Risk Factors for Cancer

Many genetic and environmental factors increase the risk of developing cancer. However, not all people who are exposed to carcinogens or who have other risk factors develop cancer.

Genes That Cause Cancer:

Abnormalities (mutations) affecting critical genes are believed to contribute to the development of cancer. These genes produce proteins that regulate growth and alter cell division and other basic cell properties. Gene mutations causing cancer may result from the damaging effects of chemicals, sunlight, drugs, viruses, or other environmental agents. In some families, these abnormal cancer-causing genes are inherited.

The two major categories of genes involved with cancer are oncogenes and tumor suppressor genes.

Oncogenes are mutated forms of genes that in their normal state regulate cell growth. If they become overactive and signal cells to divide when these cells should not, cancer may develop. The mutation of oncogenes is not entirely understood, but many factors may contribute, including x-rays, sunlight, toxins at work, in the air, or in chemicals (for example, in tobacco smoke), and infectious agents (for example, certain viruses).

Tumor suppressor genes normally suppress the development of cancers by coding for proteins that repair damaged DNA and suppress growth. Cancer is more likely when DNA damage impairs tumor suppressor gene function, allowing affected cells to divide continuously.

Family History and Genetic Factors:

Some families have a significantly higher risk of developing certain cancers. Sometimes the increased risk is due to a single gene and sometimes it is due to several genes interacting together.

An extra or abnormal chromosome may increase the risk of cancer. For example, people with Down syndrome, who have three instead of the usual two copies of chromosome 21, have a 12 to 20 times higher risk of developing acute leukemia.

Age:

Some cancers, such as Wilms' tumor, retinoblastoma, and neuroblastoma, occur almost exclusively in children. Why these cancers occur in the young is not well understood, but probably the cancers result from mutations that are inherited or that occur during fetal development. Most cancers are more common in older people. In the United States, more than 60% of cancers occur in people older than 65. The increased cancer rate is due to the combination of exposure to carcinogens and weakening of the body's immune system.

Environmental Factors:

Numerous environmental factors increase the risk of developing cancer. Example: tobacco smoke contains carcinogens which substantially increase the risk of developing cancers of the lungs, mouth, throat, esophagus, kidneys, and bladder.

Pollution in the air, whether from industrial waste or cigarette smoke, can increase the cancer risk. Many chemicals are known to cause cancer, and many others are suspected of doing so. For example, asbestos exposure may cause lung cancer and mesothelioma (cancer of the pleura), especially in smokers.

Exposure to radiation is a risk factor in the development of cancer. Extended exposure to ultraviolet radiation, primarily from sunlight, causes skin cancer. Ionizing radiation is particularly carcinogenic. Exposure to the radioactive gas radon, which is released from soil, may increases the risk of lung cancer.

Many other substances which have been investigated as possible causes of cancer, but more study is needed to identify those chemicals that increase the risk of cancer.

Geography:

The risk of cancer varies according to where people live, although the reasons for the geographic differences are often complex and poorly understood. This geographic variation in cancer risk is probably multifactorial: a combination of genetics, diet, and environment. Example: the risk of colon and breast cancers is low in Japan, yet in Japanese people who immigrate to the United States, the risk increases and eventually equals that of the rest of the American population. In contrast, the Japanese have extremely high rates of stomach cancer. When these people immigrate to the USA and eat a Western diet, the risk declines to that of the USA, although the decline may not be evident until the next generation.

Diet:

Substances consumed in the diet can increase the risk of cancer. For instance, a diet high in fat has been linked to an increased risk of colon, breast, and possibly prostate cancer. People who drink large amounts of alcohol are at much higher risk of developing esophageal cancer.

A diet high in smoked and pickled foods or in barbecued meats increases the risk of developing stomach cancer. People who are overweight or obese have a higher risk of cancer of the breast, lining of the uterus (endometrium), colon, kidneys, and esophagus.

Drugs and Medical Treatments:

Certain drugs and medical treatments may increase the risk of developing cancer. For example, estrogens in oral contraceptives may slightly increase the risk of breast cancer, but this risk decreases over time. The hormones estrogen and progestin that may be given to women during menopause (hormone replacement therapy) also increase the risk of breast cancer.

