Yttrium-90 (Y-90) radioembolization is becoming established as an effective therapeutic modality for inoperable liver tumors. For resin microspheres, the 'body surface area (BSA)' method and the partition model can both be used for Y-90 activity calculation. The BSA method is semi-empirical, but more commonly used due its simplicity. The partition model is more accurate, scientifically sound and personalized, but less popular due to its complexity. This article provides a technical comparison of both methods with an emphasis on its clinical implications. Future dosimetric techniques for Y-90 radioembolization based on emerging technologies are also discussed.

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During pre-therapy evaluation for yttrium-90 (Y-90) radioembolization, it is uncommon to find severe imaging discordance between hepatic angiography versus technetium-99m-macroaggregated albumin (Tc-99m-MAA) single photon emission computed tomography with integrated low-dose CT (SPECT/CT). The reasons for severe imaging discordance are unclear, and literature is scarce. We describe 3 patients with severe imaging discordance, whereby tumor angiographic contrast hypervascularity was markedly mismatched to the corresponding Tc-99m-MAA SPECT/CT, and its clinical impact. The incidence of severe imaging discordance at our institution was 4% (3 of 74 cases). We postulate that imaging discordance could be due to a combination of 3 factors: (1) different injection rates between soluble contrast molecules versus Tc-99m-MAA; (2) different arterial flow hemodynamics between soluble contrast molecules versus Tc-99m-MAA; (3) eccentric release position of Tc-99m-MAA due to microcatheter tip location, inadvertently selecting non-target microparticle trajectories. Tc-99m-MAA SPECT/CT more accurately represents hepatic microparticle biodistribution than soluble contrast hepatic angiography and should be a key criterion in patient selection for Y-90 radioembolization. Tc-99m-MAA SPECT/CT provides more information than planar scintigraphy to guide radiation planning and clinical decision making. Severe imaging discordance at pre-therapy evaluation is ominous and should be followed up by changes to the final vascular approach during Y-90 radioembolization.

ABSTRACT. Yttrium-90 ( 90 Y) internal pair production can be imaged by positron emission tomography (PET)/CT and is superior to bremsstrahlung single-photon emission CT/CT for evaluating hepatic 90 Y microsphere biodistribution. We illustrate a case of 90 Y imaging using first generation PET/CT technology, producing high-quality images for qualitative diagnostic purposes. We illustrate a case of 90 Y PET/CT acquired using a first generation PET/CT scanner (Biograph WO; Siemens, Erlangen, Germany), producing high-resolution images of 90 Y microsphere biodistribution (Figures 1-3). Our imaging protocol is detailed in Table 1. Total coincidences were 4.7 million over 40 min (1.2 GBq injected). No effort was made to reduce bremsstrahlung X-rays or background counts from the lutetium-based PET crystal. Background noise was visually minimised by adjusting the PET threshold. Images were analysed qualitatively for diagnostic purposes. Quantitation of

The use of the single parameter U/V2 could produce the diagnosis of renal obstruction with a high level of accuracy. This method has the potential to be used as a benchmark to distinguish the severity level of the renal obstruction.