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The SEL-I-METRY trial (EudraCT No 2015-002269-47) is the first multicentre trial to investigate the role of 123I and 131I SPECT/CT-based tumour dosimetry to predict response to radioiodine therapy. Standardised dosimetry methodology is essential to provide a robust evidence-base for absorbed dose–response thresholds for molecular radiotherapy (MRT). In this paper a practical standardised protocol is used to establish the first network of centres with consistent methods of radioiodine activity quantification. Nine SPECT/CT systems at eight centres were set-up for quantitative radioiodine imaging. The dead-time of the systems was characterised for up to 2.8 GBq 131I. Volume dependent calibration factors were measured on centrally reconstructed images of 123I and 131I in six (0.8–196 ml) cylinders. Validation of image quantification using these calibration factors was performed on three systems, by imaging a 3D-printed phantom mimicking a patient’s activity distribution. The percentage differences between the activities measured in the SPECT/CT image and those measured by the radionuclide calibrator were calculated. Additionally uncertainties on the SPECT/CT-based activities were calculated to indicate the limit on the quantitative accuracy of this method. For systems set-up to image high 131I count rates, the count rate versus activity did not peak below 2.8 GBq and fit a non-paralysable model. The dead-times and volume-dependent calibration factors were comparable between systems of the same model and crystal thickness. Therefore a global calibration curve could be fitted to each. The errors on the validation phantom activities’ were comparable to the measurement uncertainties derived from uncertainty analysis, at 10% and 16% on average for 123I and 131I respectively in a 5 cm sphere. In conclusion, the dead-time and calibration factors varied between centres, with different models of system. However, global calibration factors may be applied to the same system model with the same crystal thickness, to simplify set-up of future multi-centre MRT studies.

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.

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