Distant (code = 7)
Distant metastases are tumor cells that have broken away from the primary tumor, have traveled to other parts of the body, and have begun to grow at the new location. Distant stage is also called remote, diffuse, disseminated, metastatic, or secondary disease. The point is that in most cases there is no continuous trail of tumor cells between the primary site and the distant site.
Cancer cells can travel from the primary site in any of four ways:
- Extension from the primary organ beyond adjacent tissue into the next organ; for example, from the lung through the pleura into bone or nerve.
- Travel in lymph channels beyond the first (regional) drainage area to lymph nodes that are considered remote from the primary site. Tumor cells can be filtered, trapped and begin to grow in any lymph nodes in the body.
- Hematogenous or blood-borne metastases. Invasion of blood vessels within the primary tumor allows escape of tumor cells or tumor emboli which are transported through the blood stream to another part of the body where it lodges in a capillary or arteriole. At that point the tumor penetrates the vessel wall and grows into the surrounding tissue.
- Spread through fluids in a body cavity. Example: malignant cells rupture the surface of the primary tumor and are released into the thoracic or peritoneal cavity. They float in the fluid and begin to grow on any tissue reached by the fluid. This type of spread is also called implantation or seeding metastases. Some tumors form large quantities of fluid called ascites that can be removed, but the fluid rapidly re-accumulates. However, the presence of fluid or ascites does not automatically indicate dissemination. There must be cytologic evidence of malignant cells. For lung, pleural effusion is staged as distant.
Common sites of distant spread are liver, lung, brain, and bones, but they are not necessarily listed specifically for each scheme. These organs receive rich blood flow from all parts of the body and thus are a target for distant metastases. However, if the primary site is adjacent to the liver, lung, brain or bone, it is important to review the summary staging scheme for the primary site to assure that the stage is not regional by direct extension. An example would be liver involvement from a primary tumor in the gallbladder. It is likely that this is regional by direct extension rather than distant stage, since the gallbladder is adjacent to liver. Read the diagnostic imaging reports carefully to determine whether the cancer involves the surface of the secondary organ or whether the cancer is inside the secondary organ. If the latter is the case, the only way it could have developed in the secondary organ is if the tumor cells arrived there via the blood stream (distant hematogenous metastases). Another way to remember the difference between regional direct extension and distant metastases is whether the secondary site has tumor on the surface (most likely direct extension) or in the organ (blood-borne metastases). Hematopoietic, reticuloendothelial, immunoproliferative, and myeloproliferative neoplasms are considered distant or remote at the time of diagnosis, except as noted in the staging scheme.
As shown in the series of illustrations above, the cancer cell that invaded the blood vessel has floated to a new organ. As the blood vessels in the secondary site get smaller, the cancer cell has the ability to penetrate the capillary wall and settle in the new organ. The growth of tumor in the new organ is called a metastasis, and the location is referred to as metastatic site.