Simultaneous Ho-166/Tc-99m dual-isotope SPECT with Monte Carlo-based downscatter correction for automatic liver dosimetry in radioembolization

Background Intrahepatic dosimetry is paramount to optimize radioembolization treatment accuracy using radioactive holmium-166 microspheres (Ho-166). This requires a practical protocol that combines quantitative imaging of microsphere distribution with automated and robust delineation of the volumes of interest. To this end, we propose a dual isotope single photon emission computed tomography (SPECT) protocol based on Ho-166 therapeutic microspheres and technetium-99 m (Tc-99m) stannous phytate, which accumulates in healthy liver tissue. This protocol may allow accurate and automatic estimation of tumor-absorbed dose and healthy liver-absorbed dose. The current study focuses on a Monte Carlo-based reconstruction framework that inherently corrects for scatter crosstalk between the Ho-166 and Tc-99m imaging. To demonstrate the feasibility of the method, it is evaluated with realistic phantom experiments and patient data. Methods The Utrecht Monte Carlo System (UMCS) was extended to include detailed modeling of crosstalk interactions between Tc-99m and Ho-166. First, Tc-99m images were reconstructed including energy window-based corrections for Ho-166 downscatter. Next, Tc-99m downscatter in the 81-keV Ho-166 window was Monte Carlo simulated to allow quantitative reconstruction of the Ho-166 images. The accuracy of the Tc-99m-downscatter modeling was evaluated by comparing measurements with simulations. In addition, the ratio between Tc-99m and Ho-166 yielding the best Ho-166 dose estimates was established and the quantitative accuracy was reported. Results Given the same level of activity, Tc-99m contributes twice as many counts to the 81-keV window than Ho-166, and four times as many counts to the 140-keV window, applying a Ho-166/Tc-99m ratio of 5:1 yielded a high accuracy in both Ho-166 and Tc-99m reconstruction. Phantom experiments revealed that the accuracy of quantitative Ho-166 activity recovery was reduced by 10% due to the presence of Tc-99m. Twenty iterations (8 subsets) of the SPECT/CT reconstructions were considered feasible for clinical practice. Applicability of the proposed protocol was shown in a proof-of-concept case. Conclusion A novel Ho-166/Tc-99m dual-isotope protocol for automatic dosimetry compensates accurately for downscatter and allows for the addition of Tc-99m without compromising Ho-166 SPECT image quality.