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. 2021 Jul 23;8(1):55.
doi: 10.1186/s40658-021-00397-0.

A multicentre and multi-national evaluation of the accuracy of quantitative Lu-177 SPECT/CT imaging performed within the MRTDosimetry project

Johannes Tran-Gia  1 , Ana M Denis-Bacelar  2 , Kelley M Ferreira  2 , Andrew P Robinson  2   3   4 , Nicholas Calvert  3 , Andrew J Fenwick  2   5 , Domenico Finocchiaro  6   7 , Federica Fioroni  6 , Elisa Grassi  6 , Warda Heetun  2 , Stephanie J Jewitt  8 , Maria Kotzassarlidou  9 , Michael Ljungberg  10 , Daniel R McGowan  8   11 , Nathaniel Scott  8 , James Scuffham  2   12   13 , Katarina Sjögreen Gleisner  10 , Jill Tipping  3 , Jill Wevrett  2   12   13 ; MRTDosimetry Collaboration; Michael Lassmann  14
Collaborators, Affiliations

A multicentre and multi-national evaluation of the accuracy of quantitative Lu-177 SPECT/CT imaging performed within the MRTDosimetry project

Johannes Tran-Gia et al. EJNMMI Phys. .

Abstract

Purpose: Patient-specific dosimetry is required to ensure the safety of molecular radiotherapy and to predict response. Dosimetry involves several steps, the first of which is the determination of the activity of the radiopharmaceutical taken up by an organ/lesion over time. As uncertainties propagate along each of the subsequent steps (integration of the time-activity curve, absorbed dose calculation), establishing a reliable activity quantification is essential. The MRTDosimetry project was a European initiative to bring together expertise in metrology and nuclear medicine research, with one main goal of standardizing quantitative 177Lu SPECT/CT imaging based on a calibration protocol developed and tested in a multicentre inter-comparison. This study presents the setup and results of this comparison exercise.

Methods: The inter-comparison included nine SPECT/CT systems. Each site performed a set of three measurements with the same setup (system, acquisition and reconstruction): (1) Determination of an image calibration for conversion from counts to activity concentration (large cylinder phantom), (2) determination of recovery coefficients for partial volume correction (IEC NEMA PET body phantom with sphere inserts), (3) validation of the established quantitative imaging setup using a 3D printed two-organ phantom (ICRP110-based kidney and spleen). In contrast to previous efforts, traceability of the activity measurement was required for each participant, and all participants were asked to calculate uncertainties for their SPECT-based activities.

Results: Similar combinations of imaging system and reconstruction lead to similar image calibration factors. The activity ratio results of the anthropomorphic phantom validation demonstrate significant harmonization of quantitative imaging performance between the sites with all sites falling within one standard deviation of the mean values for all inserts. Activity recovery was underestimated for total kidney, spleen, and kidney cortex, while it was overestimated for the medulla.

Conclusion: This international comparison exercise demonstrates that harmonization of quantitative SPECT/CT is feasible when following very specific instructions of a dedicated calibration protocol, as developed within the MRTDosimetry project. While quantitative imaging performance demonstrates significant harmonization, an over- and underestimation of the activity recovery highlights the limitations of any partial volume correction in the presence of spill-in and spill-out between two adjacent volumes of interests.

Keywords: 177Lu SPECT/CT imaging; 3D printing; Harmonization of SPECT/CT imaging; International multicenter comparison exercise; Molecular radiotherapy (MRT); Phantom; Quantitative SPECT/CT; Standardization of SPECT/CT imaging; Traceability of SPECT/CT imaging.

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Conflict of interest statement

M. Lassmann has received research grants by IPSEN Pharma and Nordic Nanovector. M. Ljungberg and K. Sjögreen-Gleisner have been acting as research consultants for Fusion Pharmaceuticals Inc, Canada.

Figures

Fig. 1
Fig. 1
Two-organ ICRP 110 phantom. A Computer-aided design (purple: one-compartment spleen, green: two-compartment kidney). B 3D printed phantom inserts mounted in the support frame
Fig. 2
Fig. 2
Filling procedure for the kidney insert. A Filling of the medulla with the kidney mounted in a clamp stand. B Medulla filling holes. C Bottom-lit kidney to visualize the filling level
Fig. 3
Fig. 3
Example SPECT/CT reconstructions for a Siemens Intevo Bold system. A Coronal view of Jaczszak phantom. B Axial view of NEMA phantom. C Coronal view of Two-Organ (kidney and spleen) phantom. D Axial view of Two-Organ phantom. The red lines indicate the position of the axial cut for the Two-Organ phantom
Fig. 4
Fig. 4
Setup-specific image calibration factors (ICF). Details about the individual setups can be found in Table 1
Fig. 5
Fig. 5
Recovery curves and 95% confidence intervals fitted (to Eq. (3)) using a nonlinear weighted model for the calculated NEMA spheres recovery coefficients (black). The fitted recovery coefficients calculated for the validation data are shown in red
Fig. 6
Fig. 6
Activity ratios with (black cross) and without (blue circle) PVC applied for renal medulla, renal cortex, kidney and spleen inserts (top to bottom). For each case, the horizontal line shows the average ratio across all setups with the shaded area showing the region within one standard deviation, whilst the red line represents 100% accuracy. Note the different vertical scale for the renal medulla
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