A lung cancer screening CT is usually not optimized to measure the ascending aorta. The ascending aorta moves with every heartbeat, and if the scan is not ECG-gated, motion can make the aorta look larger than it really is. Measurements can also differ depending on whether the radiologist measured straight across an axial image or measured perpendicular to the centerline of the aorta, which is usually the better method. A CT angiogram is usually ordered because it gives a more focused look at the aorta, the vessel wall, the lumen, and the exact dimensions. If the CTA was done with the right protocol, especially if it was ECG-gated or otherwise technically adequate, I would generally put more weight on that study than on an incidental measurement from a lung screening CT. That said, I would not just ignore the discrepancy. I would ask the cardiothoracic surgeon one very direct question: “Did you personally compare the lung screening CT images with the CT angiogram images, and what was the actual measured diameter of the ascending aorta on the CTA?”

You are not making anyone mad by asking that. This is exactly the kind of question that should be asked. A reported 4.3 cm ascending aorta is not usually an emergency, but it is important enough to clarify because future follow-up depends on whether the aorta is truly enlarged and what the accurate baseline measurement is. I understand why this kind of conflicting report would be unsettling. The practical next step is not to challenge the doctors emotionally, but to ask for a clear image-to-image comparison and the exact measurement method. If the answer still does not make sense, getting a second read from an aortic center or another cardiothoracic surgeon is reasonable. Peace.