AN ANALYTICAL SOLUTION FOR THE DYNAMIC CONTROL OF MULTILEAF COLLIMATORS

All current optimization techniques in radiation therapy benefit from the use of strongly non-uniform radiation beams. The most flexible way of generating these fields under real time control is by elementary b eam scanning and/or dynamic multileaf collimation. In this work genera l analytical expressions are derived for the required motion of the co llimator leaves to achieve a desired energy fluence distribution or co llimator opening density in the patient in the shortest possible time. By simplification of the general expressions the equations of motion have been derived for both the shrinking field and the curtain shutter techniques with the associated approximations clearly quantified. The mechanical limitations on leaf motion, caused by the finite velocity and acceleration, are taken into account. It is shown that almost any desired energy fluence distribution can be created even when the limit ations on velocity and acceleration are considered. The basic rule wit h the curtain shutter technique is that when the energy fluence gradie nt along the direction of motion of the leaves is positive, the leadin g leaf should move at maximum speed and the lagging leaf should modula te the field. In regions where the gradient is negative the lagging le af should instead move at full speed and the leading leaf should modul ate the field. The overall treatment time is then proportional to the total increment in energy fluence or opening density between consecuti ve minima and maxima. For energy fluence profiles with numerous high p eaks the treatment time may therefore increase considerably over that for conventional uniform dose delivery. However, in general the treatm ent time is prolonged by a factor of about two compared to a tradition al uniform treatment. Obviously the method developed here for multilea f collimators is also suitable for simple block collimators since it c an be used to deliver arbitrary regular or irregular 'dynamic wedge' p rofiles along the direction of motion of the collimator blocks.