Dynamic multileaf collimation and three-dimensional verification in IMRT

We have developed software for constructing the leaf sequences necessary for IMRT delivery using a multileaf collimator. The algorithm used is an enhancement of the sliding window technique and supports several accelerator/MLC combinations [1,2]. Several factors are addressed in this application. The user specifies the RMS difference between the desired profile and the profile obtained with translated MLC motion. Tolerance of leaf positioning is also a user input. Transmission through MLC leaves is corrected using an iterative method. Leakage between opposing and neighboring leaves is minimized. Individual leaves are synchronized to reduce tongue and grove effect by varying their relative starting and ending times. The application supports fixed gantry step-and-shoot, fixed gantry dynamic, and fixed gantry optimized modes. During fixed gantry optimized mode, the delivery is dynamic, but switches to step-and-shoot mode in instances where MLC leaves cannot meet tolerance specification. Verifications have been performed using film dosimetry and amorphous Silicon (a-Si:H) 2D array detector acquisitions [3].