Simultaneous ¹⁶⁶Ho/^99mTc dual-isotope SPECT with Monte Carlo-based downscatter correction for automatic liver...

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.

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.

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.

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.