All cancers begin when cells in a part of the body become abnormal and start making more cells. These extra cells form a mass of tissue called a growth or tumor. If the tumor gets bigger, it can hurt nearby tissues and organs. Cancer cells  can also  break away and spread to other parts of the body.

What is cancer ?

Cells are the smallest structural unit of human body. Cells come together and form tissues and tissues come together and form organs. Specific organs have specific functions and together form the human body. Inside the human body, wear and tear is a continuous process. Older cells are continuously replaced by newer cells. the human body has a very fine control over the life cycle of these cells. When cells reach a specific age, they die naturally and this type of cell death is known as apoptosis. Cancer develops when the control over  cell growth and cell cycle is lost.

In normal process cells divide as per the need and instruction of the human body. Genetic material inside these cells are present in the form of DNA. This DNA stores the information in the form of codes which governs the process of cell growth. Change in the basic structure of the DNA is called mutation. These mutated cells never undergo the normal process of cell division and thus results in abnormal cell growth. This abnormal cell growth forms a mass of tissue known as tumor.

Figure 1

All tumors are not cancerous. They can be benign where these can be removed completely and in most of the cases they didn't come back. Tumors which are cancerous they invade the nearby structures and spread to other part of the body.

Cancer is a term for diseases in which abnormal cells divide without control and can invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymphatic systems.

Cancer is not a single entity but consist of many diseases. There are more than 100 diseases covered under this term. The main categories of cancer include:-

1. Carcinoma: Cancer begins in the skin or in the linings of the internal structure. Examples are skin cancer, head neck cancer, esophageal cancer, cancer of intestines etc.
2. Sarcoma: Cancer of soft tissues, bones and cartilage. Examples are bone cancers, muscle cancers etc.
3. Leukemia: Cancer of blood forming tissue like bone marrow.
4. Lymphoma and Myeloma: Cancer that begin in the cells of immune system.
5. Central nervous system cancers: Cancer that involves the cells of spinal cord and brain.