Optimizing 90 Y Particle Density Improves Outcomes After Radioembolization

Purpose To determine how particle density affects dose distribution and outcomes after lobar radioembolization. Methods Matched pairs of patients, treated with glass versus resin microspheres, were selected by propensity score matching (114 patients), in this single-institution retrospective study. For each patient, tumor and liver particle density (particles/cm 3 ) and dose (Gy) were determined. Tumor-to-normal ratio was measured on both 99m Tc-MAA SPECT/CT and post- 90 Y bremsstrahlung SPECT/CT. Microdosimetry simulations were used to calculate first percentile dose, which is the dose in the cold spots between microspheres. Local progression-free survival (LPFS) and overall survival were analyzed. Results As more particles were delivered, doses on 90 Y SPECT/CT became more uniform throughout the treatment volume: tumor and liver doses became more similar ( p = 0.04), and microscopic cold spots between particles disappeared. For hypervascular tumors (tumor-to-normal ratio ≥ 2.6 on MAA scan), delivering fewer particles (< 6000 particles/cm 3 treatment volume) was associated with better LPFS ( p = 0.03). For less vascular tumors (tumor-to-normal ratio < 2.6), delivering more particles (≥ 6000 particles/cm 3 ) was associated with better LPFS ( p = 0.02). In matched pairs of patients, using the optimal particle density resulted in improved overall survival (11.5 vs. 6.8 months, p = 0.047), compared to using suboptimal particle density. Microdosimetry resulted in better predictions of LPFS ( p = 0.03), and overall survival ( p = 0.02), compared to conventional dosimetry. Conclusion The number of particles delivered can be chosen to maximize the tumor dose and minimize the liver dose, based on tumor vascularity. Optimizing the particle density resulted in improved LPFS and overall survival.