Pathways of metastatic spread of malignant tumors.

In order to produce viable metastases, the neoplastic cells must first be able to survive in the fluid system that they attain and that transport them. An important circumstance that has escaped the attention of most observers is the fact that the establishment of metastatic colonies is favored by the sluggish transportation of the detached neoplastic cells. Such leisurely transport is provided by lymphatic channels and peritoneal fluid; in the peritoneal cavity, the favorable circumstance aids the spread of relatively benign tumors such as pseudomyxoma peritonei and ovarian adenomas. In the blood, the slow transport is assured only by the venous access to the vertebral vein system; this avenue to blood-borne spread is particularly favorable to well differentiated tumors with a slow rate of growth and explains the widespread bone metastases from tumors with a low degree of malignancy, such as well differentiated adenocarcinomas of the thyroid and prostate. In contrast, neoplastic cells that enter the swift arterial circulation from the left heart are not often successfully implanted, in spite of their high degree of malignancy, and more often perish. Otherwide, we should observe a greater number of metastases in the small bones of the wrists and hands and of the ankles and feet, where the arterial circulation would bring them.