Purpose: This study evaluated the ability of (18)F-FDG PET/CT imaging to predict early response to (90)Y-radioembolization in comparison with contrast-enhanced CT (CECT) using RECIST and lesion density (Choi) criteria. Progression-free survival (PFS) in patients with liver metastases at 2 years and decline in tumour markers were the primary end-points of the study.
Methods: A total of 121 liver lesions were evaluated in 25 patients (14 men, 11 women) with liver-dominant metastatic colorectal cancer who underwent (18)F-FDG PET/CT and CECT before and 6-8 weeks after treatment. Changes in SUV(max), tumour density measured in terms of Hounsfield units and the sum of the longest diameters (LD) were calculated for the target liver lesions in each patient. The patient responses to treatment were categorized using EORTC PET criteria, tumour density criteria (Hounsfield units) and RECIST, and were correlated with the responses of tumour markers and 2-year PFS using Kaplan-Meier plots and the log-rank test for comparison. Multivariate proportional hazards (Cox) regression analysis was performed to assess the effect of relevant prognostic factors on PFS.
Results: Using (18)F-FDG PET/CT response criteria, 15 patients had a partial response (PR) and 10 patients had stable disease (SD), while using RECIST only 2 patients had a PR and 23 had SD. Two patients had a PR, 21 SD and 2 progressive disease using tumour density criteria. The mean changes in SUV(max), sum of the LDs and tumour density after treatment were 2.9 ± 2.6, 7.3 ± 14.4 mm and 1.9 ± 13.18 HU, respectively. Patients who had a PR on (18)F-FDG PET/CT had a mean decrease of 44.5 % in SUV(max) compared to those with SD who had a decrease of only 10.3 %. The decreases in SUV(max) and sum of the LDs were significant (p < 0.0001, p < 0.05, respectively) while the decrease in tumour density was not (p > 0.1065). The responses on the (18)F-FDG PET/CT studies were highly correlated with the responses of tumour markers (p < 0.0001 for LDH, p = 0.01 for CEA and p = 0.02 for Ca19-9), while the responses on the CECT studies using both RECIST and tumour density criteria were not significantly correlated with the responses of tumour markers. The responses on (18)F-FDG PET/CT studies also significantly predicted PFS (the median PFS in those with a PR was 12.0 months and in those with SD was 5 months, p < 0.0001), while RECIST and tumour density did not significantly predict PFS. Multivariate analysis demonstrated that responses on (18)F-FDG PET/CT studies and decreases in SUV(max) of ≤ 2.0 were the strongest predictors of PFS.
Conclusion: Early response assessment to (90)Y-radioembolization using (18)F-FDG PET/CT is superior to RECIST and tumour density, demonstrating a correlation with tumour markers and significantly predicting PFS in patients with liver metastases. This could enable early response-adapted treatment strategies to be employed.