Background: Despite a paucity of evidence-based guidelines, the use of PET/CT (positron emission tomography/computed tomography) in the management of cancer patients is increasing. As widespread clinical application increases, unexpected radiographic findings are occasionally identified. These incidental findings are often suspicious for a second primary malignancy. The purpose of this study was to determine the clinical impact of these incidental PET/CT findings. Methods: A query of our prospectively acquired Nuclear Medicine database was performed to identify patients with a known malignancy being staged or serially imaged with PET/CT. Patients with incidental findings suggestive of a second primary malignancy were selected. Statistical analysis was performed to determine the ability of PET/CT to identify a second primary malignancy. All PET/CT were interpreted by board certified nuclear radiologists. Results: Of 3,814 PET/CT scans performed on 2,219 cancer patients at our institution from January 1, 2005, to December 29, 2008, 272 patients (12% of all patients) had findings concerning for a second primary malignancy. An invasive work-up was performed on 49% (133/272) of these patients, while 15% (40/272) had no further evaluation due to an advanced primary malignancy. The remaining 36% (99/272) had no further evaluation secondary to a low clinical suspicion determined by the treating team, a clinical plan of observation, or patients lost to follow-up. Of the 133 patients evaluated further, clinicians identified a second primary malignancy in 41 patients (31%), benign disease in 62 patients (47%), and metastatic disease from their known malignancy in 30 patients (23%). The most common sites for a proven second primary malignancy were: lung (N = 10), breast (N = 7), and colon (N = 5). Investigation of these lesions was performed using several techniques, including 24 endoscopies (6 malignant). A surgical procedure was performed in 74 patients (29 malignant), and a percutaneous biopsy was performed on 34 patients (12 malignant). The overall positive predictive value for PET/CT to detect a second primary malignancy was 31% in this subgroup. At a median follow-up of 22 months, 9 of 41 patients with a second primary were dead of a malignancy, 20 were alive with disease, and 12 had no evidence of disease. Conclusion: Incidental PET/CT findings consistent with a second primary are occasionally encountered in cancer patients. In our data, approximately half of these findings were benign, a third were consistent with a second primary malignancy or a metastatic focus, and the remainder were never evaluated due to physician and patient decision. Advanced primary tumors are unlikely to be impacted by a second primary tumor suggesting that this subset of patients will not benefit from further investigation. Our data suggests that, despite the high rate of false positivity, incidental PET/CT findings should be investigated when the results will impact treatment algorithms. The timing and route of investigation should be dictated by clinical judgment and the status of the primary tumor. Further investigation will need to be performed to determine the long-term clinical impact of incidentally identified second primary malignancies.
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