How to estimate R50% for cranial SRS/SRT cases with overlapping 50% isodose volumes: A proposed system

Abstract

Inevitably in clinical stereotactic cranial single isocenter multiple target cases treated with linac‐based multi‐leaf collimated (MLC) delivery, one encounters planning target volumes (PTVs) in close proximity with overlapping 50% isodose clouds (IDC50%). In such cases, it is very difficult to separate the IDC50% attributable to each individual target and, thus, assess the intermediate dose conformality or R50%. Such scenarios happen regardless of what metric is used to measure intermediate dose spill. Now that universal standards for intermediate dose spill have been proposed, it is important to have a consistent method for apportioning these overlapping IDC50% volumes to allow comparison with the proposed standards when multiple PTVs have overlapping IDC50%. We propose a systematic method for apportioning the IDC50% of multiple targets with overlapping IDC50% based on the relative surface area of each target volume; we call this the fair value estimate (FVE). This FVE system of apportionment is tested for reasonableness by comparing the apportionment of multiple target single isocenter stereotactic treatment with widely spaced targets where the IDC50% can be obviously assigned to demonstrate that the FVE results are very similar to the actual R50% results. We then demonstrate how the FVE system would be applied to cases with overlapping IDC50%. We propose this FVE system for consideration by the cranial stereotactic community for apportioning the intermediate dose spill when that intermediate dose spill overlaps among multiple targets.

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

“…The full application of the FVE for R50% to a cluster of PTVs with overlapping IDC50% was described by Desai and Cordrey and requires the surface area of the individual PTVs in the cluster. 6 The equation to calculate the R50% FVE is given below:…”

“…This limitation can be overcome by using a spherical PTV approximation to FVE, in which one assumes the PTV surface area is the surface area of a spherical PTV of the same volume as the PTV in question. Thus, SA PTV−sphere = 4𝜋 (r PTV ) 2 (6) Applying this approximation to Equations ( 2) and ( 3) with a little algebra, we have, respectively:…”

“…This difficulty can be overcome utilizing an equivalent spherical PTV approximation as was suggested in Desai and Cordrey. 6 This brief technical note will describe the equivalent spherical PTV approximation of FVE and apply "spherical PTV approximation FVE" to the clinical cases in the UAB dataset that used HyperArc for which the IDC50% of multiple PTVs overlapped to convert the Falloff Index into R50%.…”

“…7 Yet, to apply full FVE, one must know the individual PTV surface areas. 6 Since no commercially available radiation treatment planning system (RTPS) currently reports the surface area of a segmented structure contoured in the RTPS, FVE is challenging to apply without a user-created surface area script. This difficulty can be overcome utilizing an equivalent spherical PTV approximation as was suggested in Desai and Cordrey.…”

“…Desai and Cordrey detailed a systematic accounting system to apportion the IDC50% for those clinical cases when the IDC50% of multiple PTVs overlap, which is called the Fair Value Estimate (FVE). 6 FVE allows one to unambiguously assign an R50% value to each PTV in a clinical plan where the IDC50% of multiple PTVs overlaps. In so doing, FVE allows comparison with universal R50% standards published by Desai et al for linac-based multi-leaf collimated stereotactic radiosurgery (SRS) and stereotactic radiation therapy F I G U R E 1 A simple illustration of a 3-PTV SIMT case comprised of a single PTV and a 2-PTV cluster.…”