Assessment of treatment response with diffusion-weighted MRI and ADC in metastatic GIST as compared to 18F-FDG PET/CT: A pilot study

Objectives To evaluate the potential of diffusion-weighted MR imaging (DWI) with apparent diffusion coefficient (ADC) mapping for the assessment of gastrointestinal stromal tumor (GIST) response to targeted therapy in comparison with 18F-FDG PET/CT.

Methods Five patients (3W/2M, aged 56.4±13.4y) with a metastatic GIST underwent a 18F-FDG PET/CT (Discovery LS, GEMS) and a MRI (VIBE T1Gd, DWI [b=50,300,600] with ADC mapping) within 3.6±4d of each other, before and 4.2±2.9months (2-9 months) after a targeted therapy change. MRI and PET/CT studies were first analyzed blindly, before PET/CT images were coregistered to T1Gd-MR images for lesion detection. SUVmax and ADC were measured for the six largest lesions on MR. Changes in SUVmax and ADC values before and after therapy modification were analyzed with paired t-test and Spearman’s correlation.

Results Altogether, 24 lesions (15 hepatic and 9 non-hepatic) were analyzed on PET/CT and MR-images. Three small lesions (12.5%) were not detected on ADC maps and 4 lesions on the second PET/CT were excluded because of hepatic vascular activity. All lesions together, SUVmax decreased from 7.2±7.7g/mL to 5.9±5.9g/mL (mean difference -1.6±2.8g/mL, range -7 to 3g/mL, P=0.53) and ADC increased from (1.2±0.4)x10-3mm2/s to (1.4±0.4)x10-3mm2/s (difference (0.2±0.4)x10-3mm2/s, range (-0.4 to 0.9)x10-3mm2/s, P=0.07). Changes in SUVmax and ADC were significant for non-hepatic lesions (P=0.01 and P=0.003), but not for hepatic lesions (P=0.6 and P=0.3). Moreover, there was a significant association between SUVmax decrease and ADC increase (ρ=-0.64, P=0.004).

Conclusions Changes in ADC maps from diffusion-weighted MRI seem to reflect response of 18F-FDG-avid GIST to targeted therapy. Thus, DWI might be of interest for the assessment of targeted therapy response in GIST patients, but its exact diagnostic value needs to be investigated, as well as the effect of lesion size and time under-therapy before imaging