Comparison of predictive, pre-therapy MAA SPECT/CT dosimetry vs. post-therapy Y-90 PET/CT dosimetry

Introduction: The aim of our work is to compare predictive, pre-therapy Tc-99m macroaggregated albumin (MAA) SPECT/CT based dosimetry vs. post-therapy Y-90 PET/CT derived dosimetry in patients undergoing locoregional therapy to the liver with Y-90 microspheres.

Methods: After treatment with Y-90 microspheres 16 patients (3 female:13 male, mean age 68.2±5.3y), underwent PET/CT imaging. Images were acquired for 15 min. The low dose, non-diagnostic CT images from PET/CT were used for localization of the Y-90 microspheres and attenuation correction. The reconstruction matrix size was 200x200x75 mm and voxel size 4.07x4.07x3.00 mm. Two weeks prior to Y-90 therapy the patients also had MAA SPECT/CT performed on a dual-head Infinia SPECT/CT gamma camera (GE Medical Systems, Milwaukee, WI), with LEHR collimator and activity of 185 MBq was used. For the SPECT/CT studies, 60 views at 30 s per view were acquired and images were reconstructed using an ordered subset expectation maximization (OSEM) algorithm. Attenuation correction was performed using the CT scan, the reconstruc­tion matrix size was 128 × 128 and the pixel size was 4.4 mm. The CT was performed with 10mm axial sampling, 140 kVp, 2.5 mA, 2.6 rpm and with 256 × 256 matrix size. The data were reconstructed using the vendor's reconstruction software and were transferred to a common platform where the SPECT/CT and PET/CT images (Fig.1) were used to calculate Y-90 dosimetry using MIM 7.1software (MIM Software Inc., Cleveland, Ohio). Local deposition method with known activity of Y-90 was used to calculate dosimetry.

Results: For 16 patients, the mean liver, tumor (T) and normal tissue (NT) doses (mean ± SD) for MAA SPECT/CT studies were, 42.02±22.36 Gy, 234.72±172.54 Gy and 41.22 ±20.37Gy, respectively. For Y-90 PET/CT studies the same values were 49.85±24.88 Gy, 314.07±197.02 Gy and 46.18 ±20.52Gy, respectively. The average differences between the liver, T and NT MAA and Y-90 doses were 18%, 25% and 15%, respectively and the linear correlation between the respective doses were r= 0.86, r= 0.71 r= 0.95, respectively.

Conclusions: Our results indicate that, post-therapy Y-90 PET/CT dosimetry average values have tendency to be slightly higher in comparison with MAA SPECT/CT dosimetry values. Also, if the catheter tip positions in MAA and Y-90 studies are positioned very similar and far from major bifurcations, the main source of differences between MAA and Y-90 dosimetry values are attributed to differences in respective ROI's. This is especially case for small, less than 10cm³, tumors, where the differences can be large. The best correlation between MAA and Y-90 dosimetry was achieved for NT, providing very good estimates for not exceeding 40 Gy guideline limit for NT in Y-90 therapies.

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