Multimodality imaging can predict the metabolic response of unresectable colorectal liver metastases to radioembolization therapy with Yttrium-90 labeled resin microspheres

Selective internal radiotherapy (SIRT) using Yttrium-90 labeled resin microspheres is increasingly used for the radioembolization of unresectable liver metastases of colorectal cancer (CRC). The treatment can be simulated by scintigraphy with Tc-99m-labeled macroaggregates of albumin (MAA). The aim of the study was to develop a predictive dosimetric model for SIRT and to validate it by correlating results with the metabolic treatment response. The simulation of the dosimetry was performed by mathematically converting all liver voxel MAA-SPECT uptake values to the absolute Y-90 activity. The voxel values were then converted to a simulated absorbed dose (Gy) using simple MIRD formalism. The metabolic response was defined as the change in total lesion glycolysis (TLG) on FDG-PET. A total of 39 metastatic liver lesions were studied in eight evaluable patients. The mean administered Y90 activity was 1.69 GBq (range: 1.33-2.04 GBq). The median change in TLG of the lesions was 48%. The median (95% CI) simulated absorbed dose (Gy) was 20 Gy (1-68 Gy) and 46 Gy (22-110 Gy) in the poor (< 50% TLG change) and the good responders (TLG change > 50%), respectively. Using a simple cut-off value of 1 for the MAA-tumor-to-normal uptake ratio, a significant metabolic response was predicted with a sensitivity of 89% (17/19), a specificity of 65% (13/20), a positive predictive value of 71% (17/24) and a negative predictive value of 87% (13/15). Integrated multimodality imaging allows prediction of metabolic response post radioembolization using Y-90-resin microspheres, and should be used for patient selection.