Simulation of SPECT Imaging for Targeted Alpha-Therapy with Surrogate

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This paper describes a simulation workflow of SPECT imaging of targeted alpha-therapy with surrogates for use in imaging-system development and to aid in dosing studies for both target and off-target organs. Targeted alpha therapy isotopes emit alpha particle to treat cancerous regions within the body. Many of the isotopes also emit gamma photons; however imaging with these gamma-ray emissions is difficult due to low injected doses. In addition, these high-energy emissions leads to large levels of scattering in the patient and imaging system, as well as difficulties with collimation. This motivated the recent application of imaging surrogates in target-alpha therapy. Imaging surrogates are isotopes with similar chemical properties to alpha-emitting therapeutic isotopes, but have lack alpha decay and further emit lower-energy gamma rays that are easier to image. To verify the feasibility of imaging alpha therapy with surrogates, we proposed a workflow of calculating the decay and pharmacokinetics of both therapeutic and surrogate isotopes, and a Monte-Carlo simulation of a SPECT system to evaluate imaging performance. We simulated the biodistribution of PSMA-11 tracer with $\mathrm{Pb} 212 / \mathrm{Pb} 203$ therapy/surrogate pair in a human body. With reported two-tissue compartment model of PSMA-11, time activity curves of Pb 212 and progeny and Pb 203 are calculated, allowing us to assess dose and photon emissions under realistic image-acquisition conditions. A collimator-based SPECT system and XCAT human phantom were implemented in the Monte-Carlo model to generate output images. The model simulated photon attenuation and scattering inside the phantom and imaging system. The results showed that imaging of alpha-therapy is feasible by utilizing Pb203 surrogate with low energy emission peak (72.9 keV). Our results demonstrated the utility of our workflow to evaluate the feasibility and performance of imaging targeted alpha therapy with surrogates.

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