James Ewing lecture. The process of metastasis.

Carcinoma begins in the epithelium, and after penetrating the basement membrane, either compresses or destroys surrounding normal cells. Small numbers of tumor cells detach from this invading tumor mass to float as tumor emboli in the interstitial compartment. These tumor emboli may travel for considerable distance in the interstitial compartment and may remain dormant in that location for long periods of time, until some event such as the ingrowth of blood vessels permits the cells to grow and divide. Some of the tumor emboli floating in the interstitial fluid will be washed through clefts in the small lymphatic capillaries to be carried along in the lymph stream to the regional lymph nodes or the vascular system. Lymphatic venous anastomoses throughout the body permit systemic dissemination of the cancer cells without passing through the lymph nodes, and tumor emboli return through the arterial capillaries to the interstitial compartment. The swifter the current, the less likely the pathologist is to observe tumor cells on random sections. A demonstration on microscopic slides of tumor cells in the larger lymphatics or the veins is a measure of the quantity of cells being disseminated rather than a demonstration of the qualitative factor of aggressive behavior.