The term metastasis refers to the process that malignant cells from primary sites invade into lymph vessels, or blood vessels, or body cavities and reach remote sites continue to grow and from secondary tumors. The secondary tumors are called as metastases, and tend to grow more rapidly than the primary tumors in the same patients. Metastatic tumors are discontinuous with the primary tumors. Metastasis is the most reliable biological feature of malignant tumors. However, not all cancers have ability to Metastasize. At one extreme are basal cell carcinomas of the skin and most primary tumors of the central nervous system that are highly invasive in their primary sites of origin but rarely Metastasize. At other extreme are osteogenic sarcomas, which usually have metastasized to the lungs at the time of initial discovery. In general, the more anaplastic and the larger the primary neoplasm, the more likely is metastatic spread.
Malignant neoplasms disseminate by one or two of three pathways: lymphatic metastasis, hematogenous metastasis, seeding metastasis within body cavities.
It is more typical of carcinomas, whereas the hematogenous route is favored by sarcomas. There are numerous interconnections, however, between the lymphatic and vascular system, and so all form of cancer may disseminate through either or both systems. The pattern of lymph node involvement depends principally on the site of the primary neoplasm and the natural lymphatic pathways drainage of the site. Lung carcinomas arising in the respiratory passages metastasize first to the regional bronchial lymph nodes, then to the tracheobronchial and hilar nodes. In some cases, the cancer cell to seem traverse the lymphatic channels within the immediately proximate nodes to be trapped in subsequent lymph nodes, producing so – called skip metastases. The enlargement of lymph nodes is a clinical features of the metastasis by lymphatic pathway. But it does not always imply cancerous involvement.
This is the most feared consequence of the cancer. It is the favored pathway for sarcomas, but carcinomas used it as well. As might be expected, arteries are penetrated less readily than are veins. With venous invasion, the blood borne cells follow the venous flow draining the site of the neoplasm. The liver and the lungs are the most frequently involved secondary sites in such hematogenous dissemination. In principle all portal area drainage flows to the liver, and all caval blood flows to the lungs (fig. 5-5). Cancers arising near the vertebral column often embolize through the paravertebral plexus; this pathway probably is involved in the frequent vertebral metastasis of carcinomas of thyroid and prostate.
Certain carcinomas have a propensity for invasion of veins. Renal cell carcinoma often invades the renal vein to grow in a snakelike fashion up the inferior vena cava, sometimes reaching the right side hart. Hepatocellular carcinomas often penetrate portal and hepatic radicles to grow within them into the main venous channels. Many observations suggest that mere anatomic localization of neoplasm and natural pathway of venous drainage do not wholly explain the systemic distributions of metastases. For example, prostatic carcinomas preferentially spreads to bone, brnocgenic carcinomas tend to involve the adrenals and the brain, and neuroblastomas spread to the liver and the bones. Conversely, skeleton muscles are rarely the rarely the secondary deposits.
Seeding metastasis of cancer occurs when neoplasm invade a natural body cavity carcinomas of stomach may penetrates to the gastric wall and re implant at distant sites in the peritoneal cavity, especial the ovaries such as krukenberg tumor. This mode of dissemination is particularly characteristic of cancer of the ovary. Which often cover the peritoneal surface widely. We should remember that care must to be taken to avoid iatrogenic seeding metastasis.