Correlation of computed tomography and positron emission tomography in patients with metastatic gastrointestinal stromal tumor treated at a single institution with imatinib mesylate: proposal of new computed tomography response criteria.

PURPOSE Response Evaluation Criteria in Solid Tumors (RECIST) are insensitive in evaluating gastrointestinal stromal tumors (GISTs) treated with imatinib. This study evaluates whether computed tomography (CT) findings of GIST after imatinib treatment correlate with tumor responses by [18F]fluorodeoxyglucose (FDG) positron emission tomography (PET) and develops reliable, quantitative, CT response criteria. PATIENTS AND METHODS A total of 172 lesions selected by RECIST were evaluated in 40 patients with metastatic GISTs treated with imatinib. All patients had pretreatment and 2-month follow-up CTs and FDG-PETs. Multivariate analysis was performed using tumor size and density (Hounsfield unit [HU]) on CT and maximum standardized uptake value (SUVmax) on FDG-PET. Patients were observed up to 28 months. RESULTS Mean baseline tumor size and density on CT were 5.3 cm and 72.8 HU, respectively, and mean baseline SUVmax on FDG-PET was 5.8. Thirty-three patients had good response on FDG-PET. A decrease in tumor size of more than 10% or a decrease in tumor density of more than 15% on CT had a sensitivity of 97% and a specificity of 100% in identifying PET responders versus 52% and 100% by RECIST. Good responders on CT at 2 months had significantly longer time to progression than those who did not respond (P = .01). CONCLUSION Small changes in tumor size or density on CT are sensitive and specific methods of assessing the response of GISTs. If the prognostic value of our proposed CT response criteria can be confirmed prospectively, the criteria should be employed in future studies of patients with GIST.