The number of cells in a tissue is determined by the balance between cell division and cell death. Uncontrollable cell division leads to formation of abnormal growths called tumors.

Tumors can be benign or malignant. Benign tumors are slow-growing and constrained by surrounding connective tissue, so they do not spread to other organs. They can still be harmful or even kill by pressing on nearby nerves, blood vessels, or brain tissue.

Examples of benign tumor include pituitary tumors which may press on optic nerves and cause loss of vision.

Cancers are malignant tumors—tumors that can spread beyond the limit of the original organ where it comes from and to other organs of the body.

Cancer starts from damage in the DNA of a cell. This DNA damage is called MUTATION. Mutations happen when the cell duplicates its DNA prior to cell division and makes mistakes.

These damages are usually detected and repaired before the cell can divide, but sometimes, some of them may be ignored and transferred to daughter cells.

If the mutation is located in one of many genes that control the cell cycle, it may affect the regulation of cell cycle in the cell carrying it, and make the cell divide faster than it’s supposed to.

Usually, one mutation is not enough to cause cancer, but as it makes the cell cycle control less reliable, many more DNA damages, or mutations, would go unnoticed.

Cancer is usually the result of accumulation of many mutations of genes involved in cell cycle control and DNA repair. This commonly happens over a long period of time, over many rounds of cell divisions, and this explains why cancers are more common in older people.

Some people are said to be predisposed to cancer.

This is because they are born with a mutation that makes them more likely to develop a certain type of cancer. This mutation alone is not enough to cause cancer, but it starts the process of making the cells cancerous.

The person carrying it is one step further down the road towards developing a cancer than others who do not have the mutation.

Cancer cells do not stick together like normal
cells; they move and invade nearby tissues, organs. This is local spread. They may also
spread to further away organs by means of blood or lymph circulation.

This is systemic
spread. Metastasis is the spreading of cancers to non-adjacent organs. Cancer cells from
the original tumor, or primary cancer, can break out and may be taken up by a blood or
a lymph vessel for a ride throughout the body.

They can then squeeze out from the vessel
into other tissues and start a new tumor growth in the new location, which will become secondary
cancer.

While travelling in the bloodstream, a cancer
cell usually stops at the first place where the vessel is getting so narrow that it gets
stuck.

As blood flow from most organs goes to the capillaries of the lungs, this is where
cancers spread the most. Lungs are indeed the most common site of secondary cancers.

Likewise, while travelling in the lymphatic system, cancer cells commonly get stuck in the
nearest lymph nodes, where the vessels get narrower.

This is the reason why surgeons
usually remove nearby lymph nodes when removing tumors.

Cancer, How Cancer Starts, How Cancer Spreads, Where and Why, Animation.

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