Conformal intensity-modulated radiotherapy (IMRT) delivered by robotic linac - conformality versus efficiency of dose delivery

Intensity-modulated radiotherapy (IMRT) may be delivered with a high-energy -photon linac mounted on a robotic gantry and executing a complex trajector y. Ln a previous paper an inverse-planning technique was developed for such an application. Here the work is extended to demonstrate the dependence of conformality on the size of the elemental pencil beam, on the complexity o f the trajectory and on the sampling of azimuth and elevation of the collim ated source. The improved conformality of complex trajectories is demonstra ted and benchmarked relative to simpler trajectories, more representative o f existing non-robotic IMRT techniques. Specifically, by choosing a very fi ne pencil beam, exquisitely conformal dose distributions have been obtained . Important sampling considerations have been determined. Expressions have been derived for the dosimetry and monitor-unit efficiency of robotic IMRT. Equivalent trajectories were computed for executing the complex robotic tr ajectories instead by using a conventional linac. The work benchmarks an id eal in IMRT against which more practical and more common techniques may be measured.