A measure of SRS/SRT plan quality: Quantitative limits for intermediate dose spill (R50%) in linac‐based delivery

Abstract

Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) of multiple cranial targets using a single isocenter on conventional C-arm linear accelerators are rapidly developing clinical techniques. However, no universal guidelines for acceptable intermediate dose spill limits are currently available or widely accepted. In this work, we propose an intermediate dose spill guidance range for cranial SRS/SRT delivered on C-arm linacs with MLC collimation for single PTV plans and single isocenter multiple target plans with PTV volumes in the range 0.02-57.9 cm 3 . We quantify intermediate dose spill with the R50% metric (R50% = volume of 50% of prescription isodose cloud / volume of PTV) and test the proposed range using three clinical data sets, containing both 6 MV and 10 MV beams, previously published by other authors. Our proposed lower limit of R50% (LowerR50%) and upper limit of acceptable R50% (UpperR50%) bound over 90% of the clinical data used in this study, yet still provide a challenging benchmark for optimization and plan assessment of linac-based, MLC collimated SRS/SRT.

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

“…With the recent publication of proposed universal quality guidelines for intermediate dose spill measured with R50%, 9 the FVE accounting system allows comparison with those guidelines even when the individual IDC50% for each PTV cannot be obviously determined. Comparison with these metrics is impossible without a way to apportion the overlapping IDC50% of multiple PTV.…”

“…We propose an accounting system for intermediate dose spill in cranial stereotactic treatment for the roughly 11% of clinical single isocenter multiple target cases with PTVs having merged/overlapping IDC50% volumes. In such cases, a standardized method is needed to assign the IDC50% to the individual PTVs to allow comparison with recently proposed universal quality standards for intermediate dose spill 9 . When the IDC50% of multiple PTVs overlap, the FVE accounting system for R50% uses the ratio of the individual PTV surface area to the total surface area of all PTVs that share the IDC50% to apportion the IDC50% according to Equation ().…”

“…In such cases, a standardized method is needed to assign the IDC50% to the individual PTVs to allow comparison with recently proposed universal quality standards for intermediate dose spill. 9 When the IDC50% of multiple PTVs overlap, the FVE accounting system for R50%…”

“…"This is certainly one system for apportioning the overlapping IDC50% among the involved PTVs, yet this simple proximity-based division IDC50% could yield very large IDC50% values for small PTVs close to large PTVs. Now that universal standards for R50% in SRS/SRT have been proposed by Desai et al, 9 it is important to be able to compare the R50% of a PTV to those standards to assess plan quality. As stated earlier from the work of Popple et al, 7 roughly 11% of SRS/SRT targets likely have overlapping or bridging IDC50%, yet, for these targets,it is impossible to make a comparison to these quality metrics unless the overlapping IDC50% can be separated/apportioned to individual PTVs.…”

“…The first four PTVs(1,4,6,9) have clearly distinguishable 50% isodose clouds (ICD50%) volumes, and FVE is not used for them. The second group of three PTVs (2, 3, 5) are a cluster with overlapping IDC50% for which FVE is used to assign the IDC50% volume (V Total IDC50% ) within that cluster.…”