Cancer Introduction Chapter 4

Spreading Danger


Metastatic Cancer

Cancer spreads locally by invading neighboring tissues. It can also metastasize (spread to distant parts of the body) by entering the bloodstream or the lymphatic system. This happens when cells break away from the original, or primary, tumor, penetrate into a blood vessel or lymphatic vessel, and travel to another part of the body. Cancer cells don’t adhere to one another as well as normal cells, and it’s theorized that tumors generate substances that stimulate cancer cells to move.

Metastatic cancers may be found when the primary tumor is found, or they may be found months or even years later.


Cancer Spread: Circulatory System

Almost from time they first arise, cancerous tumors shed cells into the bloodstream. In fact, it’s estimated that a 1-cm tumor sheds more than a million cells into the circulatory system in just 24 hours. Most of these cells are killed by white blood cells or die from damage sustained while traveling in the bloodstream, but some may survive. Sometimes cancer cells stick to platelets to form clumps, giving them some protection. READ MORE

The traveling cancer cells may become stuck in a capillary and adhere to its lining. From there they penetrate into surrounding tissues or organs, where they may generate secondary tumors. Secondary tumors grow in the same way that primary tumors do, and they themselves may start to shed cells and give rise to other metastases. LESS


Cancer Spread: Lymphatic System

Cancer cells may also enter into the lymphatic system. The lymphatic system works in concert with the immune system and has several functions in the body, including combating invasive foreign bodies (like bacteria, fungi, and viruses), filtering lymph, and filtering blood. READ MORE

Lymphatic vessels form a sort of parallel circulatory system, branching throughout the tissues of the body. Lymph is a clear, colorless liquid found within the lymphatic vessels. It’s collected from tissues in all parts of the body as interstitial fluid (the fluid in the spaces between cells) and then travels to at least one lymph node for filtering before returning to the bloodstream via the veins. Lymph nodes are found in the armpits, on either side of the groin, on either side of the neck, and in the abdomen, pelvis, and chest. Lymph nodes filter the lymph fluid as it passes through them.

A cancer cell enters a lymphatic vessel in much the same way it enters a blood vessel. Then it travels in the circulating lymph fluid until it becomes lodged in the small channels inside a lymph node, where it begins to grow into a secondary cancer. LESS


Where Cancer Spreads

The most common places for cancer to spread via the circulatory system are the lungs, liver, brain, and bones. All the blood in the body passes through the lungs, so they are a common place to find secondary cancers. Blood from the organs of the digestive system goes through the liver before returning to the heart and then the lungs. Consequently, cancers of the digestive system often spread to the liver. Prostate cancer frequently metastasizes to the bones, for reasons which aren’t yet entirely understood. It is clear, though, that some types of tissue provide good environments for metastatic tumor growth. READ MORE

Cancer cells may spread to lymph nodes near the primary tumor (termed nodal involvement or regional disease). This is not considered metastasis, but it is a sign that the cancer is advancing. The lymph nodes draining an organ are likely to contain cancer cells, so during cancer surgery a surgeon will normally remove the main lymph nodes closest to the tumor. Travel through lymphatic vessels is the most common way for cancer to spread initially. LESS


How Cancer Is Named

Wherever a cancer spreads, the cancer cells are still clones of the original abnormal cell. Because of this, cancers are named after the place where they first arose. For example, breast cancer that has spread to the lungs is termed metastatic breast cancer, not lung cancer. A pathologist doing a biopsy of a tumor can usually see if the type of cell in the tissue sample is normally found in the part of the body from which it was taken. In this way, it can be determined if the tumor is primary or secondary.

Photo attribution of bone marrow smear with acute monoblastic leukemia
Copyright 2008 The Armed Forces Institute of Pathology (AFIP)

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