Feasibility of imaging (90)Y microspheres at diagnostic activity levels for hepatic radioembolization treatment planning

PURPOSE: Prior to (90)Y hepatic radioembolization, a dosage of (99m)Tc‐macroaggregated albumin ((99m)Tc‐MAA) is administered to simulate the distribution of the (90)Y‐loaded microspheres. This pretreatment procedure enables lung shunt estimation, detection of potential extrahepatic depositions, and estimation of the intrahepatic dose distribution. However, the predictive accuracy of the MAA particle distribution is often limited. Ideally, (90)Y microspheres would also be used for the pretreatment procedure. Based on previous research, the pretreatment activity should be limited to the estimated safety threshold of 100 MBq, making imaging challenging. The purpose of this study was to evaluate the quality of intra‐ and extrahepatic imaging of (90)Y‐based pretreatment positron emission tomography/computed tomography (PET/CT) and quantitative single photon emission computed tomography (SPECT)/CT scans, by means of phantom experiments and a patient study. METHODS: An anthropomorphic phantom with three extrahepatic depositions was filled with (90)Y chloride to simulate a lung shunt fraction (LSF) of 5.3% and a tumor to nontumor ratio (T/N) of 7.9. PET /CT (Siemens Biograph mCT) and Bremsstrahlung SPECT/CT (Siemens Symbia T16) images were acquired at activities ranging from 1999 MBq down to 24 MBq, representing post‐ and pretreatment activities. PET/CT images were reconstructed with the clinical protocol and SPECT/CT images were reconstructed with a quantitative Monte Carlo‐based reconstruction protocol. Estimated LSF, T/N, contrast to noise ratio of all extrahepatic depositions, and liver parenchymal and tumor dose were compared with the phantom ground truth. A clinically reconstructed SPECT/CT of 150 MBq (99m)Tc represented the current clinical standard. In addition, a (90)Y pretreatment scan was simulated for a patient by acquiring posttreatment PET/CT and SPECT/CT data with shortened acquisition times. RESULTS: At an activity of 100 MBq (90)Y, PET/CT overestimated LSF [+10 percentage point (pp)], underestimated liver parenchymal dose (−3 Gy/GBq), and could not detect the extrahepatic depositions. SPECT/CT more accurately estimated LSF (−0.7 pp), parenchymal dose (−0.3 Gy/GBq) and could detect all three extrahepatic depositions. (99m)Tc SPECT/CT showed similar accuracy as (90)Y SPECT/CT (LSF: +0.2 pp, parenchymal dose: +0.4 Gy/GBq, all extrahepatic depositions visible), although the noise level in the liver compartment was considerably lower for (99m)Tc SPECT/CT compared to (90)Y SPECT/CT. The patient’s SPECT/CT simulating a pretreatment (90)Y procedure accurately represented the posttreatment (90)Y microsphere distribution. CONCLUSIONS: Quantitative SPECT/CT of 100 MBq (90)Y could accurately estimate LSF, T/N, parenchymal and tumor dose, and visualize extrahepatic depositions.