Simultaneous 166Ho/99mTc 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 ( 166 Ho). 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 166 Ho therapeutic microspheres and technetium-99 m ( 99m Tc) 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 166 Ho and 99m Tc 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 99m Tc and 166 Ho. First, 99m Tc images were reconstructed including energy window-based corrections for 166 Ho downscatter. Next, 99m Tc downscatter in the 81-keV 166 Ho window was Monte Carlo simulated to allow quantitative reconstruction of the 166 Ho images. The accuracy of the 99m Tc-downscatter modeling was evaluated by comparing measurements with simulations. In addition, the ratio between 99m Tc and 166 Ho yielding the best 166 Ho dose estimates was established and the quantitative accuracy was reported. Results Given the same level of activity, 99m Tc contributes twice as many counts to the 81-keV window than 166 Ho, and four times as many counts to the 140-keV window, applying a 166 Ho/ 99m Tc ratio of 5:1 yielded a high accuracy in both 166 Ho and 99m Tc reconstruction. Phantom experiments revealed that the accuracy of quantitative 166 Ho activity recovery was reduced by 10% due to the presence of 99m Tc. 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 166 Ho/ 99m Tc dual-isotope protocol for automatic dosimetry compensates accurately for downscatter and allows for the addition of 99m Tc without compromising 166 Ho SPECT image quality.