Editing Metastasis (section)

=== Routes ===
Metastasis occurs by the following four routes:

==== Transcoelomic ====

The spread of a malignancy into body cavities can occur via penetrating the surface of the [[peritoneal cavity|peritoneal]], [[pleural cavity|pleural]], pericardial, or subarachnoid spaces. For example, [[ovarian cancer|ovarian tumors]] can spread transperitoneally to the surface of the liver.{{citation needed|date=September 2024}}

==== Lymphatic spread ====

Lymphatic spread allows the transport of tumor cells to regional [[lymph node]]s near the primary tumor and ultimately, to other parts of the body. This is called nodal involvement, positive nodes, or regional disease. "Positive nodes" is a term that would be used by medical specialists to describe regional lymph nodes that tested positive for malignancy. It is common medical practice to test by biopsy at least one lymph node near a tumor site when carrying out surgery to examine or remove a tumor. This lymph node is then called a [[sentinel lymph node]].
Lymphatic spread is the most common route of initial metastasis for [[carcinoma]]s.<ref name="Robbins" /> In contrast, it is uncommon for a [[sarcoma]] to metastasize via this route. Localized spread to regional lymph nodes near the primary tumor is not normally counted as a metastasis, although this is a sign of a worse [[prognosis|outcome]].
The lymphatic system does eventually drain from the [[thoracic duct]] and [[right lymphatic duct]] into the systemic venous system at the [[venous angle]] and into the [[brachiocephalic vein]]s, and therefore these metastatic cells can also eventually spread through the haematogenous route.{{citation needed|date=September 2024}}

[[File:Lymph node with metastatic melanoma.jpg|thumb|Lymph node with almost complete replacement by metastatic melanoma. The brown pigment is focal deposition of melanin]]

==== Hematogenous spread ====

This is typical route of metastasis for sarcomas, but it is also the favored route for certain types of carcinoma, such as [[renal cell carcinoma]] originating in the [[kidney]] and follicular carcinomas of the thyroid. Because of their thinner walls, veins are more frequently invaded than are arteries, and metastasis tends to follow the pattern of [[venous blood|venous flow]]. That is, hematogenous spread often follows distinct patterns depending on the location of the primary tumor. For example, [[colorectal cancer]] spreads primarily through the [[portal vein]] to the liver.{{citation needed|date=September 2024}}

==== Canalicular spread ====

Some tumors, especially [[carcinoma]]s may metastasize along anatomical canalicular spaces. These spaces include for example the bile ducts, the urinary system, the airways and the [[subarachnoid space]]. The process is similar to that of transcoelomic spread. However, often it remains unclear whether simultaneously diagnosed tumors of a canalicular system are one metastatic process or in fact independent tumors caused by the same agent ([[field cancerization]]).{{citation needed|date=September 2024}}