CT and PET: Early Prognostic Indicators of Response to Imatinib Mesylate in Patients with Gastrointestinal Stromal Tumor

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Abstract
OBJECTIVE. We report results from a pilot study aimed at optimizing the use of CT bidimensional measurements and 18F-FDG PET maximum standardized uptake values (SUVs-max) for determining response to prolonged imatinib mesylate treatment in patients with advanced gastrointestinal stromal tumors (GISTs). SUBJECTS AND METHODS. Sixty-three patients enrolled in a multicenter trial evaluating imatinib mesylate therapy for advanced GIST underwent FDG PET at baseline and 1 month after initiation of treatment. Of these 63 patients, 58 underwent concomitant CT. Time-to-treatment failure (TTF) was used as the outcome measure. Patients were followed up over a range of 23.7 to 37 months (median, 31.7 months). The predictive power of change in CT bidimensional measurements, change in PET SUVmax, and PET SUVmax at 1 month after initiation of treatment were determined, optimized, and compared. The effectiveness of combining metrics was also evaluated. RESULTS. Both a threshold PET SUVmax value of 2.5 at 1 month (p = 0.04) and the European Organization for Research and Treatment of Cancer (EORTC) criteria for partial response on FDG PET (25% reduction in PET SUVmax) at 1 month (p = 0.004) were predictive of prolonged treatment success. The Southwest Oncology Group (SWOG) criteria for partial response (3 50% reduction in CT bidimensional measurements) at 1 month were not predictive (p = 0.55) of TTF. Optimizing metrics improved results performance. An optimized PET SUVmax threshold of 3.4 (p = 0.00002), a reduction in the SUVmax of 40% (p = 0.002), and an optimized CT bidimensional measurement threshold—that is, no growth from baseline to 1 month (p = 0.00005)—outperformed the existing standards (i.e., EORTC and SWOG criteria). Combinations of metrics did not improve performance. CONCLUSION. The two best metrics were the optimized PET SUVmax threshold of 3.4 at 1 month (p = 0.00002) and the optimized CT bidimensional measurement threshold (no growth from baseline to 1 month, p = 0.00005) in this patient group.