Adjustable Prescription Dose: A New Strategy to Improve IMRT Treatment Planning

In IMRT the quality of a treatment plan depends greatly upon how well mutually-exclusive dosimetric goals for different structures within a patient are balanced and prioritized. Ideally, each voxel has to be assigned with an individual penalty factor and a prescription dose which take into account inter-voxel correlations. However, in most plans, an optimal solution is calculated by assigning a uniform importance and prescription to all voxels within a given structure volume. Previous studies have demonstrated that a significant improvement in IMRT planning can be achieved with voxel specific importance factors. On the other hand, prescription dose choice remains fixed and patient independent. We show that by removing the requirement for the prescription dose to be fixed for each voxel a significant improvement can be achieved when compared to IMRT plans with uniform fixed prescription. We explore the following approach: a spatially-dependent penalty scheme based on how well a given voxel satisfies its prescription. In this scenario both under and overdosed voxels are penalized with a penalty weight proportional to the amount of the dose discrepancy. The scheme is equivalent to the iterative adjustment of individual voxel prescriptions based on the departure from the desirable voxel doses. The method is capable of producing improved treatment plans without expert’s intervention. A clinical head and neck case was used to test this method. By adjusting voxel prescriptions to compensate for the inequalities between the actual and desired doses calculated, substantial improvements are obtained for the treatment plan as large dose reductions were achieved in almost all of the critical structures present. For instance, we demonstrate fivefold reduction in the maximum dose to the brainstem. Other organs at risk experience dose reduction ranging from 100 to 300 percents.