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To evaluate the determinants of initial 18F-fluorodeoxyglucose (18F-FDG)-PET indication following primary colorectal cancer diagnosis among patients who underwent surgery between January 2000 and December 2007 and who were observed at a single institution for at least 2 years after diagnosis.Of the 530 patients who underwent colorectal cancer resection, 113 patients received at least one 18F-FDG-PET following diagnosis. Outcome variables included indication and time of the first 18F-FDG-PET following diagnosis. Potential predictors included disease-level and patient-level characteristics. Univariate and multivariate regression analyses were performed.Patients diagnosed later in the study period and patients with higher-stage disease were more likely to receive their first 18F-FDG-PET for initial staging (P<0.001 and P=0.016, respectively). Patients with lower-stage disease were more likely to receive their initial 18F-FDG-PET for suspected recurrence on conventional imaging. When performed more than 2 years after diagnosis, 18F-FDG-PET was more likely to be ordered for suspected recurrence either on the basis of conventional imaging or on the basis of patient symptoms/tumor markers (P=0.003 and 0.031, respectively). 18F-FDG-PET demonstrated disease progression in at least 50% of patients referred for each indication (P=0.037).Higher utilization of 18F-FDG-PET may be appropriate among patients referred for a number of indications including: initial staging, particularly among those with higher-stage disease; suspected recurrence on conventional imaging among patients with lower-stage disease; and suspected recurrence more than 2 years after diagnosis. Further research is needed to verify these findings.