Optimized Decomposition of Virtual Micro Intensity Maps

Virtual Micro-Intensity Modulated Radiation Therapy (VMIMRT) is a technique that uses a 10 mm leaf width multi-leaf collimator (MLC) to deliver treatments with 5 mm × 5 mm beamlets. The virtual micro intensity map (VMIM), I(i,j), has to be decomposed into two orthogonal intensity maps. The map with the zero offset in collimator angle, Z(i,j), has 10 mm × 5 mm beamlets while the one with a ninety degree offset, N(i,j), has 5 mm × 10 mm beamlets. Figure 1 shows an example of the decomposition for a single 10 mm × 10 mm macrobeam (a group of four 5 mm × 5 mm beamlets that can be covered by a leaf pair in either collimator angle). In this example, there is only one way to perform the decomposition. However, in addition to a local gradient, G(i,j), there can be a nonzero component, U(i,j), that is locally uniform over each macrobeam. In such cases, one needs to decide how to apportion the uniform component between Z(i,j) and N(i,j) for each macrobeam. This becomes an optimization problem with as many parameters as there are macrobeams with nonzero uniform components in I(i,j), since an appropriately chosen set of parameters will minimize the total beam on time to deliver I(i,j).