Introduction
Up to 50% of extrapulmonary malignancies will have lung metastases at autopsy, and in a quarter of cases the lungs will be the only site of such metastases. This is due to several unique features of the lungs, which make them ideal “seed and soil” territory, accepting most of the extrapulmonary metastases. First, lungs receive the entire right-sided cardiac output continuously, with every heartbeat. Second, lungs have the densest capillary network in the body, which consists of delicate, easily penetrable membranes. Third, pulmonary vascular plexus receives most of the upstream lymph flow draining into the venous system. In most instances lungs are the first organs exposed to drainage of tumor cell–bearing fluids, such as lymph and blood.
Virtually any malignancy may spread to the lungs, but carcinomas (adenocarcinomas in particular) are far more common than sarcomas, lymphomas, and melanomas simply because of their overall frequency. The most common primary tumor sites are breast, colon, stomach, pancreas, kidney, melanoma, prostate, liver, thyroid, adrenals, and male and female genital systems.
The most common route of metastatic tumor spreading to lungs is hematogenous. There are two principal ways that tumor cells reach the lungs: by direct invasion into the veins or by lymphatic spreading, that is, lymph drainage eventually entering the venous blood. Hematogenous metastases are most frequently seen as bilateral nodules of variable size within middle and lower peripheral lung fields. These nodules in comparison to primary tumors are more evenly contoured, less than 3 cm in diameter, more peripherally located, and grow faster.
Lymphangitic spreading, also called lymphogenous spreading or lymphangitic carcinomatosis, is another common route of metastases, especially from breast, stomach, bronchus, and Kaposi sarcoma. The most common mechanisms of lymphangitic spreading are direct lymphatic invasion, spreading from adjacent blood vessels, and retrograde lymphangitic spreading.
Direct extension as surface spreading, through chest wall, mediastinum, and diaphragm or via permeating lymphatics, is also considered to be a mechanism of metastatic disease as long as tumor deposits are discontinuous from the primary site.
Endobronchial spreading is the least common type but is important in the differential diagnosis of primary tumors. The mechanisms of endobronchial spreading include hematogenous and lymphatic seeding, spreading from adjacent lung parenchyma, mediastinum, and tumor nodules, or by aerogenous routes. These types of metastases are commonly associated with primary carcinomas of breast, colon, kidney, lung, rectum, melanoma, cervix, and sarcomas.
In addition to main routes, the secondary mechanisms of lung metastases include intrathoracic nodal spreading, pleural metastases, and bronchopleural fistulas.