A clinical application of fair value estimate (FVE) for R50%: Separating overlapping 50% isodose volumes in single isocenter multiple targets SRS

In the treatment of single isocenter multiple targets (SIMT) stereotactic cranial cases with linac‐based, multi‐leaf collimated delivery, one encounters cases when the 50% isodose clouds (IDC50%s) of planning target volumes (PTVs) in close proximity overlap and cannot easily be separated. In such cases, it is difficult to assign an IDC50% to each individual PTV, which is necessary to allow evaluation of individual PTV intermediate dose spill for comparison to established intermediate dose spill metrics for plan quality assessment. The Fair Value Estimate (FVE) for R50% (R50%(FVE)) is a method to unambiguously apportion the overlapping volume of IDC50% to allow calculation of the intermediate dose spill metric R50% (defined as volume of IDC50% / volume of PTV). Full application of R50%(FVE) requires knowing the surface area of the PTVs. Since surface area information is not always available, we develop a spherical PTV approximation to R50%(FVE‐sphere) and compare this to R50%(FVE). Then we apply the R50%(FVE‐sphere) to clinical data from the University of Alabama, Birmingham (UAB) that catalogs 68 PTVs from various SIMT plans with overlapping IDC50%. The UAB dataset reports intermediate dose spill as Falloff Index. While Falloff Index looks mathematically equivalent to R50%, the Falloff Index attributes the “entire overlapping IDC50% of PTVs in close proximity” to each individual PTV in the cluster. R50%(FVE‐sphere) provides a value that is conceptually correct and numerically smaller relative to the Falloff Index data reported by UAB in all cases. This reprocessing of the UAB data places many of the PTVs with very high intermediate dose spill within recently proposed R50% guidelines.