Development of ⁶⁷Cu quantitative SPECT for clinical dosimetry

Objectives: ackground / Aims: Copper-67 (⁶⁷Cu) shows promise as a radionuclide therapy agent, in particular when paired with the diagnostic PET imaging agent ⁶⁴Cu. The aim of this work was to examine the potential of quantitative SPECT for ⁶⁷Cu for the aim of clinical dosimetry in patients receiving ⁶⁷Cu-SARTATE to treat somatostatin-positive neoplasia.

Methods: All work was performed on an Intevo SPECT/CT (Siemens Healthineers, USA) using medium energy collimators and imaging the 185 keV photopeak (49% abundance) with a 20% energy window. Quantitative evaluation consisted of experimental work to model the transmission dependant scatter correction technique, camera specific sensitivity and dead time. Narrow-beam attenuation correction maps were derived based on a bi-linear relationship between CT number and material density. All corrections were implemented using in-house software written in IDL (Harris Geospatial, USA). Validation of quantitative accuracy was assessed with the IEC NEMA body phantom over a period of 7 days with decaying phantom activity (1.6 - 0.4 GBq) and a sphere-to-background ratio of 8:1. Results: Scanner sensitivity was measured as approximately 86 cps/MBq. The spatial resolution on planar imaging was found to be 2.8 cm. Nine SPECT/CT phantom acquisitions were acquired over a two week period, yielding an average difference between true and measured total phantom activity and measured background concentration of -2.1% and -1.7%, respectively. Figure 1 demonstrates the image analysis of the NEMA IEC Body Phantom to derive concentration recovery coefficients. The 37mm diameter sphere suffered from 20% count losses due to the partial volume effect (PVE) when using a threshold grown VOI to derive average hot-sphere concentration. The data suggest that underestimates to concentration, and so absorbed dose, can be expected in lesions of diameter less than 5cm. Conclusions: Quantitative SPECT and clinical dosimetry of ⁶⁷Cu data is feasible and returns accurate measures in volumes unaffected by PVE. Small lesions can expect to under-estimate dose and would benefit from a resolution recovery algorithm to reduce partial volume losses.

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