A method for determining multileaf collimator transmission and scatter fordynamic intensity modulated radiotherapy

The main purpose of this work is to demonstrate a practical means of determ ining the leaf transmission and scatter characteristics of a multileaf coll imator (MLC) pertinent to the commissioning of dynamic intensity modulated radiotherapy, especially for the sweeping window technique. The data are ne cessary for the conversion of intensity distributions produced by intensity -modulated radiotherapy optimization systems into trajectories of MLC leave s for dynamic delivery. Measurements are described for two, tungsten alloy MLCs: a Mark II 80-leaf MLC on a Varian 2100C accelerator and a Millenium 1 20-leaf MLC on a Varian 2100EX accelerator. MLC leakage was measured by fil m for a series of held sizes. Measured MLC leakage was 1.68% for a 10x10 cm (2) field for both 6 and 18 MV for the 80-leaf MLC. For the 6 MV field, the 1.68% leakage consisted of 1.48% direct transmission and 0.20% leaf scatte r. Direct transmission through the 80-leaf MLC, including the rounded leaf tip, was calculated analytically taking into account the detailed leaf geom etry and a Monte Carlo-generated energy spectrum of the accelerator. The in tegrated fluence under the leaf tip was equivalent to an inward shift of 0. 06 cm of a hypothetical leaf with a flat, focused tip. Monte Carlo calculat ions of the dose to phantom beyond a closed 80-leaf MLC showed excellent ag reement with the analytic results. The transmission depends on the density of the MLC alloy, which may differ among individual MLCs. Thus, it is impor tant to measure the transmission of any particular MLC. Calculated doses fo r a series of uniform fields produced by dynamic sweeping windows of variou s widths agree with measurements within 2%. (C) 2000 American Association o f Physicists in Medicine. [S0094-2405(00)01810-1].