Pulmonary or Mediastinal Metastasis

The lungs are common sites for blood-borne metastasis. The pulmonary capillary bed is the first set of fine-vessels that a blood-borne metastasis will encounter, unless the draining veins of the primary are portal veins. Typically, the incidence and location of the masses agrees with the bulk of the lungs and the bulk of the pulmonary circulation, that is, more in the lower zones. The lungs are pyramidal in shape, after all. Existing pulmonary disease may alter local flow and the underlying probabilities. Since the vessels are displayed in the plane of the Chest X-ray, separating a metastasis from a neighbouring vessel can sometimes be easier than on CT. In both cases the vessels equidistant from major divisions at the hilum are of similar size and larger densities may be metastasis.

Lymphangitis carcinomatosa is a special case, where direct invasion of the hila and pulmonary lymphatics, produces secondary tissue density in the lungs, typically without breathlessness. Appearances are best appreciated by reference to the images.

The lymph nodes of the mediastinum are a stop on the way to the superior vena cava in the flow of lymphatics from the lung and from structures below the diaphragm, particularly when the pathological process has changed the system and forced redirection of lymph flow.