Assessment of early response to imatinib 800 mg after 400 mg progression by 18f-fluorodeoxyglucose positron emission tomography in patients with metastatic gastrointestinal stromal tumor.

Abstract

10081 Background: Imatinib is the standard first-line therapy for advanced GIST patients. 18 F- fluorodeoxyglucose positron emission tomography (FDG-PET/CT) shows a faster response than CT in nonpretreated patients. After disease progression on imatinib 400 mg, FDG-PET/CT scans were performed to evaluate early response to imatinib 800 mg. Methods: After disease progression on imatinib 400 mg, 16 patients had their dose escalated to 800 mg and tumor response was evaluated by FDG-PET/CT on days 7 and 37. Objective assessment was performed according to the recommendations of the European Organization for Research and Treatment of Cancer (EORTC) PET Study group. The primary objective was to correlate early metabolic response with progression-free survival (PFS). Secondly, metabolic assessment, Response Evaluation Criteria in Solid Tumors (RECIST) and Choi criteria were compared. Ninety-five lesions were assessed in 46 FDG-PET/CT. Results: Early metabolic assessment by FDG-PET/CT scan was not predictive of PFS in advanced GIST patients treated with imatinib 800 mg daily. Median PFS was 5.3 months. Gastric primary site and previous exposure to imatinib 400 mg for over 12 months were statistically significant positive predictive factors for PFS (p 0.02). Three partial responses were observed on day 7 FDG-PET/CT but time to progression was 2 months (range 2-3). No objective responses according to RECIST criteria were observed on Day 7 or 37. There was a trend towards an earlier identification of partial responses or disease progression with PET and Choi than RECIST. In this group of patients, OS was only influenced by gastric primary site (p 0.05). Metabolic response or other clinical-molecular variables did not impact in OS. Conclusions: The assessment of early metabolic response by FDG-PET/CT in advanced GIST patients exposed to imatinib 800 mg was not predictive of PFS or OS. No significant financial relationships to disclose.