Plain radiograph evidence of mediastinal invasion relies on demonstrating phrenic nerve paralysis. Caution is needed, however, before deciding that a high hemidiaphragm is caused by phrenic nerve invasion, because lobar collapse can also lead to elevation of a hemidiaphragm, a subpulmonary effusion may mimic it, and diaphragmatic eventration is common.
The major CT and MRI signs of mediastinal invasion include the demonstration of visible tumour deep within the mediastinal fat, particularly if tumour surrounds the mediastinal vessels, oesophagus, or proximal mainstem bronchi (Fig. 1). Associated pneumonia or atelectasis may make it very difficult to determine whether or not mediastinal contact is present. Even clear-cut contact with the mediastinum is not enough for the diagnosis of invasion, and the apparent interdigitation of tumour with mediastinal fat can be a misleading sign on both CT and MRI. Glazer et al. showed that the presence of (A) less than 3 cm of contact with the mediastinum, (B) less than 90° of circumferential contact with the aorta, or (C) a visible mediastinal fat plane between the mass and any vital mediastinal structures indicated a very high likelihood of technical resectability, even if the tumour had crossed into the mediastinum, and that most tumours in their series conforming to this description had no mediastinal invasion at surgery.73 When the question is turned round to enquire as to the criteria for unresectability, however, the answer is less certain. Tumours that obliterate fat planes or show greater contact than that described above are not necessarily unresectable, though the greater the degree of invasion and the extent of contact, the more likely it is that there is significant mediastinal involvement.
MRI does not appear to offer any advantages over CT for the routine diagnosis of mediastinal invasion, its role being limited to problem solving in specific cases. Before the advent of multidetector CT (MDCT), the multiplanar capabilities of MRI could be used to advantage to identify involvement of major mediastinal blood vessels (Fig. 2) and the tracheal carina. MDCT has largely obviated the need to proceed to MRI to take advantage of multiplanar imaging alone; however, MRI sequences optimised for evaluation of the heart and vessels may still offer advantages where there is concern about invasion of hilar or mediastinal vessels, the heart or pericardium.
FIGURE 1 ■ Mediastinal invasion. CT image (A) displayed on mediastinal windows and (B) displayed on lung windows of deep mediastinal invasion by non-small cell lung cancer. The tumour is obstructing the right main bronchus and compressing the right main pulmonary artery; it is also encasing the stented superior vena cava and the aorta. Some postobstructive atelectasis is noted on lung window.
FIGURE 2 ■ MRI of a left lower lobe tumour that has directly invaded the aortic wall, which has altered signal adjacent to the tumour.
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