Three-dimensional verification for dynamic multileaf collimated IMRT

We have developed software for constructing the leaf sequences necessary for IMRT delivery using an MLC and we have built a phantom for three-dimensional dose verification. The application requires the user to specify the RMS difference between the desired and translated profiles as well as the tolerance of leaf positioning. Transmission, interleaf and intraleaf leakage issues are also addressed. Individual leaves are synchronized to reduce tongue and groove 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. Fixed gantry optimized mode is a hybrid technique overcoming the limitation of finite leaf speed for dynamic mode. Output factor correction for small fields is found to be the most challenging phenomena for dosimetry. A cylindrical phantom is constructed for three-dimensional dosimetric verification. The phantom has 15 cylindrical inserts containing diode detectors. Each detector is connected to an electronic system and then fed to computer with specialized software. The readings of each detector ism recorded at 2-second intervals. Intensity profiles calculated by an inverse planning algorithm are applied to the phantom by doing forward calculation with 3D-calculation engine. The same profiles are translated using the leaf-sequencing algorithm and delivered to the phantom. The measured dose at the position of each diode is compared to the forward calculation.