Pathways of tumor spread through the lung: radiologic correlations with anatomy and pathology

The pathways of tumor spread through the lung are described and their significance for radiographic interpretation is illustrated. A key to understanding the spread of bronchogenic carcinoma is the realization that although the normal flow of lymph in the pulmonary lymphatics is centripetal, lymphatic obstruction can cause reversal of flow. As a result, tumor cells are commonly carried centrifugally to the periphery in lymphatics or the connective tissue around them, and remote pleural involvement, secondary parenchymal masses, or satellite nodules may develop. Failure to appreciate peripheral spread of tumor has negative consequences for tumor staging, surgery, and radiotherapy. In the absence of hilar node involvement causing obstruction, long line shadows more than 0.5 inch (1.25 cm) in length proximal to a peripheral mass very infrequently represent tumor.