Follow-up FDG PET in the evaluation of unexplained focal activity in the abdomen

PURPOSE: This study assessed the value of a follow-up FDG PET scan when the initial PET demonstrated unexplained findings of focal FDG uptake in the abdomen. METHOD: The records of 3634 patients with PET scans were retrospectively reviewed. Those patients who had follow-up PET scans after the initial PET scan showed unexplained FDG activity in the abdomen were further analyzed. The results from the second PET scan were compared with the follow-up data, which included the findings from other imaging modalities, clinical course, and biopsy or surgical pathology interpretations. RESULTS: A total of 59 patients were included in the final analysis. The average time interval between the initial and the follow-up PET scans was 4.2 + 2.3 months. The follow-up PET provided a clear-cut diagnosis in 55 (93.2%) of these patients, whereas diagnoses in only 4 patients remained indeterminate. Follow-up PET scans were negative for abdominal malignancy in 38 patients. Thirty-five of these 38 patients with negative follow-up PET were proven to be without abdominal malignancy, with a negative predictive value of 92.1% (35 of 38). The follow-up PET was positive in 17 patients. Fifteen of these 17 patients with a positive follow-up PET scans were found to have malignancy in the abdomen with a positive predicative value of 88.2% (15 of 17). All 4 patients with indeterminate follow-up scans were proven not to have malignancy. CONCLUSION: Follow-up FDG PET scan provides an effective means for diagnosing unexplained findings in the abdomen that were previously detected on initial PET scan.