18F-FDG PET/CT in response evaluation of gastrointestinal stromal tumours treated with imatinib

Objective. Several studies have demonstrated the effective use of adjuvant treatment with Imatinib mesylate for unresectable, metastatic or recurrent gastrointestinal stromal tumors (GIST). We retrospectively evaluated the role of F-18-FDG PET/CT scanning in assessing the response of GIST patients to imatinib mesylate therapy. Materials and methods. Eight consecutive patients with GIST confirmed by surgery (4 stomach, 2 small bowel, 1 small bowel and peritoneum, and 1 rectum) underwent eighteen 18F-FDG PET/CT imaging after beginning imatinib mesylate therapy (400 mg/day or greater if disease progression). PET/CT scan was acquired 60-90 minutes after the intravenous injection of 333-707 MBq of F-18-FDG. Visual and semi-quantitative (standardized uptake value [SUV]) analysis of images was performed. Response to therapy was assessed according to EORTC recommendations for PET Results were confirmed by clinical follow-up, radiographic findings or histological analysis. Results. Complete response to imatinib mesylate was observed in 5 patients. Four had abdominal lymph nodes, associated with liver metastases in 2, and the other had a residual tumour mass. Partial response (reduction in SUV and in the extent of FDG uptake) was demonstrated in a patient with lung nodules. Disease progression was observed in one patient who had developed new liver metastases on the PET/CT scan. One patient with multiple peritoneal implants and abdominal mass was a non-responder and died 2 months after the F-18-FDG PET/CT. Conclusion. F-18-FDG PET/CT scan identified the degree of GIST response to imatinib therapy. Patients who responded to therapy showed normalisation of FDG uptake or a decrease in the SUV of lesions.