What is cancer?
Cancer is a disease resulting from the uncontrollable proliferation of abnormal cells which can break off, travel, and take hold at other sites in the body.

Usually, normal cells in the body grow and divide to form new cells replacing the old, damaged, and dying cells. If the cell division process is not regulated and the mutated cells multiply rapidly, abnormal lumps of tissues or tumors are formed. The tumorous growths may be benign or malignant.

Benign or non-cancerous tumors can reach large size, they do not invade adjacent tissues. Once removed, they do not grow back. Despite not being cancerous, benign tumors can sometimes be life-threatening such as those growing in the brain.

Malignant or cancerous tumors can recur after surgical excision. They can invade or spread into surrounding tissues or distant organs. This is called metastasis. Some cancers, for example, the leukemias, do not form solid tumors.

Differences between and Normal Cells and Cancer Cells

• Normal cells divide only when they receive a set of appropriate signals whereas cancer cells divide themselves despite the absence of those signals, and they are resistant to the signals telling them to self-destruct, known as apoptosis or programmed cell death.
• Cancer cells fuel their growth with different nutrients than normal cells and some utilize different pathway to transform those nutrients into energy, allowing them to proliferate faster than normal cells. For instance, tumor can induce new blood vessels formation (Angiogenesis), bringing in more nutrients supply.
• Normal cells stop dividing when touching other cells. They usually do not migrate to other areas. Cancer cells, on the contrary, invade into surrounding tissues and spread to other organs.
• Cancer cells can evade our immune system elements which normally eliminate abnormal or invading cells. They can co-opt our immune system to help them proliferate. For instance, cancer cells can disguise as normal cells via antigen expression on the cell membrane.
• Cancer cell genes can be amplified, deleted, or altered. Their chromosomes can be reshuffled.

The proliferation of cancer cells depends on their altered characteristics, so specific treatment has been developed to interfere and stop how cancer cells sustain themselves. Some cancer therapies, for instance, blocking the formation of blood vessels supplying oxygen and nutrients to the tumor.

What causes cancer?
The alteration in our genes that control how cells grow, divide, and die is the main driver of cancer. Factors that can cause those changes include:

• Heredity
• Cell division errors
• DNA damage due to harmful chemicals in the environment

Our body can remove cells with damaged DNA to prevent them from becoming cancerous. However, as we get older, this natural ability deteriorates. The elderly has higher risk of developing cancers. Once a cell becomes cancerous, its genes continue to mutate. It is possible for a single cancer mass to consist of clone of cells with different genomes.

Metastatic cancer
Metastasis is the process when cancer cells spread to other distant organs. If lung cancer spreads to the brain, it is designated a metastasis lung cancer, not a brain cancer. Metastatic cancer cells look like cancer cells of the original site and have similar but not identical chromosome changes.

Treatment of metastatic cancer is to limit cancer spread and relieve symptoms caused by the metastatic cancer. People with cancer most commonly die from metastatic cancer.

Non-cancerous tissue changes
Some tissue changes are not cancerous, but they should be monitored or treated to prevent cancer development.

• Hyperplasia is an increase in the number of cells resulting from faster proliferation than of normal cells. These cells are organized and normal appearing under a microscope. Chronic irritation is one of the causes of hyperplasia.
• Dysplasia is an abnormal growth and development of altered cells. The cells look and organize abnormally. A dysplastic nevus is an abnormal mole which sometimes can turn into melanoma.

Carcinoma in situ is also called a stage 0 cancer, but it does not invade the surrounding tissues. It needs treatment because it may become an invading cancer.