Abstract
PURPOSE
Combined positron emission tomography (PET)-computed tomography (CT) using fluorine-18 fluorodeoxyglucose (FDG) is a well-established technique for diagnostic workups of multiple myeloma that can detect intramedullary as well extramedullary disease. However, PET-CT frequently reveals incidental findings, often necessitating additional workup. The present study is focused on clinically significant incidental findings on PET-CT scans in patients with plasma cell dyscrasias, leading to additional diagnostic measures with the resulting outcome.
PATIENTS AND METHODS
We reviewed medical records of consecutive patients with plasma cell dyscrasias that were diagnosed from 2012 to 2017 and that had a staging PET-CT at diagnosis. Only the initial PET-CT report was reviewed. Clinically significant incidental finding (CSIF) is defined as any finding that was new and needed further investigation or follow up. Myeloma-related osseous findings and extramedullary plasmacytomas were not considered as incidental findings. All CSIFs were further classified as unequivocal if the finding was previously unknown, definitive and did not require any workup or equivocal if it required further workup, such as review of previous medical records, imaging or tissue diagnosis.
RESULTS
A total of 470 patients consented and had completed follow-up. Of these, 15.5% (n=73) showed CSIF; 2.7% (n=13) were unequivocal and 12.8% (n=60) were classified as equivocal. Based on review of the medical records, 10 were true positive (most common findings were abdominal aortic aneurysm and hiatal hernia) and 3 were false positive (Flow Sheet). In 12.8% (n=60), findings were equivocal and these were further classified into those requiring clinical correlation/review of previous imaging (4.5%; n=21) and those which required subsequent investigative workup, including imaging procedures and biopsies (8.3%; n=39)(Flow Sheet). Among the 21 cases that required clinical correlation or review of previous imaging, 17 were falsely positive and 4 showed diagnosis of Hepatic angioma, PCP pneumonia, thyroid nodule, gastritis. Thirty-nine cases of equivocal findings required further investigation. Twenty-three cases had non-biopsy-based investigations yielding 12 truly positive and 11 falsely positive findings. The most common true positive finding was hydronephrosis requiring emergent nephrostomy tube and pituitary macroadenoma. Sixteen patients proceeded to a tissue diagnosis, with 10 truly positive and 6 falsely positive. The most common true positive finding on biopsy was thyroid malignancy. Incidental discovery of other primary malignancies due to PET-CT included lobular carcinoma of the breast, prostate adenocarcinoma, papillary thyroid carcinomas and neuroendocrine tumor of the pancreas. Overall, only 1% (5/470, 95% CI 0-2%) of all patients evaluated with PET-CT scan as initial workup for plasma cell dyscrasia were found to have another primary malignancy.
CONCLUSION
In conclusion, incidental findings on PET scans may be related to a benign process, infection, malignancy or may be falsely positive but finding another primary malignancy is rare. High positive predictive value (58.3%) among equivocal findings in our study suggests that it is important to investigate whenever clinical suspicion is strong. The overall rate of incidental finding was consistent with other studies evaluating the use of PET-CT scans in malignancies. Further prospective studies will be required to identify uniform standards for the further evaluation of incidental findings of PET-CT scans.
No relevant conflicts of interest to declare.
