3-DIMENSIONAL CONFORMAL TREATMENT PLANNING WITH MULTILEAF COLLIMATORS

Purpose: Three dimensional conformal radiation treatments are complex, often involving large numbers of blocked or multileaf collimated fiel ds that shape regions of high dose to conform to the treatment volume. As manual definition and digitization of aperture shapes and their co rresponding multileaf configurations can be impractically time consumi ng, it was necessary to integrate the planning of multileaf fields int o an existing three dimensional treatment planning system and improve the efficiency of treatment delivery to make these treatments feasible on a routine basis. Methods and Materials: A subfunction of the Beam' s Eye View (BEV) component can be used to automatically generate a con tinuous aperture shape with a margin around the tumor to account for b eam penumbra, and excluding any normal structures to be spared (each w ith its own margin). To convert a continuous aperture shape into one d efined by the multileaf collimator (MLC), a leaf coverage mode is chos en to determine how Leaves are fitted to aperture shapes. The conversi on process also considers parameters of the specific MLC system, e.g., leaf thickness and the number of leaves. If normal structures to be s hielded split the target into multiple regions, more than one multilea f aperture can result. An interactive leaf adjustment routine is also provided to allow for modification of individual leaf positions. Dose calculation programs then take into account multileaf apertures for co mputation of dose distributions using a pencil beam convolution model. Finally, prescription files specifying Leaf and jaw configurations ar e prepared in treatment machine specific formats and downloaded to the computers driving the multileaf collinators and other components of t he treatment machines. Results and Conclusion: An example is presented of a prostate treatment plan, with MLC configurations, dose distribut ions, and treatment delivery description, along with discussion of cli nical implementation at Memorial Hospital.