Monitor unit calculation for an intensity modulated photon held by a simple scatter-summation algorithm

An important issue in intensity modulated radiation therapy (IMRT) is the v erification of the monitor unit (MU) calculation of the planning system usi ng an independent procedure. Because of the intensity modulation and the dy namic nature of the delivery process, the problem becomes much more involve d than that in conventional radiation therapy. In this work, a closed formu la for MU calculation is derived. The approach is independent of the specif ic form of leaf sequence algorithms. It is straightforward to implement the procedure using a simple computer program. The approach is illustrated by a simplified example and is demonstrated by a few CORVUS (NOMOS Corporation , Sewickley, PA) treatment plans. The results indicate that it is robust an d suitable for IMRT MU verification.