Imatinib mesylate for the treatment of gastrointestinal stromal tumours: best monitored with FDG PET

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Abstract
The new anti-cancer drug imatinib mesylate inhibits the tyrosine kinase growth factor receptor, c-KIT, and has shown spectacular activity in patients with gastrointestinal stromal tumours (GISTs). To assess whether fluorodeoxyglucose positron emission tomography (FDG PET) is suitable for response evaluation of this new type of tumour treatment. Sixteen consecutive patients with irresectable or metastasized GIST or another c-KIT (CD117) positive mesenchymal tumour underwent FDG PET before and 1 week after the start of treatment with imatinib mesylate (Glivec®). Visual findings and standard uptake values (SUVs) were compared with the overall response to treatment, based on clinical and radiological response. PET visualized all known and some unknown tumour locations. The separation by PET after 1 week of treatment in ‘PET responders’ (11/16 patients, mean SUV reduction 65%) versus ‘PET non-responders’ (5/16 patients, mean SUV increase 16%) appeared to match almost perfectly with overall treatment response and proved correct in 14/15 patients (prediction sensitivity 93%). FDG uptake changes after 1 week of treatment were of greater magnitude than tumour volume changes on computed tomography at 8 weeks. Progression-free survival was significantly better in patients with a PET response (P=0.002). PET response predicted treatment outcome better than the radiological response. Finally, PET was helpful during follow-up, in discriminating side effects from tumour progression. FDG PET is a valuable tool in patients with gastrointestinal stromal tumours treated with imatinib mesylate. It improves staging, accurately separates responders from non-responders in an early phase, and is helpful during follow-up.