Abstract PURPOSE: Thymic epithelial tumors (TETs) are rare tumors of the mediastinum, with an estimated incidence of about 3 cases per 100,000 inhabitants. Although anthracycline- and platinum-based chemotherapy is an active treatment for TETs, novel systemic therapeutic options are especially needed for metastatic disease, which is virtually incurable. On the basis of the radiographic response obtained with imatinib (Novartis Pharma, Basel, Switzerland) in a patient with thymic carcinoma harboring the V560del c-KIT mutation, a phase II trial was initiated at the Department of Molecular and Clinical Oncology and Endocrinology of University "Federico II of Naples" with the purpose to test imatinib in TETs. METHODS: Imatinib was daily delivered at the dose of 400 mg to patients affected by TETs, who had progressed after at least one chemotherapy regimen. Positivity of c-KIT on immunohistochemistry was not mandatory for study entry. Radiographic responses were measured by CT scans performed every 3 months, according to the RECIST criteria. Toxicity was graded according to the Common Toxicity Criteria of the National Cancer Institute, version 3.0. RESULTS: Fifteen patients with advanced TETs were enrolled from March 2008 to May 2010. Three patients presented with thymic carcinomas. Two of these three patients presented c-kit expression on immunohistochemistry. No patient harbored a known c-kit activating mutation. Imatinib was very well tolerated, with no toxicity-related death. Diarrhea and migraine were the most frequent events, occurring both in 20% of patients, but were manageable and mild. No radiographic responses were recorded. Median progression-free survival was 3 months (interquartile range, 2.5-4). Median overall survival was not reached. The study was terminated before it reached its target accrual of 42 patients, because of the lack of responses and low accrual rate. CONCLUSIONS: This trial indicates the lack of effectiveness of imatinib in unselected patients with thymic epithelial tumors. Nevertheless, imatinib may represent a valuable option in selected patients with TETs, such as those harboring the V560del c-KIT mutation.