Principles of Oncology and Main Factors of Cancer Manifestation

The international Union Against cancer has defined cancer as a disturbance of growth characterized primarily by excessive proliferation of cells without apparent relation to the physiological demands of the organs involved. Oncology deals with the prevention, diagnosis, treatment and research aspects of cancer. As the life expectancy increases, the incidence of cancer also increases. There are several factors involved in development of cancer. Let's see what these factors are.

Chemical carcinogens: These act cumulatively to bring about carciogenesis. Food additives, coloring agents, aflatoxins and n-nitrosocompounds are common carcinogens. The incidence of lung cancer is about 11 times more in cigarette smokers compared with non-smokers. Cancer may be produced:
1. At the site of exposure to the carcinogen, e.g, skin cancers in tar workers and buccal cancer in tobacco chewers;
2. At the site of metabolism, e.g, Liver cancer in aflatoxicosis; or
3. At the site of elimination, e.g, bladder cancer in workers using aromatic amines.

Viruses: Several viruses have been implicated in many cancers. Carcinoma cervix is more common in women with genital herpes infection. Epstein-Barr virus has been identified as the causative factor for Burkitt's lymphoma and there is a strong suggestive evidence to attribute hepatitis B virus as the cause of hepatic carcinoma.

Physical agents: Agents such as ionizing radiation cause a marked increase in cancer incidence in later life. Exposure to X-rays in fetal life increases the risk of developing leukemia in later life. The risk of developing cancer in a population exposed to ionizing radiation in a dose of 1 rad/ year is 40 per million annually. Indiscriminate use of diagnostic X-rays constitutes a health hazard, and has to ba avoided. Chronic irritation by heat lead to cancer of the abdominal wall and the palate in different populations. Bilharziasis is associated with higher risk of bladder neoplasms.

Genetic factors: many neoplasms show evidence of genetic predisposition. Some of these are:
1. Retinoblastoma, multiple polyposis or the colon and, carcinoma breast run in families.
2. Mongolism is associated with a ten-fold increase in the risk of leukemia.
3. Blood group A is associated with a higher risk of gastric carcinoma, compared to blood group B and O.
4. Hodgkin's disease is more frequent in subjects with HLA B18.

Several chromosomal changes have been demonstrated in malignant cells. Many of then are characteristic to be of diagnostic help, while others are not, for example, Ph' chromosome is diagnostic of chronic myeloid leukemia. More and more examples of chromosomal abnormalities in cancers are being described.

Immunological changes: Malignant cells show different degrees of de-differentiation. Several functional genes are present which are active in fetal life, but which are repressed in later life leading to the disappearance of several fetal antigens from the surface of the adult cells. I malignant cells Embryonal antigens reappear on the cell surface due to de-repression of the genes, and are termed onco-fetal antigens. E.g, carcino-embronic antigen (CEA) in colon cancer and alpha-feto-protein (AFP)) in liver cancer. Several cancers show overlap in this regard and therefore detection of these antigens is not helpful in specific diagnosis, but estimation of these antigens help to monitor the course of such cancers. Malignant cells show several biochemical abnormalities compared to normal cells. Some of them have been exploited in formulating cancer-chemotherapy, e.g, use of 1-asparaginase for acute lymphatic leukemia.

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