In hepatic 90 Y radioembolization, pretreatment 99m Tc-macroaggregated albumin ( 99m Tc-MAA) nuclear imaging is used for lung shunt analysis, evaluation of extrahepatic deposition, and sometimes for treatment planning, using a partition model. A high level of agreement between pretreatment 99m Tc-MAA distribution and final 90 Ymicrosphere distribution is assumed. The aim of this study was to investigate the value of pretreatment 99m Tc-MAA SPECT to predict intrahepatic posttreatment 90 Y-microsphere distribution.

BackgroundAfter yttrium-90 (90Y) microsphere radioembolization (RE), evaluation of extrahepatic activity and liver dosimetry is typically performed on 90Y Bremsstrahlung SPECT images. Since these images demonstrate a low quantitative accuracy, 90Y PET has been suggested as an alternative. The aim of this study is to quantitatively compare SPECT and state-of-the-art PET on the ability to detect small accumulations of 90Y and on the accuracy of liver dosimetry.Methodology/Principal FindingsSPECT/CT and PET/CT phantom data were acquired using several acquisition and reconstruction protocols, including resolution recovery and Time-Of-Flight (TOF) PET. Image contrast and noise were compared using a torso-shaped phantom containing six hot spheres of various sizes. The ability to detect extra- and intrahepatic accumulations of activity was tested by quantitative evaluation of the visibility and unique detectability of the phantom hot spheres. Image-based dose estimates of the phantom were compared to the true dose. For clinical illustration, the SPECT and PET-based estimated liver dose distributions of five RE patients were compared. At equal noise level, PET showed higher contrast recovery coefficients than SPECT. The highest contrast recovery coefficients were obtained with TOF PET reconstruction including resolution recovery. All six spheres were consistently visible on SPECT and PET images, but PET was able to uniquely detect smaller spheres than SPECT. TOF PET-based estimates of the dose in the phantom spheres were more accurate than SPECT-based dose estimates, with underestimations ranging from 45% (10-mm sphere) to 11% (37-mm sphere) for PET, and 75% to 58% for SPECT, respectively. The differences between TOF PET and SPECT dose-estimates were supported by the patient data.Conclusions/SignificanceIn this study we quantitatively demonstrated that the image quality of state-of-the-art PET is superior over Bremsstrahlung SPECT for the assessment of the 90Y microsphere distribution after radioembolization.

In clinical practice, lung absorbed doses are significantly overestimated by pretreatment diagnostic (99m)Tc-MAA imaging. Pretreatment diagnostic (166)Ho-microsphere SPECT/CT imaging accurately predicts lung absorbed doses after (166)Ho radioembolization.

The evaluation of radiation absorbed doses in tumorous and healthy tissues is of increasing interest for 90 Y microsphere radioembolization of liver malignancies. The objectives of this work were to introduce and validate a new reconstruction method for quantitative 90 Y bremsstrahlung SPECT to improve posttreatment dosimetry. Methods: A fast Monte Carlo simulator was adapted for 90 Y and incorporated into a statistical reconstruction algorithm (SPECT-MC). Photon scatter and attenuation for all photons sampled from the full 90 Y energy spectrum were modeled during reconstruction by Monte Carlo simulations. The energy-and distance-dependent collimator-detector response was modeled with precalculated convolution kernels. The National Electrical Manufacturers Association 2007/International Electrotechnical Commission 2008 image quality phantom was used to quantitatively evaluate the performance of SPECT-MC in comparison with those of state-of-the-art clinical SPECT reconstruction and PET. The liver radiation absorbed doses estimated by SPECT, PET, and SPECT-MC were evaluated in 5 patients consecutively treated with radioembolization. Results: In comparison with state-of-the-art clinical 90 Y SPECT reconstruction, SPECT-MC substantially improved image contrast (e.g., from 25% to 88% for the 37-mm sphere) and decreased the mean residual count error in the lung insert (from 73% to 15%) at the cost of higher image noise. Image noise and the mean count error were lower for SPECT-MC than for PET. Image contrast was higher in the larger spheres (diameter of $28 mm) but lower in the smaller spheres (#22 mm) for SPECT-MC than for PET. In the clinical study, mean absorbed dose estimates in liver regions with high absorbed doses were consistently higher for SPECT-MC than for SPECT (P 5 0.0625) and consistently higher for SPECT-MC than for PET (P 5 0.0625). Assessment of the 90 Y microsphere distribution can be performed by imaging bremsstrahlung photons with a SPECT camera or by imaging annihilation photons with a PET camera. Posttreatment dosimetry with 90 Y PET has advantages over SPECT, mainly because of higher resolution and image contrast (6,7). However, the low positron branch (32 · 10 26 ) in 90 Y decay requires a stateof-the-art lutetium-(yttrium)-orthosilicate time-of-flight PET/CT scanner to obtain images with sufficiently high quantitative accuracy for dosimetry purposes (8,9). Posttreatment imaging with a standard SPECT/CT system may be a more widely available and cost-effective option for most centers, but the image quality (IQ) of state-of-the-art clinical 90 Y bremsstrahlung SPECT is still limited (7). The wide range (0-2.3 MeV) and continuous nature of the 90 Y bremsstrahlung photon spectrum prohibit the use of simple energy window-based scatter rejection and correction techniques, hinder attenuation correction based on single-photon energy, and require compensation for collimator-and detector-related imagedegrading effects, such as collimator scatter, lead x-rays, septal penetration, camera (back)scatter, an...