PET/CT- Based Dosimetry in 90Y-Microsphere Selective Internal Radiation Therapy: Single Cohort Comparison With Pretreatment Planning on 99mTc-MAA Imaging and Correlation With Treatment Efficacy

Y-90 PET/CT can be acquired after Y-90-microsphere selective radiation internal therapy (SIRT) to describe radioactivity distribution. We performed dosimetry using Y-90-microsphere PET/CT data to evaluate treatment efficacy and appropriateness of activity planning from Tc-99m-MAA scan and SPECT/CT.Twenty-three patients with liver malignancy were included in the study. Tc-99m-MAA was injected during planning angiography and whole body Tc-99m-MAA scan and liver SPECT/CT were acquired. After SIRT using Y-90-resin microsphere, Y-90-microsphere PET/CT was acquired. A partition model (PM) using 4 compartments (tumor, intarget normal liver, out-target normal liver, and lung) was adopted, and absorbed dose to each compartment was calculated based on measurements from Tc-99m-MAA SPECT/CT and Y-90-microsphere PET/CT, respectively, to be compared with each other. Progression-free survival (PFS) was evaluated in terms of tumor absorbed doses calculated by Tc-99m-MAA SPECT/CT and Y-90-microsphere PET/CT results.Lung shunt fraction was overestimated on Tc-99m-MAA scan compared with Y-90-microsphere PET/CT (0.0600.037 vs. 0.018 +/- 0.026, P<0.01). Tumor absorbed dose exhibited a close correlation between the results from Tc-99m-MAA SPECT/CT and Y-90-microsphere PET/CT (r=0.64, P<0.01), although the result from Tc-99m-MAA SPECT/CT was significantly lower than that from Y-90-microsphere PET/CT (135.4 +/- 64.2 Gy vs. 185.0 +/- 87.8 Gy, P<0.01). Absorbed dose to in-target normal liver was overestimated on Tc-99m-MAA SPECT/CT compared with PET/CT (62.6 +/- 38.2 Gy vs. 45.2 +/- 32.0 Gy, P=0.02). Absorbed dose to out-target normal liver did not differ between Tc-99m-MAA SPECT/CT and Y-90-microsphere PET/CT (P=0.49). Patients with tumor absorbed dose >200 Gy on Y-90-microsphere PET/CT had longer PFS than those with tumor absorbed dose 200 Gy (286 +/- 56 days vs. 92 +/- 20 days, P=0.046). Tumor absorbed dose calculated by Tc-99m-MAA SPECT/CT was not a significant predictor for PFS.Activity planning based on Tc-99m-MAA scan and SPECT/CT can be effectively used as a conservative method. Post-SIRT dosimetry based on Y-90-microsphere PET/CT is an effective method to predict treatment efficacy.