Thoracic aneurysm is a silent disease with a potential mid-term high risk of death or major complications. Few data are available on the real incidence of major complications in case of small and moderate thoracic aneurysm. Different factors are supposed to increase the risk of aortic enlargement as high blood pressure and sleep disorder breathing. The modality of imaging and clinical follow-up are well defined. In this prospective observational study, the aim to assess the incidence of of major complications during follow-up in a population of patients with a small or moderate thoracic aneurysm. The study will also try to identify systemic factors influencing aneurysm evolution.