New conformity indices based on the calculation of distances between the target volume and the volume of reference isodose

Abstract

Objective: To present conformity indices (CIs) based on the distance differences between the target volume (TV) and the volume of reference isodose (V RI ). Methods: The points on the three-dimensional surfaces of the TV and the V RI were generated. Then, the averaged distances between the points on the TV and the V RI were calculated (CI distance ). The performance of the presented CIs were evaluated by analysing six situations, which were a perfect match, an expansion and a reduction of the distance from the centroid to the V RI compared with the distance from the centroid to the TV by 10%, a lateral shift of the V RI by 3 cm, a rotation of the V RI by 45°and a spherical-shaped V RI having the same volume as the TV. The presented CIs were applied to the clinical prostate and head and neck (H&N) plans. Results: For the perfect match, CI distance was 0 with 0 as the standard deviation (SD). When expanding and reducing, CI distance was 10 and 210 with SDs ,1.3, respectively. With shifting and rotating of the V RI , the CI distance was almost 0 with SDs .11. The average value of the CI distance in the prostate and H&N plans was 0.13 67.44 and 6.04 623.27, respectively. Conclusion: The performance of the CI distance was equal or better than those of the conventional CIs. Advances in knowledge: The evaluation of target conformity by the distances between the surface of the TV and the V RI could be more accurate than evaluation with volume information.The goal of radiation therapy is the conformal delivery of a prescription dose to whole target volumes (TVs) homogeneously, while minimizing the dose delivered to adjacent normal tissues. [1][2][3][4] In order to achieve this goal, various state-of-the-art techniques, such as intensitymodulated radiation therapy and volumetric-modulated arc therapy (VMAT) have gained popularity.5-12 A tool to compare the quality of different plans in terms of target conformity was required, thus the conformity index (CI) was proposed by the Radiation Therapy Oncology Group (RTOG) in 1993 and described in Report 62 of the International Commission on Radiation Units and Measurements. 13,14The CI is an indicator that assesses the degree of congruence between a shape of the reference isodose volume (V RI ) and the shape of the TV. Two concepts are included in the CI, which are target coverage and degree of normal tissue sparing in the proximity of the target. Although this could be verified by a manual review of dose distributions calculated on patient CT images slice by slice, detailed comparisons among several treatment plans would be inconvenient.2 The conformity could also be verified by reviewing dose-volume histograms (DVHs) of each structure calculated by the treatment planning system. Since the pre-requisite of the evaluation with DVHs is the contouring of organs at risk (OARs), healthy tissues crossed by the beam could not be taken into account owing to the difficulties of contouring and the absence of sufficient data concerning the dose-volumetric tolerance information of these ti...

Analysis of different evaluation indexes for prostate stereotactic body radiation therapy plans: conformity index, homogeneity index and gradient index

“…Park et al . also presented a new CI based on the distance between the surfaces of the TV and reference dose volume, which not only evaluates the shape prescription isodose, but also the target coverage …”

Optimal collimator rotation based on the outline of multiple brain targets in VMAT

“…For the target volumes, the evaluated dose-volumetric parameters were the mean dose, maximum dose, minimum dose, dose received by at least 99% volume of the target volume (D 99% ), D 95% , D 5% , D 1% , conformity index (CI), homogeneity index (HI), and gradient measure (GM). The CI, HI, and GM are defined as follows [ 2 , 17 – 19 ]: …”

Dosimetric effects of sectional adjustments of collimator angles on volumetric modulated arc therapy for irregularly-shaped targets

“…For the target volume, the evaluated dose-volumetric parameters were the mean dose, maximum dose, minimum dose, dose received by at least 97% volume of the target volume (D 97% ), the percent volume of the target volume irradiated by at least 95% of the prescription dose (V 95% ), V 97% , conformity index suggested by Paddick (CI Paddick ), homogeneity index (HI), and gradient measure (GM). The CI Paddick , HI, and GM are defined as follows [18–22]: where, the TV prescription dose is the target volume irradiated by the prescription dose, TV is the target volume, V prescription dose is the volume of the prescription dose, and R x is the sphere radius of which the volume is the same as the volume of isodose of x .…”