Considerations of target surface area and the risk of radiosurgical toxicity

Abstract

ObjectiveThe goal of this study was to explore conceptual benefits of characterizing delineated target volumes based on surface area and to utilize the concept for assessing risk of therapeutic toxicity in radiosurgery.Methods and materialsFour computer-generated targets, a sphere, a cylinder, an ellipsoid and a box, were designed for two distinct scenarios. In the first scenario, all targets had identical volumes, and in the second one, all targets had identical surface areas. High quality stereotactic radiosurgery plans with at least 95% target coverage and selectivity were created for each target in both scenarios. Normal brain volumes V12Gy, V14Gy and V16Gy corresponding to received dose of 12 Gy, 14 Gy and 16 Gy, respectively, were computed and analyzed. Additionally, V12Gy and V14Gy volumes and values for seven prospective toxicity variables were recorded for 100 meningioma patients after Gamma Knife radiosurgery. Multivariable stepwise linear regression and best subset linear regression analyses were performed in two statistical software packages, SAS/STAT and R, respectively.ResultsIn a phantom study, for the constant volume targets, the volumes of 12 Gy, 14 Gy and 16 Gy isodose clouds were the lowest for the spherical target as an expected corollary of the isoperimetric inequality. For the constant surface area targets, a conventional wisdom is confirmed, as the target volume increases the corresponding volumes V12Gy, V14Gy and V16Gy also increase. In the 100-meningioma patient cohort, the best univariate model featured tumor surface area as the most significantly associated variable with both V12Gy and V14Gy volumes, corresponding to the adjusted R2 values of 0.82 and 0.77, respectively. Two statistical methods converged to matching multivariable models.ConclusionsIn a univariate model, target surface area is a better predictor of spilled dose to normal tissue than target largest dimension or target volume itself. In complex multivariate models, target surface area is an independent variable for modeling radiosurgical normal tissue toxicity risk.

Expanded Radiosurgery Capabilities Utilizing Gamma Knife Icon™

“…Although patients are commonly treated based on the risk groups defined in RTOG 0539 [ 22 ], we prefer to treat progressive World Health Organization (WHO) grade I meningiomas with 15-16 Gy and higher-grade meningiomas in a single fraction or multiple fractions, prescribing 30 Gy and five fractions. Although toxicity predictions and normal tissue constraints continue to evolve, tumor location and tumor surface area may prohibit safe SRS [ 23 ]. In these instances, we prefer SRT with 25 Gy in five fractions in an attempt to mitigate toxicity.…”

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

“…This portion of V IDC50% will be referred to as the fair value estimate (FVE). Previous published work studying the “surface area effect” and the derivation of R50% Analytic implies an accounting system based on PTV surface area (SA PTV ) may prove useful 10–12 . We can assign the $$V_{{\rm{IDC50\% }}}^{{\rm{Remaining}}}$$ according to the surface area ratio.…”

“…Previous published work studying the "surface area effect"and the derivation of R50% Analytic implies an accounting system based on PTV surface area (SA PTV ) may prove useful. [10][11][12] We can assign the V Remaining IDC50% according to the surface area ratio.…”