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

Abstract

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