The production of metastases is poorly understood and is just receiving the attention it deserves in the laboratory. Describing cancer of the lung as early "angio-invasive" does not illuminate the mechanism by which cancer cells gain access to the body circulation.
The microcirculation of neoplasms has been investigated by a variety of techniques, all of which purport to show an increased vasculature as compared to most normal tissues. The rapid growth of new vessels to keep pace with the accelerated metabolic demands of neoplasms often results in an imperfect vasculature with defective endothelial lining. In fact, the cancer cells may form the lining of the capillary bed and irregularity of vessel patterns can lead to sinusoidal dilatations and arteriole-venous shunts. From correlative histopathological and microangiographic studies, it is difficult to conceive of a tumor circulation as a closed circulation rather than an open one. In an open circulation, as in
Rubin P. Comment: Metastases—Experimental Facts and Clinical Fancies. JAMA. 1966;196(10):853-854. doi:10.1001/jama.1966.03100230097027