Author: Jianrong Dai, Jun Dang, Enzhuo Quan, Yong Sang, Jianan Wu 👨🔬
Affiliation: Cancer Hospital Chinese Academy of Medical Sciences, Shenzhen Center, National Cancer Center / National Clinical Research Center for Cancer / Cancer Hospital & Shenzhen Hospital / Chinese Academy of Medical Sciences and Peking Union Medical College, Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College 🌍
Purpose: The current conformity index (CI) formula is distorted for radiotherapy plans with multiple, non-enclosing targets. We redefined the volume of reference isodose (VTV) parameter in the CI formula to more accurately calculate the CI value for individual targets in multi-target plans, providing a useful dosimetric evaluation tool for clinical applications.
Methods: The new VTV formula for PTVi is a delicate calculation of the intersection of PTVi's expanded volume and its prescription isodose volume, aiming to better reflect the true conformity of individual targets in the CI calculation values for multi-target planning. We selected 15 breast cancer (BC) plans with two target volumes and 15 nasopharyngeal cancer (NPC) plans with five target volumes. For each plan, VTVnew and CInew were calculated using the proposed formula, while VTVold and CIold were calculated using traditional formulas. Paired, two-tailed T-Test and Pearson's correlation analyses were conducted.
Results: The CI values calculated using the proposed method shows better agreement with visual conformity evaluation for individual targets. For BC, VTV and CI values calculated by the two methods for the two targets showed statistically significant differences, with VTVnew and CInew being lower and higher respectively than those calculated by traditional methods. Similar results were found for four of the targets in NPC, but not for the one that encloses other targets. There was a positive correlation between the supra-clavical target for BC (r=0.640, p<0.05) and the RPN target for NPC (r=0.660, p<0.05).
Conclusion: The new VTV formula eliminates the influence of other target doses, retaining only the prescription dose range of the analysis target. Its calculated CI more accurately reflects target conformity compared to traditional formulas. This formula is also applicable to single target volumes and is recommended for calculating CI values in multi-target plans like BC and NPC.