CT Evaluation of the Response of Gastrointestinal Stromal Tumors After Imatinib Mesylate Treatment: A Quantitative Analysis Correlated with FDG PET Findings

OBJECTIVE: We correlated changes in tumor density on CT with changes in glucose metabolism, or the maximum standardized uptake value (SUV(max)), on FDG PET and sought to develop CT imaging criteria that can be used to objectively evaluate tumor response in patients with metastatic gastrointestinal stromal tumors (GISTs) who undergo treatment with imatinib mesylate. MATERIALS AND METHODS: Using the criteria established by the Response Evaluation Criteria in Solid Tumors (RECIST) group, we selected 173 tumors (in 36 patients) for study. Tumor size and density were determined objectively, and overall tumor response (OTR) was evaluated subjectively on CT images. The changes in these parameters before and after treatment were correlated with changes in SUV(max). RESULTS: Significant decreases were seen in both tumor density (mean, 12.3 H 16.5%; p < 0.0001) and SUV(max) (mean, 3.43 64.9%; p < 0.0001). OTR evaluated subjectively, correlated well with changes in SUV(max) (p < 0.0001). No statistically significant association was found between changes in tumor density and changes in SUV(max) (p = 0.3088), but 70% (14/20) of the patients with tumors that showed response on FDG PET exhibited at least a partial response by a change in tumor density. Tumor size was found to have decreased significantly 2 months after treatment (p = 0.0070). However, in 75% of the patients, the disease was stable according to the traditional tumor response criteria of RECIST. CONCLUSION: FDG PET is sensitive and specific for evaluating tumor response but cannot be used in patients whose baseline FDG PET results are negative for tumors. Although subjective evaluation was a better indicator of treatment response than was tumor density alone, the tumor density measurement is a good indicator and provides a reliable quantitative means of monitoring the tumor. RECIST, using only tumor size, was unreliable for monitoring GISTs during the early stage of imatinib mesylate treatment.