 mm, which it should be considered in the treatment of tumors and spare the healthy cells.