Calculation of x-ray transmission through a multileaf collimator

A ray tracing based method has been developed to calculate the x-ray transm ission through a multileaf collimator (MLC) for beam delivery Verification and dose calculation in intensity modulated radiotherapy (IMRT). The path l ength of a ray line in the MLC is accurately calculated using the exact geo metry of the MLC leaves. The fluence distribution of an IMRT field is calcu lated first using a point source. The fluence distribution for a realistic beam model is obtained, as an approximation, by convolving the point source fluence distribution with the distribution of source strength. Full ray tr acing calculations are performed using analytic and Monte Carlo simulated b eam models to verify the accuracy of the convolution method. The calculatio n is in better agreement with measurements using either film or a beam imag ing system (BIS) than previous calculations for MLC transmission using a si mplified model. This ray tracing calculation can be applied to the problem of verifying dynamic MLC leaf sequences as part of a patient-specific quali ty assurance process for IMRT. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)00508-3].