What is Cancer and Etiology of carcinogenesis

Cancer is a cellular tumour that, unlike benign tumour cells, can metastasize and invade the surrounding and distant tissues. Cancer has been a major cause of death in the USA for the past few decades, being second only to cardiac diseases. Approximately 20 per cent of all deaths in America are due to cancer.
Approximately 20 per cent of all deaths in America are due to cancer.

There are at least fifty different types of malignant tumours being identified. More than 50 per cent of the newly diagnosed cancers occur in five major organs: (i) lungs, (ii) colon/rectum, (iii) breast, (iv) prostate and (v) uterus. Cancers of the lungs, colon/rectum and prostate are the principal leading causes of deaths in males and in females, breast, colorectal and uterine cancers are the most common.

Environmental factors play a very important part. In Japan, death rate from cancer of stomach is about seven times more than that in the USA. Other examples are:
• Increased risk of certain cancers with occupational exposures to asbestos, naphthylamine, etc.
• Association of cancers of oropharynx, larynx, oesophagus and lungs with tobacco chewing and cigarette smoking.

Cancer cells are characterised by three important properties:

1. Diminished or unrestricted control of growth.
2. Capability of invasion of local tissues, and
3. Capable of spreading to distant parts of body by metastasis.
Factors which can contribute to the development of cancer (carcinogenesis)

1. Age: Cancer can develop in any age, though it is most common in those over 55 years of age. Certain cancers are particularly common in children below 15 years of age, viz.
• Retinoblastomas • Neuroblastomas • Wilms’ tumours • certain tumours of haemopoietic tissues as lymphomas and leukaemias. • Sarcomas of bones and skeletal muscles.

2. Heredity: Heredity plays an important role in carcinogenesis. Certain precancerous conditions are inherited.
Examples are:
• Susceptibility to childhood retinoblastomas is inherited as an autosomal dominant trait and approximately 40 per cent of retinoblastomas are familial.
• Susceptibility to multiple colonic polyposis is inherited as autosomal dominant trait and almost all cases develop into adenocarcinomas in later life.
• Chromosomal DNA instability may be inherited as an autosomal recessive trait. Conditions are characterised by some defect in DNA repair.
• In xeroderma pigmentosa, a skin condition, the affected individuals develop carcinomas of skin in areas exposed to UV rays of sunlight.

3. Environmental factors: Statistically it has been shown that 80 per cent of human cancers are caused by environmental factors, principally chemicals, viz.
• Lifestyle: Cigarette smoking, tobacco chewing.
• Dietary: Groundnuts and other foodstuffs infected with fungus like Aspergillus produce aflatoxin B1 which is carcinogenic.
• Occupational: Asbestos, benzene, naphthylamines, beryllium, etc.
• Iatrogenic: Certain therapeutic drugs may be carcinogenic.

4. Acquired precancerous disorders: Certain clinical conditions are associated with increased risk of developing cancers.
Examples are:
• Leukoplakia of oral mucosa and genital mucosa developing into squamous cell carcinomas.
• Cirrhosis of liver: A few cases can develop hepatoma (hepatocellular carcinoma).
• Ulcerative colitis: Can produce adenocarcinoma of colon.
• Carcinoma in situ of cervix: Can produce squamous cell carcinoma of cervix. (b) 

Carcinogenic Agents (Agents Causing Cancer): Carcinogens that cause cancer can be divided into three main broad groups:

1. Physical: Radiant energy
2. Chemicals: Variety of chemical compounds can cause cancer. Some of these can act directly and others can act as procarcinogens
3. Biological: Oncogenic viruses.

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