PET-CT evaluation of gastrointestinal stromal tumors (GIST) treated with imatinib mesylate.

Objectives to correlate the changes in tumor density (HU units) and in SUV with the changes in size (RECIST: Response Evaluation Criteria in Solid Tumors) in metastatic lesions of GIST.

Methods we selected 50 lesions (36 in the liver, 12 abdominal and 2 peri-anastomosis) were evaluated on PET-CT before and 2 months after treatment (imatinib mesylate). We measured change in SUV of each lesion before and after treatment.The percentage of change in the SUV was thengraded on a scale of 1-4: 4: 61-100% decrease, 3: 15-60% decrease; 2: decrease of less than 15% or increase of less than 25%; and 1: increase of 25% or more (these grades were modified from EORTC criteria 1999).For each lesion we measured the CT attenuation coefficient (density) before and after treatment, in Hounsfield units.The percentage of change in tumor density was graded on a scale 1-4: grade 4: decrease of more than 30%; grade 3: 11-30% decrease; grade 2 decrease of 10% or less, or increase of 10% or less; and grade 1 increase of more than 10%. We comparated before and after treatment in each lesion the changes of size (RECIST), SUV uptake (EORTC criteria modified) and density.

Results for all 50 lesions RECIST showed: disease stable in 30/50 (60 %),disease progression in 18/50 and (36 %) and 2/50 partial response (4%).For the percentage of change in tumor density and SUV : 26/50 (52 %) patients was grade 1, 16/50 ( 32 %) grade 2, 4/50 grade 3 (8%) , and 4/50 grade 4 (8%).

Conclusions There was a high correlation between the response based on CT-HU and SUV max.But the changes of tumor size by the RECIST not showed high correlation with PET response.