Field Shaping for Three-Dimensional Conformal Radiation Therapy and Multileaf Collimation.

Three-dimensional (3-D) conformal treatments are complex and involve many fields, virtually all of which are irregularly shaped and, in the not too distant future, may be intensity-modulated arbitrarily. The multileaf collimator (MLC) is an essential tool for making the delivery of these treatments practical. The characteristics of MLCs of different manufacturers vary significantly, which has an impact on the resulting dose distributions and their clinical applicability. The jagged edges of the fields shaped with MLCs has raised concern about the dose distributions. To study the dosimetric consequences of the jagged boundary, accurate methods of predicting dose distributions are necessary. Preliminary dosimetric and clinical studies show that, for a vast majority of sites and techniques, the MLC is an appropriate replacement for continuous alloy blocks and that the uneasiness about the MLC-shaped boundaries is unwarranted. To fully realize the efficiency possible with a MLC, it is important that it be integrated into the 3-D conformal treatment planning system. Schemes and criteria for positioning leaves relative to the continuous boundary are being studied and often depend on the treatment site, the surrounding normal critical structures, and the treatment technique. Optimum use of the MLC for the efficient delivery of complex 3-D conformal treatments is possible only when treatments are delivered under computer control. Computer-controlled radiation therapy with the MLC is evolving rapidly. Currently, only boundaries are shaped automatically. It is expected that in the near future intensity-modulated treatments will also become practical. General quality assurance and safety requirements of a treatment machine. In many ways, MLCs are more precise and safer than conventional field-shaping devices. However, it is important that the MLC configuration for each field be verified by a record and verify system to ensure that the correct field shape has been selected for treatment.