The study investigates response assessment of gastrointestinal stromal tumors using Dual Energy CT (DECT) in patients undergoing targeted therapy with a TKI Inhibitor.

Patients treated with targeted therapies by tyrosine kinase inhibition (TKI) almost always show different response patterns than tumors treated with cytotoxic chemotherapy. Tumor manifestations treated with targeted therapies often show only minor measurable changes in tumor size despite inhibition of tumor proliferation. Decrease in tumor size fulfilling the criteria of a partial remission according to RECIST may occur at a delayed stage of treatment after several months (PMID 19620483). Sometimes, even an initial increase of maximum tumor diameters may be observed although those patients are treatment responders with respect to 'hard' study endpoints like overall survival or progression free survival. As a result, patients often are misclassified too early as non-responders. As a consequence, successful treatment is not recognized or decisions towards a less effective second-line therapy are made.

Dual Energy CT (DECT) is a new and robust CT method that allows to exactly quantify the intra-tumoral amount of intravenously injected iodinated contrast material PMID 21822784). Thus, the technique can be considered as a simple and reliable surrogate of tumor Perfusion (PMID 21822784, 21934517, 20498609, 18677487). Initial studies have demonstrated a high accuracy of DECT in the differentiation of benign and malignant tumors in the kidney or lung (PMID 20498609, 18677487, 18796658). Furthermore, it has been demonstrated that the amount of iodine in lung tumors correlates with tumor glucose metabolism (PMID 21822784). These findings confirm the results of previous studies that have demonstrated the relationship between tumor perfusion, tumor microvessel density and glucose metabolism (PMID 15304661).

The hypothesis of the study is that DECT is the more accurate diagnostic tool to evaluate immediate therapeutic response in patients with GIST undergoing TKI inhibitor therapy (in most cases Glivec ®) than evaluating by RECIST, World Health Organization (WHO) and Choi criteria.

Inclusion Criteria:

• General inclusion criteria for TKI treatment according to general medical standard procedures

Exclusion Criteria:

• General inclusion criteria for TKI treatment according to general medical standard procedures

gastrointestinal stromal (GIST) patients under tyrosine kinase inhibitor (TKI) treatment