MRI-based biodistribution assessment of holmium-166 poly(L-lactic acid) microspheres after radioembolisation

OBJECTIVES: To demonstrate the feasibility of MRI-based assessment of the intrahepatic Ho-PLLA-MS biodistribution after radioembolisation in order to estimate the absorbed radiation dose. METHODS: Fifteen patients were treated with holmium-166 ((166)Ho) poly(L-lactic acid)-loaded microspheres (Ho-PLLA-MS, mean 484 mg; range 408–593 mg) in a phase I study. Multi-echo gradient-echo MR images were acquired from which R(2)(*) maps were constructed. The amount of Ho-PLLA-MS in the liver was determined by using the relaxivity r(2)(*) of the Ho-PLLA-MS and compared with the administered amount. Quantitative single photon emission computed tomography (SPECT) was used for comparison with MRI regarding the whole liver absorbed radiation dose. RESULTS: R(2)(*) maps visualised the deposition of Ho-PLLA-MS with great detail. The mean total amount of Ho-PLLA-MS detected in the liver based on MRI was 431 mg (range 236–666 mg) or 89 ± 19 % of the delivered amount (correlation coefficient r = 0.7; P < 0.01). A good correlation was found between the whole liver mean absorbed radiation dose as assessed by MRI and SPECT (correlation coefficient r = 0.927; P < 0.001). CONCLUSION: MRI-based dosimetry for holmium-166 radioembolisation is feasible. Biodistribution is visualised with great detail and quantitative measurements are possible. KEY POINTS: • Radioembolisation is increasingly used for treating unresectable primary or metastatic liver tumours. • MRI-based intrahepatic microsphere biodistribution assessment is feasible after holmium-166 radioembolisation. • MRI enables quantification of holmium-166 microspheres in liver in a short imaging time. • MRI can estimate the whole liver absorbed radiation dose following holmium-166 radioembolisation.