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Cancer is a class of diseases/disorders characterized by abnormal or uncontrolled division of cells and the capability of these to spread, either by undeviating growth into adjacent tissue, or by implantation into distant sites by metastasis. Cancer arises when cells become abnormal and keep dividing and forming more cells without control or order (Figure 1). All organs of the human body are made up of cells. In general, cells divide to produce new cells only when the body needs them. If cells divide while new ones are not needed, they form an accumulation of excess tissue, called a tumor. Tumors can be benign or non-benign/malignant. Benign tumors are not cancer. They do not spread to the other parts of the body and they are rarely a threat to life. Usually benign tumors can be removed by surgery, and they are not expected to return. Malignant tumors are cancer. They have tendency to spread, infect and demolish nearby organs and tissues. Cancer cells can also separate from a malignant tumor and pass through the bloodstream (leukemia) or the lymphatic system (lymphoma) to form new tumors in other parts of the body and is called metastasis. The abnormal cellular division of this is not subject to normal growth controls.1 It is one of the most frightful diseases due to incurable affliction that insidiously hit persons of all cultures and ages. While, cancer is an antique disease, the inability to treat cancer has persisted even though impressive advances in medical techniques and scientific knowledge.
Figure 1. Normal and cancer cell divisions
The radiotherapy is the first symbol of multi-disciplinary advance to cancer treatment and the improvement in surgical reflects in diagnosing at an earlier and improvements in treatment. According to the World Health Organization (WHO), cancer is the second-leading cause of death worldwide1 and is estimated that 12 million deaths will happen from cancer in 2030. Presently cancer is responsible for about 25% of all deaths3 and 0.5% of the population is diagnosed with cancer on a yearly basis. The rate of its initiation throughout the world is growing every year, making it a challenging area for major focus for both physicians as well as scientists.
Factors responsible for cause of cancer
Cancer causing factors5 have been classified into three major groups include physical, chemical and viral carcinogens. Physical carcinogens include UV light, soft and hard X-rays, asbestos fibres and various other external agents. Chemical carcinogens are supposed to be by far the greatest source of cancer. These are substances that interact indirectly or directly with DNA, causing changes in the sequence of genetic code. Viral carcinogens are positively linked to the onset of specific cancers, which for decades were considered not to cause cancer but simply capture the benefit of a weakened cell. Birkett's lymphoma, among Africans and South East China has been found to be caused by the Epstein-Barr virus. Cervical cancer is also believed to be connected with the Herpes Simplex Virus Type II, and certain lymphomas and leukemias are caused by the Human T-cell Leukemia virus.
Figure 2. Cancer threat factors with percentages.
Types of Tumors
Cancer types can be grouped into broader categories. The main categories4 of cancer include:
Carcinoma - cancer that begins in the skin or in tissues that cover or line internal organs.
Sarcoma - cancer that starts in bone, cartilage, muscle, fat, blood vessels, or other supportive or connective tissues.
Lymphoma and myeloma - cancers that commence in the cells of the immune system.
Leukemia - cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
Central nervous system cancers - cancers that begin in the tissues of the brain and spinal cord.
Treatment of Cancer
Cancer can be treated by chemotherapy, surgery, radiation therapy, monoclonal antibody therapy immunotherapy and or other methods. The selection of therapy depends upon the location and the stage of the disease and grade of the tumour, as well as the general condition of the patient. There are a number of experimental cancer treatments under development. The complete removal of the cancer without damage to the rest of the body is the target of treatment.
In theory, cancers can be cured if completely removed by surgery, but this is not always possible. While the cancer has metastasized to other parts of the body prior to surgery, complete surgical removal is usually not possible. Like surgical procedures for cancer include prostatectomy for prostate cancer and mastectomy for breast cancer.
2. Monoclonal Antibody Therapy
Immune mechanisms are used against tumours in immunotherapy. These are used in various forms of cancer, such as leukemia (gemtuzumab ozogamicin/ Mylotarg) and breast cancer (trastuzumab/Herceptin). The agents are monoclonal antibodies directed against proteins that are characteristic to the cells of the cancer in question, or cytokines that modulate the immune system's response.
The other and more contemporary methods for generating non-specific immune response against tumours include intravesical Bacille Calmette-Guerin (BCG) immunotherapy for superficial bladder cancer, and use of interleukin and interferon. Vaccines to generate non-specific immune responses are the topic of intensive research for a number of tumours, notably renal cell carcinoma and malignant melanoma.
4. Radiation Theraphy
Radiation therapy also called as radiotherapy, irradiation or X-ray therapy is the use of ionizing radiation to destroy cancer cells and shrink tumours. Radiation therapy can be directed externally via external beam radiotherapy (EBRT) or internally via brachytherapy. This therapy injures or destroys cells in the area being treated by damaging their genetic material.
5. Hormonal Suppression
The development of some cancers can be inhibited by providing or blocking definite hormones. Common examples of hormone-sensitive tumours consist of certain types of prostate and breast cancers. Removing or blocking testosterone or estrogen is often an important additional treatment.
Chemotherapy5 is a specifically different approach than radiation therapy and surgery to treat cancer. Rather than physically removing a tumor or a part of it, chemotherapy uses chemical agents (cytotoxic or anti-cancer drugs) to interact with cancer cells to destroy or control the growth of cancer. Cells divide by going through a cell cycle, following an controlled set of events that include the synthesis of DNA (S-phase), mitosis (M-phase), culminating in cell growth and division into two daughter cells. Normal cells grow and die in a specifically controlled way while cancer occurs while the process becomes abnormal, with cells dividing and forming more cells devoid of control and order.
In chemotherapy, drugs that interfere mainly with DNA synthesis and mitosis (the S and M phases of the cell cycle) are used to kill cancer cells. Different agents work through different mechanisms, some damage a cell's genetic material (DNA); some prevent the cell from dividing. While chemotherapeutic drugs cannot distinguish between cancer cells and normal cells, both types of cells are affected by chemotherapy. Toxicity of chemotherapeutic agents to normal cells is the source of unpleasant side effects. Though, the value of chemotherapy lies in the reality that the killing effect of chemotherapeutic agents has a specific selectivity for cancer cells over normal host cells. The notorious side effects of chemotherapy are caused by the cell killing consequence of anti-cancer drugs. Cancer cells typically grow and divide more quickly than the normal cells. The fast-growing normal cells most probable to be affected by chemotherapy are blood cells forming in the bone marrow, and cells in the reproductive system, digestive tract, and hair follicles. The most common side effects of chemotherapy consist of fatigue, diarrhea, nausea, mouth sores, anemia and hair loss.
Chemotherapeutic drugs are divided into some categories based on how they affect specific chemical substances within the cancer cells, which cellular processes or activities the drug hinders with, and which specific phases of the cell cycle the drug affects. These include DNA interactive agents, DNA topoisomerase I and II inhibitors, tubulin polymerization inhibitors, CDK inhibitors, carbonic anhydrase (CA) inhibitors, antimetabolites, antimitotic agents and miscellaneous agents.