RADIOLOGY: VASCULAR: Case# 24: PSEUDOANEURYSM DISSECTING THROUGH CHEST WALL. The patient is a 70 year old white male who presents with a left back soft tissue "pulsatile" mass. This mass underwent biopsy at outside hospital. The descending thoracic aorta is enlarged with a diameter of 5 cm. A saccular aneurysm originates off of the left posterolateral descending thoracic aorta. The posterior portion of this saccular aneurysm gives rise to a pseudoaneurysm which, in turn, supplies a chest wall hematoma. This case demonstrates the advantage of cross-sectional imaging prior to undertaking an invasive procedure (or, rather, the hazards of not having the imaging as a guide). Most likely, the saccular aneurysm was present before the biopsy. The pseudoaneurysm may have been present before the procedure or developed as a consequence of the biopsy.