2265 - A Concept of Equivalent-Volume Conformity Index Used in Target Volumes Comparison

Q. Du¹, Q. Zhong¹, J. Li¹, X. Ou¹, H. Yue², H. Zhu¹, X. Li¹, Q. Liang¹, Y. Xie², and W. Liu²; ¹The Second Affiliated Hospital of Guangxi Medical University, Nanning, China, ²Department of Radiation Oncology, The Second Affiliated Hospital of Guangxi Medical University, Nanning, China

Purpose/Objective(s): To present a concept of equivalent-volume conformity index to distinguish different intersection conditions in target volume comparison. Materials/

Methods: By using uniform expansion and Boolean operation, the non-overlapped volumes were segmented by a certain spatial distance (d) from the margin of standard volume (V_Std) or reference volume (V_Ref). The volume of V_Std minus V_I (the intersection volume of V_Std and V_Ref) was defined as V_Std-I, and the volume of V_Ref minus V_I was defined as V_Ref-I. Equivalent volume (EV) increased with the increasing spatial distance (EV_Std-I= ?V_{Std-I_i}(d_i/d₁)ⁿ; EV_Ref-I = ?V_{Ref-I_i}(d_i/d₁)ⁿ, d₁=1cm) , and the increase velocity was modulated by distance-penalty parameter n which was determined by the adjacent dose gradient. The gross tumor volumes (GTV) of 20 nasopharyngeal-carcinoma cases were delineated, and the treatment plans were designed with volumetric modulated arc therapy (VMAT). The DVH of whole body was exported in each plan, and the dosimetric parameters of V_PCT (the volume covered by a specific percentage of prescription dose) were extracted. The radius (R) of equivalent sphere for V_PCT was calculated by the equation: R_PCT = (3V_PCT/4p)^1/3, and the average distance from V_PCT to V_100% could be calculated by the equation: d_PCT= R_PCT-R_100%. The distance-penalty parameter was determined by the exponential relationship between the dose percentage and d_PCT. Equivalent-volume conformity index (CI_EV) was analyzed as an agreement rate for diagnostic test (CI_EV = V_I/(V_I+EV_Ref-I+EV_Std-I)), provided that the volume beyond V_Std and V_Ref was ignored. A case with different spatial location target volumes was designed to show the difference between CI_EV and traditional index.
Results: The DVH of VMAT could be divided into prescription dose region, rapid decline region, slow decline region and low dose region according to dose gradient. The volume of V_50% minus V_100% was contained in the rapid decline region, and the average d_50% was 1.91cm (1.65cm-2.26cm). The volume of GTV was correlated with d_50% (P<0.01). In the volume of V_50% minus V_100%, dose percentage was lineal correlated with d_PCT (R² ranged from 0.990 to 0.994), so parameter n was set as 1. CI_EV distinguished different intersection conditions in the example and produced reasonable results.
Conclusion: For VMAT plans, the dose in vivo is generally inversely proportional to the distance in the volume adjacent to GTV, although its distribution would be affected by the GTV size. A monitoring distance of about 2cm and a distance-penalty parameter of 1 may be reasonable for CI_EV in clinic practice. CI_EV based on the analysis of spatial distance and dose gradient fully takes into account the location of non-overlapped volumes without complex formulas and additional software.