Purpose: The quality of image-based, voxel-level, radiopharmaceutical therapy (RPT) dosimetry is dependent on the accuracy of images. Artifacts caused by metallic implants can affect both the image upon which dose calculations are performed and the corresponding SPECT image if CT based attenuation correction is utilized. This work investigates the impact of CT artifacts upon RPT dosimetry estimates for a phase I clinical trial in humans investigating the combination of external beam radiotherapy (EBRT) and RPT.
Methods: A patient with metallic dental fillings underwent both Iodine-131 SPECT/CT imaging at multiple timepoints and a treatment planning CT (TPCT) for EBRT planning. Six scenarios were investigated to examine dependencies on portions of the dosimetry workflow. Affected slices of the low dose CT images were modified using density overrides and reconstructed using GE Xeleris 4.0 to correct artifacts. SPECT/CT images were registered to the TPCT using Elastix deformable image registration modules in 3D Slicer. Absorbed doses were calculated using a Monte Carlo based RPT dosimetry engine, RAPID.
Results: For a TPCT scenario isolating the effect of reconstruction, the mean activity concentration and mean dose to the GTV were underestimated by 1.92% and 1.89%, respectively. The PTV mean dose was underestimated by 1.74%, likely attributable to an attenuation under correction. For normal tissues, voxel-level dose overestimations occurred in the mandible (0.2 Gy, 8.2% of mean dose) and the oral cavity (0.45 Gy,13.4% of mean dose), likely attributable to attenuation overcorrection. For low dose CT based dosimetry scenarios, small differences were noted but the mean dose to the PTV was largely unchanged.
Conclusion: Instances where artifacts are present can affect RPT dosimetry. If tumor volumes or critical structures are in close proximity to an artifact affected area, care should be taken to correctly account for artifacts in the final dosimetry evaluation.
Funding Support, Disclosures, and Conflict of Interest: This project is supported by the Specialized Program of Research Excellence (SPORE) program, through the NIH National Institute for Dental and Craniofacial Research (NIDCR) and National Cancer Institute (NCI), grant P50DE026787. BB and JG are co-founders of Voximetry, Inc., a nuclear medicine dosimetry company in Madison, WI.
IM/TH- Radiopharmaceutical Therapy: Dose estimation: Monte Carlo