Interplay of MLC, gantry and respiratory motion during DCAT delivery

Abstract

This study investigated the possible interplay effects arising from the treatment of moving targets using the dynamic conformal arc therapy (DCAT) technique. Dose from a modulated test beam was measured, with and without phantom motion and with and without a 30° arc rotation, using a diode array placed on a sinusoidally moving platform. Measurements were repeated at five different collimator angles (0, 22.5, 45, 67.5 and 90°), at two different dose rates (300 and 600 MU/min). Results showed that the effect of respiratory motion on the measured dose distribution increased slightly when the beams were delivered as arcs, rather than with a static gantry angle, and that this effect increased substantially as the collimator angle was increased from 0° (MLC motion perpendicular to respiratory motion) to 90° (MLC motion parallel to respiratory motion). The dose oscillations arising from interplay between phantom and MLC motion were found to increase in magnitude when the dose rate was increased. These results led to the development of simple recommendations for minimizing the negative effects of motion interplay on DCAT dose distributions.