Cancer is a tumor or a mass resulting from uncontrolled abnormal growth of cells. Cancer cells multiply rapidly despite restriction of space or nutrients and fail to respond to signals to stop reproducing. Cancer cells usually look different and do not function normally. They can spread to adjoining organs or through blood vessels to other parts of the body where again they grow and reproduce to form masses called metastasis.

Cancer cells develop because of damage to DNA in the nucleus of the cell. This substance directs all cell activities. Most of the time when DNA becomes damaged the body is able to repair it. In cancer cells, the damaged DNA is not repaired. People can inherit damaged DNA, which accounts for inherited cancers. More often, though, a person's DNA becomes damaged by exposure to something in the environment, like smoking.

A few basic terms to understand cancer are defined below:

• A tumor means a swelling or mass. A tumor can be benign or malignant.
• Benign refers to a tumor which does not have the potential to spread beyond the organ it arises.
• Malignant refers to a tumor which has the ability to spread or metastasize beyond the organ from which it arises, to other organs of the body. The cells within malignant tumors have the ability to invade neighboring tissues and organs, thus spreading the disease. It is also possible for cancerous cells to break free from the tumor and enter the blood stream, and spreading the disease to other organs.

When cancer has metastasized and has affected other areas of the body, the disease is still referred to the organ of origination. For instance, if cervical cancer spreads to the lungs, it is still called cervical cancer, not lung cancer.

Broadly speaking, cancers can be divided into the following major categories:

1. Carcinoma is a cancer which is derived from the lining cells, or epithelium, of an organ. Of the four major types of epithelia in the body (Glandular, squamous, transitional, and pseudostratified), some types are only found in a few select organs such as the lung (pseudostratified) or urinary bladder (transitional). Carcinomas can arise from any of these epithelial types. For example, breast carcinoma is most commonly derived from the lining cells of the milk producing glands. A carcinoma with a glandular growth pattern is an adenocarcinoma. Common adenocarcinomas include prostate, colon, and breast. A carcinoma with a growth pattern resembling the squamous lining cells is termed a squamous cell carcinoma. Common squamous cell carcinomas are seen in the esophagus and skin. However, any of these organs may have either type of carcinoma arising from it, although these latter diagnoses are exceedingly rare.
2. Sarcoma is a cancer derived from the soft tissues of the body. Soft tissues include the fat, muscle, nerves, and connective tissue support. Although not usually soft these also include bone and cartilage tumors.
3. Lymphoma is a cancer derived from the white blood cells present in the lymphoid tissues of the body. These sites most commonly include the lymph nodes and spleen. However, lymphomas may arise from any organ and body site.
4. Melanoma is a cancer derived from melanocytes which are the pigment producing cells present in the skin. A mole is a benign growth of melanocytes.
5. Leukemia is a cancer that starts in the blood-forming tissues such as the bone marrow and causes abnormal production of large numbers of abnormal blood cells which enter the blood. These can then be detected in peripheral blood on a blood test.

Lymphoma and Myeloma are cancers that begin in the cells of the immune system or lymphohematopoeitic system. Lymphomas usually appear as enlarged glands (lymph nodes) in the body. They are of two main types: Hodgkins and Non Hodgkins. Hodgkins lymphoms have more favourable outcome on chemotherapy. Non Hodgkins lymphoma has variable outcome depending on the subtype and stage of the disease.

Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord. After biopsy diagnosis they are treated by surgery and/or radiotherapy.

Cancers may sometimes pose diagnostic problems when cancer cells display such a distorted and grotesque growth that a recognizable growth pattern such as a carcinoma or sarcoma cannot be given. In these cases, the pathologist may diagnose the tumor as a poorly differentiated or undifferentiated malignancy and additional studies including special stains or immunohistochemistry markers on the processed biopsy tumor tissue may aid in the final diagnosis. Some of the markers are done to select appropriate chemotherapy drug to which the tumor will respond. For eg:-Estrogen and Progestrone receptors

As diagnostic capabilities advance, pathologists have identified earlier and earlier forms of cancer. Some of these cancers are termed 'in situ', designating a stage of cancer development where the cancer has not spread beyond its usual confines of the basement membrane. An in situ cancer cannot metastasize. In addition, there are precancerous conditions termed dysplasia which in some cases, have a higher risk of progressing to cancer.

The advancing knowledge of causes, genesis and progression of cancer aims to recognize pre-cancerous conditions and early stage cancer so that appropriate and prompt curative treatment can be offered.