Metastasis is the spread of tumors by invasion so that discontinuous secondary tumors are formed at the site of lodgement. This is the characteristic by which malignancy is different from benign tumors because benign tumors do not metastasise.
Malignancy of Cancers spread to distant sites through the following pathways:
- Lymphatic pathway: Generally carcinoma metastasize through this route. Very few types of sarcoma follow this pathway. There may be lymphatic permeation of the lymphatic channels since lymphatic walls are readily invaded by cancer cells. OR, The malignant cells may be able to detach from the origin to form tumor emboli and carried along the lymph to the next draining lymph nodes and get lodged in the subcapsular sinuses and start growing over there.
Thereafter, the whole lymph node get replaced by metastatic tumor. In some cases, lymphatic metastasis fail to develop in the lymph nodes nearest to the tumor because of Venous Lymphatic Anastomoses or due to Obliterated Lymphatics. Due to obstruction of Lymphatics by tumor cells, there is a disturbance in the flow of the lymph causing Retrograde Metastasis at unusual sites as in the case of prostate cancer. Usually, regional lymph nodes draining the tumor cells are involved in producing Regional Nodal Metastasis as when breast carcinoma spread to the axillary Lymphatics. Another type of nodal metastasis spreading to the supraclavicular lymph nodes from the abdominal organs is called Virchow’s Lymph node.
- Hematogenous pathway: This is the route of metastasis for sarcomas especially those of the lungs, breasts, renal, prostatic and cerebral. This is a type of blood – borne route. Frequently, cancer of head, neck and limbs metastasize to the lungs through systemic veins. Portal veins drain the blood from spleen and pancreas into the liver causing secondary tumor in the hepatocytes. Blood carried by the pulmonary veins may carry cancer cells from the lungs to the left side of the heart and finally into the systemic circulation forming secondary masses of tumor cells in different parts of the body. In case of vertebral metastasis in thyroid gland and prostate cancer, there may be Venous obstruction leading to Retrograde Metastasis at unusual sites of the body.
- Pathways through body cavity and Natural passages: Sometimes, peritoneal cavities and serosal walls of coelomic cavities carry cancer cells and implant them elsewhere in the body as in the case of Gastric carcinoma, Ovarian carcinoma, Breast carcinoma or Bronchi carcinoma. In case of Krukenberg tumor, gastric carcinoma may metastasize to the ovaries in females through this pathway. Sometimes, in exceptional cases, malignant cells may spread through the oviducal funnel from the endometrial mass to the ovaries or from the kidneys down to the lower urinogenital tract. Malignant cells of the ependyma and leptomeninges may spread through the CSF.
HENCE, WE CAN SAY THAT METASTASIS IS THE TRANSFORMING OF RESIDENCE FROM ONE SITE TO DIFFERENT PARTS OF THE BODY.