Monitor unit calculation for intensity modulated photon field

A pressing issue in intensity modulated radiation therapy (IMRT) is the verification of the monitor unit (MU) calculation of the planning system using an independent procedure. Because of the intensity modulation and the dynamic nature of the delivery process, the problem becomes much more involved than that in conventional radiation therapy. In this work, a closed formula for MU calculation in IMRT is derived. The formalism is quite general and independent of the specific form of leaf sequence algorithm or delivery machine. The dose at an arbitrary spatial point (either on the central axis or off-axis) is expressed as a summation of the contributions from all the beamlets. The dose from each individual beamlet can be calculated using a variety of methods, as simple as a Clarkson type of approach or as complex as Monte Carlo simulation. The method is demonstrated using a simple scatter-summation. Some specific issues related to the clinical implementation of the formalism, such as effect of skin contour variation, off-axis dose calculation, effect of tissue density inhomogeneity, and the verification in the low dose region, are discussed. In addition, the implementation of the technique with different delivery schemes (step-and-shoot, sliding-window, ...) is addressed. The approach is illustrated by a simplified example and several CORVUS (NOMOS corporation, Sewickley, PA) treatment plans. The results indicate that it is robust and suitable for IMRT MU verification