Simultaneous Ho/Tc 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). 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 therapeutic microspheres and technetium-99 m (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 Ho and 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 Tc and Ho. First, Tc images were reconstructed including energy window-based corrections for Ho downscatter. Next, Tc downscatter in the 81-keV Ho window was Monte Carlo simulated to allow quantitative reconstruction of the Ho images. The accuracy of the Tc-downscatter modeling was evaluated by comparing measurements with simulations. In addition, the ratio between Tc and Ho yielding the best Ho dose estimates was established and the quantitative accuracy was reported.

RESULTS

Given the same level of activity, Tc contributes twice as many counts to the 81-keV window than Ho, and four times as many counts to the 140-keV window, applying a Ho/Tc ratio of 5:1 yielded a high accuracy in both Ho and Tc reconstruction. Phantom experiments revealed that the accuracy of quantitative Ho activity recovery was reduced by 10% due to the presence of 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 Ho/Tc dual-isotope protocol for automatic dosimetry compensates accurately for downscatter and allows for the addition of Tc without compromising Ho SPECT image quality.