THE EFFECTS OF INTRA-FRACTION ORGAN MOTION ON THE DELIVERY OF DYNAMICINTENSITY MODULATION

Computer-optimized treatment plans, aimed to enhance tumour control an d reduce normal tissue complication, generally require non-uniform bea m intensities. One of the techniques for delivering intensity-modulate d beams is the use of dynamic multileaf collimation, where the beam ap erture moves and the held shape changes during irradiation. When inten sity-modulated beams are delivered with dynamic collimation, the probl em of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static held treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra , the interplay of the movement of the beam aperture and the movement of the patient anatomy can create 'hot' and 'cold' spots throughout th e field. The mechanism for creating these effects is not well understo od. This paper provides a simple analytical model which illustrates th e fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiat ion. Numerical analysis was carried out which calculates the cumulativ e primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variation s in the target can be greater than 100% of the desired beam intensity . The magnitude of the photon intensity variations is strongly depende nt on the speed of the beam aperture relative to the speed of the targ et motion, and the width of the scanning beam relative to the amplitud e of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction o rgan motion are discussed.