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The onset of cancer usually begins as
a solitary tumor

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in a specific area of the body.

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If the tumor is not removed,

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cancer has the ability to 
spread to nearby organs,

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as well as places far away
from the origin, such as the brain.

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So how does cancer move to new areas,

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and why are some organs more likely
to get infected than others?

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The process of cancer spreading across the
body is known as metastasis.

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It begins when cancer cells from an initial
tumor invade nearby normal tissue.

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As the cells proliferate,

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they spread via one of the three 
common routes of metastasis:

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transcoelomic, lymphatic, 
or hematogenous spread.

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In transcoelomic spread, malignant cells
penetrate the covering surfaces

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of cavities in our body.

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These surfaces are known as 
peritoneum

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and serve as walls to segment 
the body cavity.

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Malignant cells in ovarian cancer, 
for example,

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spread through peritoneum, 
which connects the ovary to the liver,

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resulting in metastasis on 
the liver surface.

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Next, cancerous cells invade blood vessels
when they undergo hematogenous spread.

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As there are blood vessels almost 
everywhere in the body,

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malignant cells utilize this to reach
more distant parts of the body.

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Finally, lymphatic spread occurs
when the cancer invades the lymph nodes,

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and travels to other parts of the body
via the lymphatic system.

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As this system drains many
parts of the body,

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it also provides a large network
for the cancer.

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In addition, the lymphatic vessels empty
into the blood circulation,

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allowing the malignant cells to undergo
hematogenous spread.

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Once at a new site, the cells once again
undergo proliferation,

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and form small tumors known as
micrometastases.

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These small tumors then grow 
into full-fledged tumors,

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and complete the metastatic process.

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Different cancers have been known to have
specific sites of metastasis.

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For example, prostate cancer commonly
metastasizes to the bone,

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while colon cancer metastasizes 
to the liver.

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Various theories have been proposed to
explain the migration pattern

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of malignant cells.

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Of particular interest are 
two conflicting theories.

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Stephen Paget, an English surgeon,

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came up with the seed and soil
theory of metastasis.

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The seed and soil theory stated that
cancer cells die easily

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in the wrong microenvironment,

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hence they only metastasize to a location
with similar characteristics.

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However, James Ewing, the first professor
of pathology at Cornell University,

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challenged the seed and soil theory,

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and proposed that the site of metastasis
was determined by the location

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of the vascular and lymphatic channels
which drain the primary tumor.

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Patients with primary tumors that were
drained by vessels leading to the lung

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would eventually develop lung metastases.

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Today, we know that both theories contain
valuable truths.

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Yet the full stories of metastasis is much
more complicated

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than either of the two proposed theories.

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Factors like the cancer cell's properties,

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and the effectiveness of the immune system
in eliminating the cancer cells,

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also play a role in determining
the success of metastasis.

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Unfortunately, many questions about 
metastasis remain unanswered until today.

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Understanding the exact mechanism holds
an important key

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to finding a cure for
advanced stage cancers.

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By studying both the genetic and
environmental factors,

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which contribute to successful metastasis,


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we can pinpoint ways to shut down
the process.

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The war against cancer is 
a constant struggle,

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and scientists are hard at work developing
new methods against metastasis.

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Of recent interest is immunotherapy,

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a modality which involves harnessing the
power of the immune system

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to destroy the migrating cells.

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This can be done in different ways,

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such as training immune cells to recognize
cancerous cells via vaccines.

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The growth and activity
of the immune cells

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can also be stimulated by injecting
man-made interleukins,

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chemicals which are usually secreted by
the immune cells of the body.

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These two treatments are only the
tip of the iceberg.

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With the collaborated research efforts of 
governments, companies and scientists,

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perhaps the process of metastasis will
be stopped for good.