Positron emission tomography with 18F-fluorothymidine for monitoring therapeutic effect after chemoradiation therapy

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The aim of this study was to assess the prevalence of discrepancy and which yielded more accurate diagnoses in patients who underwent both FDG-PET and FLT-PET. Thirteen patients underwent both FDG-PET and FLT-PET scans after they have completed neoadjuvant chemotherapy or chemo-radiation therapy. Of 38 lesions, 32 were true positive lesions. There were 10 lesions (26%) with discrepant findings. For these lesions, FDG-PET yielded 4 true-positives and 2 true-negatives, and FLT-PET showed 2 true-positives and 2 true-negatives. Overall, the lesion-based sensitivity, specificity, positive predictive value, negative predictive value and accuracy of FDG-PET were 50%, 33%, 80%, 11% and 47%, respectively, while those of FLT-PET were 38%, 67%, 86%, 17% and 42%, respectively. In addition, 13 patients had FDG-PET and FLT-PET scans for the purpose of differentiation, staging or restaging. Of 29 lesions, 26 were true positive lesions. There were 9 lesions (31%) with discrepant findings. For these lesions, FDG-PET yielded 5 true-positives and FLT-PET showed 2 true-positives and 2 true-negatives. Overall, the sensitivity, specificity, positive predictive value, negative predictive value and accuracy of FDG-PET were 62%, 33%, 89%, 9%, and 59%, respectively, while those of FLT-PET were 50%, 100%, 100%, 19%, and 50%, respectively. Based on these preliminary data, we concluded (i) FDG-PET tended to yield more false positive findings, while FLT-PET false negative findings, (ii) the diagnostic discrepancy occurred in one-fourth to one-third of the lesions, with the half accuracy of each study, and (iii) the positive predictive value of FLT-PET was reasonably high. FLT-PET could be helpful for detecting non-FDG-avid metastatic lesions.