Radioembolization and the dynamic role of Y-90 PET/CT

Before the advent of tomographic imaging, it was postulated that decay of Y-90 to the 0(+) excited state of Zr-90 may result in emission of a positron electron pair. While the branching ratio for pair-production is small (similar to 32 x 10(-6)), PET has been successfully used to image Y-90 in numerous recent patients and phantom studies. Y-90 PET imaging has been performed on a variety of PET/CT systems, with and without time-of-flight (TOF) and/or resolution recovery capabilities as well as on both bismuth-germanate and lutetium yttrium orthosilicate (LYSO)-based scanners. On all systems, resolution and contrast superior to bremsstrahlung SPECT has been reported. The intrinsic radioactivity present in LYSO-based PET scanners is a potential limitation associated with accurate quantification of Y-90 However, intrinsic radioactivity has been shown to have a negligible effect at the high activity concentrations common in Y-90 radioembolization. Accurate quantification is possible on a variety of PET scanner models, with or without TOF, although TOF improves accuracy at lower activity concentrations. Quantitative Y-90 PET images can be transformed into 3-dimensional (3D) maps of absorbed dose based on the premise that the Y-90 activity distribution does not change after infusion. This transformation has been accomplished in several ways, although the most common is with the use of 3D dose-point-kernel convolution. From a clinical standpoint, Y-90 PET provides a superior post-infusion evaluation of treatment technical success owing to its improved resolution. Absorbed dose maps generated from quantitative PET data can be used to predict treatment efficacy and manage patient follow-up. For patients who receive multiple treatments, this information can also be used to provide patient specific treatment-planning for successive therapies, potentially improving response. The broad utilization of Y-90 PET has the potential to provide a wealth of dose response information, which may lead to development of improved radioembolization treatment-planning models in the future.