P23. Simulation and measurement of CT parameters affecting Hounsfield units stability for radiotherapy planning

Abstract

Introduction Stability of Hounsfield units determines accuracy of dose calculation in radiotherapy. Variation of ±20 HU for soft dense tissues such as lung or ±50 UH for dense tissues like bone can cause error in dose calculation of 2%. Today scanners offer more numerous and complex acquisition and reconstruction tools. The objective of this study was to evaluate by simulation and measurement, parameters which have significant impact on the Hounsfield stability. Our goal was to establish most appropriate protocols with better image quality and acceptable patient dose. Methods During installation of CT Siemens sensation AS40, we wanted to improve protocols with better image quality, optimized patient exposure linked to constancy in the Housnfield units. We initially modeled our scanner and density Gammex CIRS phantom 062 with Speckcalc and Imasim and simulation tools. We modified acquisition (extended field of view, voltage, slice thickness, dose modulation) and reconstruction (reconstruction increment, extended Hounsfield scale…) settings. We compared these results with measurements. Once optimized settings by anatomy region done, we did a quantitative analysis with CT performance phantom CIRS and quality analysis of images obtained thanks to radiation oncologists. Results In addition to voltage influence, reconstructions parameters could lead to variations >10 HU of Hounsfield units primarily for low densities. In parallel we have defined the optimal image quality for each anatomy region. Conclusions This work allowed us to define a unique CT table in our treatment planning software with better image quality and optimized patient exposure. CT scans are more sophisticated and have tools that cannot be used without prior assessment of their impact on dose calculation. Role of manufacturers is compulsory to provide information on parameters affecting the stability of Hounsfield units.