Abstract
The purpose of this study was to investigate the effect of dose perturbations for two metallic spinal screw implants in proton beam therapy in the perpendicular and parallel beam geometry. A 5.5 mm (diameter) by 45 mm (length) stainless steel (SS) screw and a 5.5 mm by 35 mm titanium (Ti) screw commonly used for spinal fixation were CT‐scanned in a hybrid phantom of water and solid water. The CT data were processed with an orthopedic metal artifact reduction (O‐MAR) algorithm. Treatment plans were generated for each metal screw with a proton beam oriented, first parallel and then perpendicular, to the longitudinal axis of the screw. The calculated dose profiles were compared with measured results from a plane‐parallel ion chamber and Gafchromic EBT2 films. For the perpendicular setup, the measured dose immediately downstream from the screw exhibited dose enhancement up to 12% for SS and 8% for Ti, respectively, but such dose perturbation was not observed outside the lateral edges of the screws. The TPS showed 5% and 2% dose reductions immediately at the interface for the SS and Ti screws, respectively, and up to 9% dose enhancements within 1 cm outside of the lateral edges of the screws. The measured dose enhancement was only observed within 5 mm from the interface along the beam path. At deeper depths, the lateral dose profiles appeared to be similar between the measurement and TPS, with dose reduction in the screw shadow region and dose enhancement within 1–2 cm outside of the lateral edges of the metals. For the parallel setup, no significant dose perturbation was detected at lateral distance beyond 3 mm away from both screws. Significant dose discrepancies exist between TPS calculations and ion chamber and film measurements in close proximity of high‐Z inhomogeneities. The observed dose enhancement effect with proton therapy is not correctly modeled by TPS. An extra measure of caution should be taken when evaluating dosimetry with spinal metallic implants.PACS number: 87.50.sj
Highlights
334 Jia et al.: Metallic implants in proton therapy more patients receiving proton radiation therapy with high-Z implants such as dental alloys, hip prosthesis, prostate gold markers, and spinal metallic screws.Modern radiotherapy uses X-ray computed tomography (CT) for most treatment planning
Several groups have studied the dose perturbation in proton radiotherapy from various fiducial markers such as gold, stainless steel, and titanium.[2,3,4,5,6,7] They demonstrated that metal fiducials cause dose perturbation in dose distribution and that the dose perturbation in proton therapy is dependent on marker size, placement depth, and orientation with respect to proton beam axis
Streaking artifacts in the planning CT images were significantly suppressed with orthopedic metal artifact reduction (O-MAR), which is consistent with published reports.[15,19] Besides Philips, many vendors have included metal artifact reduction in their CT software, such as the Smart Metal Artifact Reduction (SMART) provided by GE, the Metal Deletion Technique (MDT) provided by ReVision Radiology, and the iterative metal artifact reduction algorithm on Siemens platforms
Summary
334 Jia et al.: Metallic implants in proton therapy more patients receiving proton radiation therapy with high-Z implants such as dental alloys, hip prosthesis, prostate gold markers, and spinal metallic screws.Modern radiotherapy uses X-ray computed tomography (CT) for most treatment planning. It has been reported that knowing the size and shape of a high-Z heterogeneity is more important than the accuracy of knowing the electron density and, the relative stopping power.[1] The problem is enhanced in proton therapy if a metal artifact-contaminated CT dataset is used for dose calculations. Several groups have studied the dose perturbation in proton radiotherapy from various fiducial markers such as gold, stainless steel, and titanium.[2,3,4,5,6,7] They demonstrated that metal fiducials cause dose perturbation in dose distribution and that the dose perturbation in proton therapy is dependent on marker size, placement depth, and orientation with respect to proton beam axis. Examples of potential severe proton beam range errors and target underdosage in the presence of metal have been reported.[8,9]
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