The role of ¹⁸F-FDG PET in staging and early prediction of response to therapy of recurrent gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are gaining the interest of researchers because of impressive metabolic response to the targeted molecular therapeutic drug imatinib mesylate. Initial reports suggest an impressive role for ¹⁸F-FDG PET in follow-up of therapy for these tumors. However, the role of ¹⁸F-FDG PET versus that of CT has not been established. Therefore, we compared the roles of ¹⁸F-FDG PET and CT in staging and evaluation of early response to imatinib mesylate therapy in recurrent or metastatic GIST. Methods: The study included 54 patients who underwent ¹⁸F-FDG PET and CT scans within 3 wk before initiation of imatinib mesylate therapy. Forty-nine of these patients underwent repeat scans 2 mo after therapy. The numbers of sites or organs containing lesions on ¹⁸F-FDG PET and CT scans were compared. Corresponding lesions on ¹⁸F-FDG PET and CT scans or those confirmed to be malignant in appearance by other imaging modalities or on follow-up were considered true positives. Lesions seen on ¹⁸F-FDG PET or CT scans but not seen or confirmed to be of benign appearance with other imaging modalities or on follow-up were considered false positives. Measurements of the maximum standard uptake value (SUV) on ¹⁸F-FDG PET scans and tumor size on CT scans were used for quantitative evaluation of early tumor response to therapy. Results: A total of 122 and 114 sites and/or organs were involved on pretherapy ¹⁸F-FDG PET and CT scans, respectively. The sensitivity and positive predictive values (PPVs) for CT were 93% and 100%; whereas these values for ¹⁸F-FDG PET were 86% and 98%. However, the differences between these values for CT and ¹⁸F-FDG PET were not statistically significant (P = 0.27 for sensitivity and 0.25 for PPV). This suggests comparable performance of ¹⁸F-FDG PET and CT in staging GISTs. Repeat scans at 2 mo after therapy showed agreement between ¹⁸F-FDG PET and CT scans in 71.4% of patients (57.1% having a good response to therapy and 14.3% lacking a response). Discrepant results between ¹⁸F-FDG PET and CT were recorded for 28.6% of the patients. ¹⁸F-FDG PET predicted response to therapy earlier than did CT in 22.5% of patients during a longer follow-up interval (4-16 mo), whereas CT predicted lack of response to therapy earlier than ¹⁸F-FDG PET in 4.1%. One patient did not undergo long-term follow-up. These findings suggest that ¹⁸F-FDG PET is superior to CT in predicting early response to therapy in recurrent or metastatic GIST patients. Conclusion: The performances of ¹⁸F-FDG PET and CT are comparable in staging GISTs before initiation of imatinib mesylate therapy. However, ¹⁸F-FDG PET is superior to CT in predicting early response to therapy. Thus, ¹⁸F-FDG PET is a better guide for imatinib mesylate therapy.