Differentiating macrovascular tumor thrombus from bland thrombus in thromboembolic disease can be very challenging on CT angiography. The primary obstacle is that bland thrombus is common in patients with cancer, whereas tumor thrombus is rare. When the large pulmonary arteries are involved, certain factors can lead the physician to the diagnosis, such as the size of the involved pulmonary artery and the morphology and enhancement properties of the filling defect.5 A pulmonary artery involved with tumor thrombus is more likely to be expanded by the dividing tumor. Even when tumor embolism is microvascular, an important finding is identifying beading of the small arteries. Meanwhile, bland thrombus should not cause arterial dilation at the site of obstruction. As well, bland thrombus often has irregular or jagged edges, whereas tumor thrombus tends to be more rounded and smoothed (Fig 4). Enhancement characteristics of an embolism represent the least reliable method for differentiating bland from tumor thrombus. Often, tumor thrombi occur in small vessels, and an attenuation reading cannot be reliably obtained. Streak artifact from surrounding intravascular contrast also artificially suppress attenuation measurements. Finally, precontrast images usually are not obtained, so quantitative enhancement by subtracting the precontrast from the postcontrast attenuation value is not possible.