[Oral Communication] Analysis of Differences Between 99mTc-Macroaggregated Albumin-SPECT and 90Y-Microsphere-PET Dosimetry

Marilyne Kafrouni^1,2,4, Marjolaine Fourcade¹, Sébastien Vauclin², Alina-Diana Ilonca¹, Denis Mariano-Goulart^1,3,4, Fayçal Ben Bouallègue^1,3,4
1 Department of Nuclear Medicine, Montpellier University Hospital, Montpellier, France
2 DOSIsoft SA, Cachan, France
3 PhyMedExp, INSERM, CNRS, Montpellier, France
4 University of Montpellier, Montpellier, France

Presented at SNMMI 2018

ABSTRACT
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 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 (D_avg), 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. 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. 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.
Preliminary results indicate that the largest dose discrepancies can be explained by clinical differences in terms of administration procedure and particle distribution. This will be further analyzed with spatial distribution analysis.

Conclusion: The overall good correlation observed between predictive and post-treatment dosimetry confirm the MAA predictive value. The ⁹⁰Y-microsphere-PET tendency to underestimate ^99mTc-MAA-SPECT doses was also observed on phantom measurements and seem related mainly to imaging factors.