Dosimetric impact of CT-to-ED curves on high atomic number heterogeneity irradiations

With the evolution of planning systems and the reduction of dose calculation uncertainties, the correction of heterogeneities became necessary. The presence of high density metal alloys is still one of the limitations and source of inaccuracy in dose calculation. Although accurate dose calculation algorithms exist, the main difficulty lies in establishing CT-to-ED correction curves for appropriate HU values. Most current CT scanners operate in 12 bits, saturating the Hounsfield number around 3000 HU, which is insufficient for most materials with a high atomic number. However, there are already devices that operate with 16 bits and that do not saturate in the presence of these materials, resulting in more complete information on physical properties. The objective of this work is to evaluate, in irradiations of high Z heterogeneities, the dosimetric impact when correction curves that include high HU values are used. The factors that influence the construction of the CT-to-ED curve and the possible sources of process uncertainty were analyzed. The curves were introduced in the planning system and the results compared with experimental measurements. The results indicate a dependence on the energy of the tomograph when high atomic number materials are included in the construction of the CT-To-ED curve. The results also show a greater agreement between measured and calculated data when expanded CT-to-ED curves are used in treatment planning systems. (author)