Gastrointestinal Stromal Tumors Treated with Imatinib Mesylate: Apparent Diffusion Coefficient in the Evaluation of Therapy Response in Patients

Purpose: To prospectively investigate the use of the apparent diffusion coefficient (ADC) as an early response indicator in patients with gastrointestinal stromal tumors (GISTs) treated with imatinib mesylate. Materials and Methods: This study was approved by the institutional review board and written informed consent was obtained from all patients. Diffusion-weighted magnetic resonance (MR) imaging was performed in 32 patients with GISTs before and 1, 4, and 12 weeks after treatment with a tyrosine kinase inhibitor, imatinib mesylate. The measurable lesions were classified as having responded well or poorly according to size alterations at clinical evaluation following the first round of treatment (3 months). A linear mixed-effects model was applied to analyze changes in the ADCs of tumors during treatment and to compare the variation and slopes of the time-dependent ADC curves between the good-and poor-response groups. Results: There were 56 lesions in the good-response group and 35 in the poor-response group. An early (1 week after therapy) noticeable and statistically significant (P < .001) increase in the ADC was observed in the good-response group (median ADC increase, 44.8%) but not in the poor-response group (median ADC increase, 1.5%). The time-dependent ADC variation was significantly different between the good- and poor-response groups, with a sharper median ADC increase displayed in the former (week 1: 44.8% vs 1.5%; week 4: 80.4% vs 7.8%; week 12: 89.6% vs 16.7%; F = 25.78, P < .001). The largest difference in the weekly percentage increase in ADC between the good-and poor-response groups was observed at 1 week after therapy (week 0-1: 44.8% vs 1.5%; week 1-4: 7.0% vs 2.8%; week 4-12: 1.6% vs 0.7%). The pretherapy mean ADC (+/- standard deviation) of lesions in the good-response group (1.06 [x 10(-3) mm(2)/sec] +/- 0.27) was signifi cantly lower than that in the poor-response group (1.24 [x 10(-3) mm(2)/sec] +/- 0.32) (F = 8.34, P = .005). Conclusion: Comparatively low pretherapy ADC and marked ADC increase at 1 week after therapy is associated with good response to imatinib mesylate in patients with GISTs. (C)RSNA, 2010