Very early detection of response to imatinib mesylate therapy of gastrointestinal stromal tumours using 18fluoro-deoxyglucose-positron emission tomography.

Objective: To investigate whether 18F-FDG-PET allows early assessment of response to imatinib mesylate in GIST patients.

Methods: Five gastrointestinal stromal tumour (GIST) patients and one patient with a KIT-positive small cell cancer were studied. Treatment consisted of 400 mg imatinib mesylate daily. 18F-deoxyglucose-positron emission tomography (18F-FDG-PET) scans were done before and one week after starting imatinib mesylate therapy.

Results: Metabolic responses were detected after only one week of therapy in all GIST patients (four partial and one complete response). The mean decrease of the standardised uptake value was 60% (range 43 to 77%). In contrast, the tumour of the non-GIST patient was metabolically stable. Four of the 5 GIST patients achieved a partial response on CT after a mean duration of 23 weeks (range 6 to 48 weeks).

Conclusions: 18F-FDG-PET is a valuable method for the detection of response to imatinib mesylate in patients with KIT-positive tumours as early as one week after starting therapy.