If the tumor is not removed, cancer has the ability to spread to nearby organs, as well as places far away from the origin, such as the brain.

如果腫瘤沒有被切除癌症就有辦法傳播到附近的器官，或是距離發源地較遠的地方，例如腦部。

So how does cancer move to new areas, and why are some organs more likely to get infected than others?

那麼癌症是如何移動到新的區域的呢？又是為什麼有些器官比其他器官更易受到感染呢？

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

癌症在身體裡擴散的形式叫做「轉移」。

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

這從癌細胞由原本的腫瘤入侵附近的一般組織時開始。

As the cells proliferate, they spread via one of the three common routes of metastasis: transcoelomic, lymphatic, or hematogenous spread.

當癌細胞開始擴散，它們會透過三種常見的轉移路徑進行擴散： 移植性轉移、淋巴性轉移與血行性轉移。

In transcoelomic spread, malignant cells penetrate the covering surfaces of cavities in our body.

在移植性轉移中，惡性細胞會穿透身體裡覆蓋體腔的防護表層。

These surfaces are known as peritoneum and serve as walls to segment the body cavity.

這些表層就是所謂的腹膜 (腹腔中的一層黏膜)，用以區隔身體裡的腔室。

Malignant cells in ovarian cancer, for example, spread through peritoneum, which connects the ovary to the liver, resulting in metastasis on the liver surface.

舉例來說，卵巢癌的惡性細胞透過連接卵巢與肝臟的腹膜擴散，導致癌細胞轉移到肝臟表面。

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

接下來，癌細胞在血行性轉移就會侵入血管。

As there are blood vessels almost everywhere in the body, malignant cells utilize this to reach more distant parts of the body.

正因為血管幾乎遍及全身，惡性細胞便藉此移動到更遠的身體部位。

Finally, lymphatic spread occurs when the cancer invades the lymph nodes, and travels to other parts of the body via the lymphatic system.

最後，當癌症入侵到淋巴結時就會發生淋巴性轉移，並且透過淋巴系統轉移到身體其他部位。

As this system drains many parts of the body, it also provides a large network for the cancer.

正因此系統流遍了身體的許多部位，它也就提供了癌症一個很大的流通網路。

In addition, the lymphatic vessels empty into the blood circulation, allowing the malignant cells to undergo hematogenous spread.

此外，當淋巴管注入血液循環系統後就使得惡性細胞得以進行血行性擴散。

Once at a new site, the cells once again undergo proliferation, and form small tumors known as micrometastases.

一旦細胞開始在新的地點再次增值並形成小腫瘤時，就是所謂的轉移。

These small tumors then grow into full-fledged tumors, and complete the metastatic process.

這些小腫瘤接下來長成成熟的腫瘤進而完成轉移的過程。

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

我們已知不同癌症會有特定的、不同的轉移地點。

For example, prostate cancer commonly metastasizes to the bone, while colon cancer metastasizes to the liver.

例如，前列腺癌通常會轉移到骨頭，而結腸癌會轉移到肝臟。

Various theories have been proposed to explain the migration pattern of malignant cells.

為了解釋惡性細胞的轉移模式已經有各式的理論被提出來。

Of particular interest are two conflicting theories.

其中最著名的是兩個相互衝突的理論。

Stephen Paget, an English surgeon, came up with the seed and soil theory of metastasis.

史蒂芬 ‧ 佩吉特，一位英國的外科醫生，提出了一個「種子與土壤」的轉移理論。

The seed and soil theory stated that cancer cells die easily in the wrong microenvironment, hence they only metastasize to a location with similar characteristics.

種子與土壤理論主張癌細胞在錯誤的微環境中容易死亡，因此它們只會轉移到具有相似特性的部位。

However, James Ewing, the first professor of pathology at Cornell University, challenged the seed and soil theory,

然而，詹姆斯 ‧ 尤因，康乃爾大學的首位病理學教授，卻挑戰了種子與土壤理論，

and proposed that the site of metastasis was determined by the location of the vascular and lymphatic channels which drain the primary tumor.

並提出轉移取決於血管和淋巴管傳送原發性腫瘤到的地點。

Patients with primary tumors that were drained by vessels leading to the lung would eventually develop lung metastases.

如果病人的血管將原發性腫瘤引導到肺臟，那最終就會導致肺轉移。

Today, we know that both theories contain valuable truths.

今天，我們知道兩個理論皆包含可貴的事實。

Yet the full stories of metastasis is much more complicated than either of the two proposed theories.

然而轉移的完整過程遠比 這兩個提出的理論更為複雜。

Factors like the cancer cell's properties, and the effectiveness of the immune system in eliminating the cancer cells, also play a role in determining the success of metastasis.

影響的因素諸如癌症細胞的性質以及免疫系統在剷除癌細胞時的效能，都會影響轉移的成敗。

Unfortunately, many questions about metastasis remain unanswered until today.

不幸的是，直到今日很多關於轉移的問題仍未得到解答。

Understanding the exact mechanism holds an important key to finding a cure for advanced stage cancers.

了解背後真實的原理是找到後期癌症治療方法的重要關鍵。

By studying both the genetic and environmental factors, which contribute to successful metastasis, we can pinpoint ways to shut down the process.

藉由研究遺傳以及環境因素中何者有助於轉移的成功，我們可以精準地找到中止轉移過程的方法。

The war against cancer is a constant struggle, and scientists are hard at work developing new methods against metastasis.

對抗癌症的過程相當艱辛，科學家們也努力要找出對付癌症轉移的新方法。

Of recent interest is immunotherapy, a modality which involves harnessing the power of the immune system to destroy the migrating cells.

目前最有利的是免疫療法，它的形式是透過加強免疫系統能力來消滅轉移的細胞。

This can be done in different ways, such as training immune cells to recognize cancerous cells via vaccines.

這可以透過許多不同的方式達成，例如經由疫苗訓練免疫細胞辨識癌細胞。

The growth and activity of the immune cells can also be stimulated by injecting man-made interleukins, chemicals which are usually secreted by the immune cells of the body.

免疫細胞的成長及活動也能經由注射人工介白素來刺激，而介白素是一種通常由免疫細胞所分泌的化學物質。

These two treatments are only the tip of the iceberg.

而這兩項療法只是冰山一角。

With the collaborated research efforts of governments, companies and scientists, perhaps the process of metastasis will be stopped for good.

藉由政府、企業以及科學家合作的研究或許就能永遠阻止癌細胞的轉移了。