Metastasis
Metastasis is the spread of a primary tumor to a physically discontinuous site.
For example, if a malignant tumor arising in the breast spreads to the liver, the tumor in the liver represents a metastatic deposit.
Metastasis is an important feature of malignant tumors. Benign tumors do not metastasize. However, some malignant tumors, such as gliomas and basal cell carcinoma, metastasize only infrequently.
Why is Metastasis Important?
Metastasis is a complex process that depends on the interaction between the tumor and the host.
It is clinically important because metastatic spread:
- Indicates malignant behavior
- Has an important influence on prognosis
- Is important for cancer staging
- Influences treatment planning
- Limits the possibility of cure
An important point to remember is that growth rate alone does not predict metastatic behavior. Some slowly growing tumors may metastasize widely, while some rapidly growing tumors may remain localized at the time of diagnosis.
Routes of Metastasis
There are three major routes of metastatic spread:
- Lymphatic spread
- Hematogenous spread
- Seeding of body cavities
1. Lymphatic Spread
Lymphatic spread is the most common route of spread of carcinomas.
Tumor cells enter the lymphatics and usually spread along the natural lymphatic drainage of the organ to the regional lymph nodes.
Sentinel Lymph Node
The sentinel lymph node is the first draining lymph node from a primary tumor.
It need not necessarily be the lymph node anatomically closest to the tumor.
Evaluation of the sentinel lymph node can provide important information for staging, prognosis, and treatment planning.
Remember that an enlarged lymph node does not always indicate metastasis. Lymph nodes draining a tumor may also enlarge because of a reactive process.
2. Hematogenous Spread
Hematogenous spread is particularly important in sarcomas, although carcinomas can also spread through the bloodstream.
Tumor cells tend to invade veins more commonly than arteries.
The pattern of hematogenous spread depends on:
- Venous drainage of the tumor
- Biology of the tumor
The liver and lungs are common sites of hematogenous metastasis.
3. Seeding of Body Cavities
Tumors may spread by seeding across natural body cavities and serosal surfaces.
Body cavities that may be involved include:
- Peritoneal cavity
- Pleural cavity
- Pericardial cavity
The peritoneal cavity is an important site for this form of spread, particularly in malignant tumors of the ovary.
Extensive tumor deposits over the peritoneal surfaces may produce peritoneal carcinomatosis.
Routes of Metastasis at a Glance
| Route | Important Association |
|---|---|
| Lymphatic spread | Common route for carcinomas |
| Hematogenous spread | Common route for sarcomas |
| Seeding of body cavities | Spread across body cavities and serosal surfaces |
Important Exceptions
The usual routes of spread are useful general rules, but exceptions do occur.
Some sarcomas can spread through lymphatics, while some carcinomas show prominent hematogenous spread.
Therefore, every tumor has its own characteristic metastatic behavior.
Key Take-Home Points
- Metastasis is spread of a primary tumor to a physically discontinuous site.
- Benign tumors do not metastasize.
- The three major routes are lymphatic spread, hematogenous spread, and seeding of body cavities.
- Lymphatic spread is particularly associated with carcinomas.
- Hematogenous spread is particularly associated with sarcomas.
- The sentinel lymph node is the first draining lymph node from a primary tumor.
- The route and pattern of metastasis are important for staging, prognosis, and treatment planning.
- Remember the general rules, but also remember that exceptions occur.
Want to Understand This Better?
The video explains these concepts with examples, patterns of spread, sentinel lymph nodes, skip metastasis, important exceptions, and illustrations of metastatic disease.
▶️ Watch the complete video on ILovePathology:
