$^{177}$Lu SPECT Imaging in the Presence of $^{90}$Y: Does $^{90}$Y Degrade Image Quantification? A Simulation Study

TL;DR

This simulation study demonstrates that the presence of $^{90}$Y does not significantly degrade the accuracy of $^{177}$Lu SPECT image quantification, supporting dual-isotope imaging in radiopharmaceutical therapy.

Contribution

The paper provides a comprehensive simulation analysis showing that $^{90}$Y does not impair $^{177}$Lu quantification, with implications for dual-isotope SPECT imaging accuracy.

Findings

Quantification error remains within ±6% with $^{90}$Y present.

CNRs are unaffected by $^{90}$Y but improve with wider scatter windows.

Scatter window width slightly influences activity recovery by 1-2%.

Abstract

This work aims to investigate the accuracy of quantitative SPECT imaging of $^{177}$Lu in the presence of $^{90}$Y, which occurs in dual-isotope radiopharmaceutical therapy (RPT) involving both isotopes. We used the GATE Monte Carlo simulation toolkit to conduct a phantom study, simulating spheres filled with $^{177}$Lu and $^{90}$Y placed in a cylindrical water phantom that was also filled with activity of both radionuclides. We simulated multiple phantom configurations and activity combinations by varying the location of the spheres, the concentrations of $^{177}$Lu and $^{90}$Y in the spheres, and the amount of background activity. We investigated two different scatter window widths to be used with triple energy window (TEW) scatter correction. We also created multiple realizations of each configuration to improve our assessment, leading to a total of 540 simulations. Each configuration was imaged using a simulated Siemens SPECT camera. The projections were reconstructed using the standard 3D OSEM algorithm, and errors associated with $^{177}$Lu activity quantification and contrast-to-noise ratios (CNRs) were determined. In all configurations, the quantification error was within $\pm$6% of the no-$^{90}$Y case, and we found that quantitative accuracy may slightly improve when $^{90}$Y is present because of reduction of errors associated with TEW scatter correction. The CNRs were not significantly impacted by the presence of $^{90}$Y, but they were increased when a wider scatter window width was used for TEW scatter correction. The width of the scatter windows made a small but statistically significant difference of 1-2% on the recovered $^{177}$Lu activity. Based on these results, we can conclude that activity quantification of $^{177}$Lu and lesion detectability is not degraded by the presence of $^{90}$Y.