SU‐E‐T‐404: Evaluation of the Effect of Spine Hardware for CyberKnife Spinal Stereotactic Radiosurgery

Abstract

Purpose:Spine hardware made of high‐Z materials such as titanium has the potential to affect the dose distribution around the metal rods in CyberKnife spinal stereotactic radiosurgery (SRS) treatments. The purpose of this work was to evaluate the magnitude of such effect retrospectively for clinical CyberKnife plans.Methods:The dose calculation was performed within the MultiPlan treatment planning system using the ray tracing (RT) and Monte Carlo (MC) method. A custom density model was created by extending the CT‐to‐Density table to titanium density of 4.5 g/cm3 with the CT number of 4095. To understand the dose perturbation caused by the titanium rod, a simple beam setup (7.5 mm IRIS collimator) was used to irradiate a mimic rod (5 mm) with overridden high density. Five patient spinal SRS cases were found chronologically from 2010 to 2015 in our institution. For each case, the hardware was contoured manually. The original plan was re‐calculated using both RT and MC methods with and without rod density override without changing clinical beam parameters.Results:The simple beam irradiation shows that there is 10% dose increase at the interface because of electron backscattering and 7% decrease behind the rod because of photon attenuation. For actual clinical plans, the iso‐dose lines and DVHs are almost identical (<2%) for calculations with and without density override for both RT and MC methods. However, there is a difference of more than 10% for D90 between RT and MC method.Conclusion:Although the dose perturbation around the metal rods can be as large as 10% for a single beam irradiation, for clinical treatments with complex beam composition the effect of spinal hardware to the PTV and spinal dose is minimal. As such, the MC dose algorithm without rod density override for CyberKnife spinal SRS is acceptable.