Correct!
2. The chest radiograph shows the “hilum overlay” sign
The frontal chest radiograph shows the “hilum overlay” sign. Originally described as a means to distinguish a mediastinal mass (often anterior) from an enlarged heart and enlarged pulmonary arteries, this chest radiographic sign may be applied more generally and can be said to be present when a lesion detected at chest radiography resides anterior or posterior to the pulmonary hilum, but not within the hilum itself. When a lesion arises from the pulmonary hilar structures, the lesion will be in contact with the hilar pulmonary vessels, and will therefore exclude the air in the lung that normally surrounds these vessels and creates the tissue density difference that allows the pulmonary hilar vessels to be visualized at chest radiography. Therefore, a true hilar mass will obscure the margins of hilar vessels. When a lesion appears to be related to the hilum, but in fact the hilar vessel contours can clearly be seen “though” the lesion, then it follows that the lesion cannot arise from the hilum because lung air must still invest the hilar vessels- this situation represents the “hilum overlay” sign. The mass on the chest radiograph (Figure 1) shows this relationship- the right interlobar pulmonary artery can be clearly seen “thorough” the lung mass. The “incomplete border” sign is present when an opacity on chest radiography shows a relatively circumscribed border along one side, but the other side shows an indistinct border. This imaging finding may be seen with extraparenchymal lesions. The “V sign of Naclerio” refers to V-shaped lucency projected over the basal medial left mediastinum, resulting from pneumomediastinum: one limb of the “V” is caused by air interposed between the parietal pleura and medial left diaphragm, whereas the other limb of the “V” results from air extending cranially along the medial left mediastinal border. The “Luftsichel” sign may be seen in patients with left upper lobe collapse. Luftsichel in German means “air crescent,” (“luft” = air, “sichel”= sickle) which refers to the lucency that may outline the aortic arch when the superior segment of the left lower lobe becomes hyperinflated and extends more cranially than usual in response to left upper lobe collapse. The “cervico-thoracic sign describes the location of a lesion at the thoracic inlet. Because the posterior lung apex normally extends more cranially than the anterior lung apex, a lesion in the posterior mediastinum located at the cervico-thoracic junction will show a contour abnormality that extends more superiorly than an anterior or middle mediastinal lesion. This occurs because a posterior mediastinal lesion in this location will maintain contact with the lung parenchyma to a greater superior extent [generally cranial to the clavicles] than either anterior or middle mediastinal lesions at the cervico-thoracic junction, thereby creating a visible contour, whereas anterior or middle mediastinal lesions at the cervico-thoracic junction will lose their visible contour as they enter the soft tissues of the caudal neck.
Clinical course: The patient subsequently underwent unenhanced thoracic CT (Figure 2).
Figure 2: Axial contrast-enhanced thoracic CT.
Which of the following is the leading consideration for the likely etiology of the lesion in this patient? (Click on the correct answer to proceed to the fourth of 6 panels)