Early cancer therapy response assessment by quantitative F-18 FDG PET/CT

Purpose: Reliable noninvasive quantitative methods for assessing early response to chemotherapy are vital for development of more personalized cancer therapy management, by allowing earlier decisions regarding therapy changes. The CT RECIST method is widely used; however, CT does not provide information on the physiologic properties of the tumor. In this study, F-18 FDG (FDG) PET is compared to CT in predicting response to chemotherapy using changes in imaging biomarkers of tumor metabolism and size early after initiation of therapy compared to baseline (BL) and follow-up.

Materials and Methods: Nine patients with malignant cancer (lung, colorectal, or lymphoma) entered this early therapy response study. All patients received a BL scan before the initiation of chemotherapy, an early therapy assessment (ETA) scan between 2 to 7 weeks after initiation of therapy, and a study end point (FTA) scan between 3 to 9 months. The average length of time between ETA and FTA scans was 4 months. All PET/CT scans were performed on GE ST PET/CT scanner, approximately 60 min after patients received IV injection of approximately 15 mCi of F-18 FDG. All PET/CT images were registered and analyzed on MIMvista workstation to calculate FDG SUV values and CT RECIST measurements. By comparing BL and FTA scans, patients were classified as responders or non-responders based on RECIST. To compare FDG PET and CT data, the largest tumor was identified on CT at BL and its area and SUV were measured at ETA and FTA in each patient.

Results: At FTA, responders had an average (FTA CT area)/(BL CT area) of 0.40 and average (FTA SUV)/(BL SUV) of 0.25. For non-responders, these values were 1.55 and 1.27 respectively. At ETA, for responders, (ETA CT area)/(BL CT area) averaged 0.65 and (ETA SUV)/(BL SUV) averaged 0.26. For non-responders, these values were 1.19 and 1.15 respectively.

Conclusion: FDG uptake reduction at ETA compared to BL can be used to distinguish responders from non-responders, classified based upon RECIST at FTA. This early change in FDG uptake is more sensitive than early CT changes in predicting response to chemotherapy at FTA.

Research Support: This research was supported in part by grant U01 CA140230 from the National Cancer Institute to Dr. Mountz.