Unified dosimetry index (UDI): a figure of merit for ranking treatment plans.

We have formulated a unified dosimetry index (UDI) that computes, for any given treatment plan, its deviations in terms of dose coverage, conformity, homogeneity, and dose gradient vis-à-vis an ideal plan (which we define as a dosimetry plan of perfect dose coverage, conformity, homogeneity, and step-wise fall-off to zero dose outside the planning target volume). In order to validate the UDI scoring system, 21 stereotactic cranial radiosurgery cases were evaluated retrospectively. The cases were planned on the BrainSCAN treatment planning system (BrainLAB, Feldkirchen, Germany) using 6 to 8 non-coplanar static beams collimated with the micro multi-leaf collimator (mMLC). We suggest a technique for creating a ranking system that can be utilized for plan evaluation and comparison between multiple plans. Under this system treatment plans are classified as "excellent", "good", "average", or "poor". The proposed ranking system can be utilized as a general guide for generating an optimal dosimetry plan for external beam radiation therapy.