SU‐E‐T‐450: Dose Perturbation of Spinal Metal Implants in Proton Beam Therapy

Abstract

Purpose: To investigate the effect of dose perturbations for different spinal metallic screws in proton beam therapy. Methods: A 5.5mm (diameter) × 45mm (length) stainless steel (SS) screw and a 5.5mm × 35mm titanium (Ti) screw used for spinal fixation were CT‐scanned in a small water phantom. The CT data was processed with a metal artifact reduction (MAR) software. Two treatment plans were generated using Eclipse treatment planning system (TPS) for each metal screw with a proton beam parallel and perpendicular to the longitudinal axis of the metal, respectively. The calculated dose profiles were compared with measured ion chamber and EBT2 films results. Results: With the perpendicular setup, the measured dose immediately downstream from the screw exhibited dose enhancement up to 12% and 8% for SS and Ti respectively, but such dose perturbation was not observed outside the lateral shadow 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 1cm outside of the lateral shadow of the screws. The measured dose enhancement was only observed within 5mm 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 1–2cm outside of the lateral edge of the metals. For the parallel setup, no significant dose perturbation was detected at lateral distance beyond 3 mm away from both screws. Conclusion: Streaking artifacts in CT images are significantly reduced with MAR but significant dose discrepancies between measurements and TPS in close proximity of the high‐Z inhomogeneities are clearly visible. Dose enhancement effect is not correctly modeled by TPS, thus extra caution should be given for dosimetry with metallic implants.