Abstract
BackgroundHigh activities of holmium-166 (166Ho)–labeled microspheres are used for therapeutic radioembolization, ideally directly followed by SPECT imaging for dosimetry purposes. The resulting high-count rate potentially impacts dead time, affecting the image quality and dosimetric accuracy. This study assesses gamma camera performance and SPECT image quality at high 166Ho activities of several GBq. To this purpose, the liver compartment, including two tumors, of an anthropomorphic phantom was filled with 166Ho-chloride, with a tumor to non-tumorous liver activity concentration ratio of 10:1. Multiple SPECT/CT scans were acquired over a range of activities up to 2.7 GBq. Images were reconstructed using a commercially available protocol incorporating attenuation and scatter correction. Dead time effects were assessed from the observed count rate in the photopeak (81 keV, 15% width) and upper scatter (118 keV, 12% width) window. Post reconstruction, each image was scaled with an individual conversion factor to match the known total activity in the phantom at scanning time. The resulting activity concentration was measured in the tumors and non-tumorous liver. The image quality as a function of activity was assessed by a visual check of the absence of artifacts by a nuclear medicine physician. The apparent lung shunt fraction (nonzero due to scatter) was estimated on planar and SPECT images.ResultsA 20% count loss due to dead time was observed around 0.7 GBq in the photopeak window. Independent of the count losses, the measured activity concentration was up to 100% of the real value for non-tumorous liver, when reconstructions were normalized to the known activity at scanning time. However, for tumor spheres, activity concentration recovery was ~80% at the lowest activity, decreasing with increasing activity in the phantom. Measured lung shunt fractions were relatively constant over the considered activity range.ConclusionsAt high 166Ho count rate, all images, visually assessed, presented no artifacts, even at considerable dead time losses. A quantitative evaluation revealed the possibility of reliable dosimetry within the healthy liver, as long as a post-reconstruction scaling to scanning activity is applied. Reliable tumor dosimetry, instead, remained hampered by the dead time.
Highlights
High activities of holmium-166 (166Ho)–labeled microspheres are used for therapeutic radioembolization, ideally directly followed by single photon emission computed tomography (SPECT) imaging for dosimetry purposes
In 166Ho radioembolization, single photon emission computed tomography (SPECT)/CT is used for dosimetry purposes both for treatment planning and post-treatment evaluation, relying on the additional 80.6 keV gamma emission of 166Ho
The two spectra were normalized for the sum of counts in the scatter window centered at 118 keV
Summary
High activities of holmium-166 (166Ho)–labeled microspheres are used for therapeutic radioembolization, ideally directly followed by SPECT imaging for dosimetry purposes. When using 166Ho-microspheres, approximately 3.8 GBq of 166Ho is injected per kilogram of treated liver mass [3] in order to deliver 60 Gy to the injected liver portion, as indicated by the manufacturer, and assuming a homogenous uptake throughout the liver This typically results in injected activities of 5–10 GBq. the abundancy of the 80.6 keV 166Ho photopeak is only 6.7%, the count rate immediately after treatment is very high. There is a considerable amount of bremsstrahlung, caused by β− (Emax:1.77 Mev/49%, 1.85MeV/50%) interactions within the patient [4, 5] This potentially impacts image quality and quantitative accuracy because of gamma camera dead time effects.
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