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.
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
Since US Food and Drug Administration approval of 18-fluorodeoxyglucose as a positron tracer, and the development of hybrid positron emission tomography/ computed tomography machines, there has been a great increase in clinical application and progress in the field of nuclear molecular imaging. However, not underestimating the value of 18 F, there are known limitations in the use of this cyclotron-produced positron tracer. We hence turn our focus to an emerging positron tracer, 68 Ga, and examine the advantages, current clinical uses and potential future applications of this radioisotope. THE DEVELOPMENT OF POSITRON EMISSION TOMOGRAPHYPositron emission tomography (PET) imaging is essentially a story of a technique in wait of a technology. Since the discovery of positron emission in 1933 by Thibaud and Joliot et al, and the subsequent report of the coincident nature of emissions by Klemperer and Beringer, it has in effect taken close to half a century for the full realization of PET imaging in mainstream medical practice [1] . The history of PET development is a fascinating look into the technological advances in molecular medicine, and the account of Terry Jones provides fascinating reading [1] . The first use of positron tracers was likely performed in the 1940s using 11 CO in animal models [2] , with its first possible use in humans performed in the 1950s at the Hammersmith Hospital in London, United Kingdom using 15 O2 in studies of lung ventilation [3] . This was followed by increasing use of positron tracers in the physiological assessment of lung function, which resulted in the installation of the world's first hospital-based cyclotron in 1955. Subsequently, development shifted into myocardial perfusion [4] , cerebral perfusion [5][6][7] , and of course, glucose metabolism [8][9][10][11] . This development in positron tracers mirrored the progress in positron imaging. In the 1970s, Massachusetts General Hospital developed, what was then, the most advanced coincidence positron camera system, with a spatial resolution of approximately 1 cm [12] . This was followed by developments predominantly in single photon emission computed tomography (SPECT) with work done by Kuhl, Budinger and Gullberg, and in 1974, there were reports of the development of a dedicated single-plane positron emission transaxial tomograph, which was the precursor to the current PET systems.These developments explain the slow implementation of PET into clinical practice, as synchronous developments in both tracer and detector technology were reWorld Journal of Radiology W J R
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