A dosimetric leaf-setting strategy for shaping radiation fields using a multileaf collimator.

A dosimetric leaf-setting strategy of using multileaf collimators (MLC) for shaping radiation fields has been developed. Existing MLC leaf-setting strategies are all based upon geometric criteria. This new approach, however, matches a prescribed field contour with a MLC using clinically consistent dosimetric criteria. The leaf positions are determined using an iterative optimization algorithm. An empirical dose model was developed to compare the dosimetric-based leaf-setting strategy with the geometric-based leaf-setting strategies. Differences up to half a centimeter in the leaf positions and isodose lines were found between setting the MLC geometrically and setting the MLC dosimetrically. The dosimetric leaf-setting strategy provides the ability to achieve better dose conformation for a clinically desired isodose line. Since the desired isodose line that covers a treatment volume is typically higher than 50% of the maximum dose, the scalloping effects due to the finite leaf width at the leaf edge or 50% isodose lines are much reduced. Another benefit of the dosimetric leaf-setting is that it separates the leaf-setting process from the treatment planning process, and this frees the treatment planning vendors from developing detailed dose models for various existing types and future upgrades of MLC systems.