Analysis of differences between 99mTc-Macroaggregated Albumin-SPECT and 90Y-Microsphere-PET dosimetry

Objectives: The aim of this study is to analyze the differences between 99mTc-MAA-SPECT and 90Y-microsphere-PET based dosimetry at the voxel level using both patient and phantom dose data. Particularly, distinguishing the uncertainties related to SPECT/PET imaging differences (spatial resolution, partial volume effect, noise) and to clinical aspects (catheter positioning, modified vascularization, MAA/microsphere distribution). Methods Sixteen hepatocellular carcinoma patients treated with 90Y-glass microsphere radioembolisation were included in this study. For each patient, predictive and in vivo absorbed doses to tumor and normal liver were calculated. The mean (± standard deviation) administered activity was 3.8 GBq (±1.6) for a mean tumor volume of 575 mL (±436mL). A phantom study was also conducted on a 3D-printed liver phantom (1) to assess imaging-related dose differences. Measurements were performed with 99mTc and 90Y injected each time into two 40 mm diameter spheres: one considered as a simple homogeneous lesion and one considered as a necrotic lesion with an inner 25 mm diameter solid sphere. Doses for both patient and phantom studies were calculated using a dedicated software (PLANET® Dose, DOSIsoft, Cachan, France). Doses were computed using a convolution method based on voxel-S factors. In addition to the tumor average dose (Davg), dose metrics extracted from dose volume histograms were analyzed: minimum dose to 70% and 50% of the tumor volume (D70 and D50) and percentage of the tumor volume receiving at least 205 Gy (V205), as recommended in the literature for glass-microsphere treatments (2). Difference and correlation between predictive and absorbed doses to tumor and normal liver were assessed using a paired Student's t test and Pearson's correlation coefficient. Spatial concordance analysis at the voxel level between MAA and microsphere distributions on the patient cohort is ongoing. This should provide additional information to better understand the part of dose differences related to clinical factors. Results Dose results showed a good correlation between predictive and post-treatment dosimetry (Table 1). However, difference between 99mTc-MAA SPECT and 90Y-microsphere PET based dosimetry was significant for all tumor and normal liver dose metrics considered. The 90Y-microsphere-PET tendency to underestimate 99mTc-MAA-SPECT based doses was also observed on the liver phantom. Conclusions The overall good correlation observed between pred