Mathematical analysis of gamma index on pretreatment patient-specific quality assurance with octavius 4D rotational...

Gamma index is one of the widely used metrics to evaluate the patient-specific quality assurance parameters for the delivery of accepted treatment plans. The dimensionality of gamma metric has changed from two-dimensional to three-dimensional with due course of time, and presently, we are in a verge to assess the three-dimensional metric in a volumetric scale. The present study aimed at determining the mathematical relationship between 2D, 3D, and 3D volumetric gamma index for patient-specific quality assurance parameters using the Octavius 4D rotational phantom.

Twenty-five patients planned with volumetric modulated radiotherapy for different sites were selected for the study to analyze the pretreatment parameters correlation using 2D- Array 1500 rotational Octavius phantom. The data were collected in both local and global dose modes at 5%/5 mm, 4%/4 mm, 3%/3 mm, and 2%/2 mm for 2D, 3D, and Volmetric 3D gamma index. The passing criterion of Gamma <=1.0 is applicable throughout the study.

A significant jump in the passing rate percentage was found from 2D planar to 3D planar mode analysis. Decreased passing percentage values in both local and global modes from 3D planar to 3D volumetric mode analysis were recorded. The 2D and 3D planar analysis shows a significant level of correlation which is less pronounced in 3D planar to 3D volumetric gamma metric evaluation.

The radiotherapy centers having the license for 2D planar gamma index analysis can retrace the results into 3D planar analysis, which is always a better passing percentage rate, and no such implication of 3D volumetric analysis was found as the passing rate significantly shows a high degree of entropy in results. The effective use of 3D volumetric gamma metric can be appreciable if the CT overlay parameter is used in assessment and qualitatively the quantification can be predicted.