Background: Malignant gastrointestinal stromal tumours (GISTs) are a rare subset of aggressive mesenchymal tumours specific to the gastrointestinal system. They are both locally aggressive and can metastasize. The aim of this analysis was to report on our experience of the utility of coincidence positron emission tomography (co-PET) based on an 18F-FDG gamma camera in assessing treatment response to imatinib using CT as the comparator and the final clinical outcome as the end point.
Methods: We compared the results of CT and PET scans as predictors of outcome in a consecutive series of patients treated at the Prince of Wales hospital. All patients had biopsy-proven malignant GIST and were on treatment with the targeted pharmacotherapeutic agent imatinib. The majority of the patients were receiving treatment as part of the randomized trial of the European Organization for Research and Treatment of Cancer, the Australian Gastrointestinal Trials Group and the Italian Sarcoma Group, comparing 400 with 800 mg (400 mg b.i.d.). The monitoring of tumour response was achieved by serial CT measurements according to the RECIST criteria. Concurrent 18F-FDG co-PET studies were performed within a mean of 2.8 days from the CT scan and were interpreted by a consensus panel of 2 nuclear medicine physicians.
Results: A total of 18 patients were recruited into the study, with a total of 74 lesions. There were 47 liver lesions, 31 of which were identified on the initial 18F-FDG co-PET scans (63%). There were 10 primary lesions (4 stomach, 4 duodenal, 2 small bowel), 9 of which were demonstrated on initial 18F-FDG co-PET examinations (90%). There were 17 extrahepatic metastatic sites, 15 of which were visualized on the initial 18F-FDG study (88%). Of the 18 patients, 8 showed concordant improvement on both CT and (18)F-FDG co-PET criteria. One patient showed concordant worsening and 1 was not FDG avid. Eight patients initially showed discordance. 18F-FDG co-PET studies demonstrated changes preceding CT findings in all 8 patients with subsequent concordant improvement.
Conclusion: 18F-FDG co-PET is a useful modality to monitor treatment response to imatinib in patients with malignant GIST. Although there is a relatively reduced sensitivity when compared with CT for the detection of lesions especially in the liver, co-PET changes in several instances precede the changes on CT scanning. This modality has the potential to influence clinical decision making and should be considered as part of the standard care of patients on imatinib.
Copyright (c) 2005 S. Karger AG, Basel.