Semiquantitative analysis of dual-time-point FDG PET data increased specificity for pediatric malignancies compared with single-time-point PET imaging, according to a study published in the February issue of American Journal of Roentgenology .
Although FDG PET provides an effective modality for imaging soft-tissue malignancies in pediatric patients, FDG is not tumor specific and may accumulate in benign entities, resulting in false-positive or equivocal results. Previous studies of dual-time-point FDG PET/CT have indicated that FDG intensity increases in malignant lesions between two scans, whereas FDG intensity remains stable or decreases in benign entities.
Danny L. Constantini, MD, from the University of Toronto in Ontario, and colleagues devised a study to assess the utility of dual-time-point FDG PET/CT in differentiating benign from malignant lesions in pediatric patients.
The study population was comprised of 21 patients with a suspected malignancy who underwent dual-time-point PET/CT. Patients underwent a second delayed PET-only scan at a mean time of 121 minutes after an initial whole-body PET/CT exam. Imaging results were compared to pathologic findings, if available, or conventional imaging and clinical follow-up.
All 13 malignant lesions had a standardized uptake value (SUV) of 2.5 or higher on the initial scan, and three of the 10 benign lesions had an early SUV of less than 2.5. Among patients with malignant lesions, SUV measured between the first and second scans increased so that the mean early and delayed SUVs were 7.3 and 10.9 respectively.
In contrast, these values remained stable among patients with benign lesions. The mean delayed SUV was observed to be significantly greater among patients with malignant tumors compared with those with benign lesions.
Constantini and colleagues applied a retention index of 10 percent or higher between the two exams, which correctly identified 10 of 13 neoplastic lesions and correctly diagnosed eight of 10 benign lesions. The sensitivity, specificity and accuracy for the dual-point method were 76.9 percent, 80 percent and 78.2 percent, respectively.
When using an early or delayed SUV of 2.5 or higher as the cutoff for distinguishing between malignant and benign lesions with the single-time point method, the sensitivity, specificity and accuracy were 100 percent, 30 percent and 69.5 percent, and 100 percent, 40 percent and 73.9 percent, respectively.
The researchers acknowledged “considerable overlap” in the early or delayed SUV max among most benign and malignant lesions. “The degree of overlap became less evident with delayed imaging, likely because the uptake of FDG continues to increase in malignant tissues for several hours after FDG injection, whereas benign lesions show a decrease or remain stable over time,” wrote Constantini and colleagues.
The advantages of dual-time-point FDG PET/CT, according to Constantini et al, include the acquisition of additional information to differentiate benign and malignant lesions and the ease of incorporating the study into daily workflow as it requires 10 to 15 minutes of camera time. However, the researchers cautioned that not all malignant lesions revealed increasing SUVs over time, thus the exam may have shortcomings and should be considered complementary to conventional exams and clinical workup.
The authors are planning additional studies with more cases, including those with different pathologic subtypes, in the future.