90Y microsphere treatment of unresectable liver metastases:: changes in 18F-FDG uptake and tumour size on PET/CT

Purpose: The intra-arterial administration of Y-90 microspheres is a new palliative treatment option for unresectable liver metastases. The aim of this study was to quantitatively assess changes in FDG uptake and tumour size following Y-90 microsphere treatment (SIR-Spheres) using F-18-fluorodeoxyglucose (FDG) PET/CT imaging. Methods: Five patients with unresectable liver metastases who had failed multiple prior chemotherapy regimens received seven 90Y microsphere treatments to a single liver lobe. All patients underwent a baseline PET/CT scan prior to treatment, as well as up to four follow-up PET/CT scans. The tumour area of 30 liver metastases was measured on CT and the FDG uptake was semiquantitatively assessed by calculation of standardised uptake values (SUVs). A total of 18 FDG-PET/CT scans were performed. Results: The SUVs in the 30 treated liver metastases decreased from 6.5 +/- 2.3 at baseline to 4.2 +/- 1.8 after the first follow-up PET/CT scan (p = 0.001). In contrast, the SUVs of untreated metastases increased slightly from 7.2 +/- 2.3 to 8.0 +/- 0.8. There was no difference in FDG uptake in treated versus untreated normal liver tissue. Using a previously defined threshold of 20% decrease in SUV from baseline to determine response, 20 out of 30 liver metastases were considered to have responded at the first follow-up PET/CT scan approximately 1 month after treatment. In these metastases, the SUV decreased by 47 +/- 12%, compared with a slight increase by 5.9 +/- 19% in ten non-responding metastases (p = 0.0001). The changes in tumour size did not correlate with changes in FDG uptake. On the first follow-up PET/CT scan, the tumour area on CT increased by 3.1 +/- 57% in treated metastases compared with 23.3 +/- 32% in untreated metastases. A wide range of post-treatment changes of target lesions was observed on CT, including an increase in the size of hypodense lesions, necrotic features and complete resolution of CT abnormalities. Conclusion: The metabolic information obtained from FDG-PET/CT seems to provide a more accurate and earlier assessment of therapy response following Y-90 microsphere treatment than does the anatomical CT information.