Long-term use of anabolic steroids may slightly increase the risk of liver and prostate cancer. Treatment of cancer with chemotherapy drugs and with radiation therapy may increase the risk of people developing a second cancer years later.

Infections:

Several viruses are known to cause cancer in humans, and several others are suspected of causing cancer. The human papillomavirus (HPV, which causes genital warts) is one cause of cervical cancer in women. Hepatitis B virus or hepatitis C virus can cause liver cancer. Some bacteria also may cause cancer. Helicobacter pylori, which cause stomach ulcers, can increase the risk of stomach cancer and lymphomas.

Some parasites can cause cancer. Schistosoma haematobium can cause chronic inflammation and scarring of the bladder, which may lead to cancer. Another type of parasite, Opisthorchis sinensis has been linked to cancer of the pancreas and bile ducts.

Inflammatory Disorders:

Inflammatory disorders often increase the risk of cancer. Such disorders include ulcerative colitis (which can result in colon cancer).

List of Some Carcenogens

There are a number of items throughout the world that are known, or suspected, to cause cancer. These items are cause carcinogens and can be nearly anything.

A carcinogen is any substance, radionuclide or radiation, that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for example gamma rays and alpha particles, which they emit. Common examples of carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke.

There are a number of radioactive substances which are considered carcinogens. The carcinogenic activity of these substances is actually attributed to the radiation which they emit, rather than the substance itself. The radiation can be through gamma rays or alpha particles.

Common examples of carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke. All of these items, and many more, are known to cause cancer in individuals. Other carcinogens include coal tar creosote and x-rays from x-ray machines.

Carcenogen	Types of cancer
Environmental & Industrial
Arsenic	Lung
Asbestos	Lung Pleura
Aromatic amines	Bladder
Benzene	Leukemia
Chromates	Lung
Diesel exhaust	Lung
Ionizing radiation	Leukemia
Nickel	Lung Nasal sinuses
Pesticides	Lung
Radon	Lung
Ultraviolet radiation	Skin
Vinyl chloride	Liver
Associated with lifestyle
Betel nuts	Mouth Throat
Tobacco	Bladder Esophagus Kidney Lung Mouth Throat
Used in medicine
Chemotherapy drugs (such as topoisomerase inhibitors)	Bladder Leukemia
Diethylstilbestrol	Breast (in women who took the drug and in women exposed before birth) Cervix (when exposed before birth) Vagina (when exposed before birth)
Oxymetholone	Liver
Radiation therapy	Sarcomas

Development of Cancer

Introduction:

Imagine a healthy tissue containing thousands of cells. Each cell serves the greater good, which is the continuation of a person’s life. Each cell is programmed so that when the cell is old or no longer needed, it dies a peaceful and timely death. This death is called apoptosis. All cells are in communication, which allows for the smooth repair and replacement of tissues and other aspects of cell behavior. Communication takes place either indirectly, via exchange of messenger compounds such as hormones and growth factors, or directly, via cell-to-cell contact. Contact allows cells to respond to the “feel” of neighboring cells, via cell adhesion molecules, and to exchange messenger molecules through cell-to-cell portals called gap junctions. With the help of proper communication, appropriate cells proliferate when new cells are needed, and when enough new cells have been produced, cell division stops.

The elderly are more susceptible to cancer than other groups because it is a multistage process that may take 15-45 years before clinical signs are evident. Cancerous growth forms as the result of a sequence of events over a period of time. The events are associated with the exposure of DNA to harmful molecules originating from within or outside of the body. Researchers suggest that an average of five or six chemical insults to susceptable genes will lead to cancer. Some people are genetically susceptible to cancer because enzymes which help to prevent cancer growth are not efficient or enzymes that may contribute to cancer formation are very active.

A cancer is an abnormal growth of cells (usually derived from a single cell). The cells have lost normal control mechanisms and thus are able to expand continuously, invade adjacent tissues, migrate to distant parts of the body, and promote the growth of new blood vessels from which the cells derive nutrients. Cancerous (malignant) cells can develop from any tissue within the body.

As cancerous cells grow and multiply, they form a mass of cancerous tissue—called a tumor—that invades and destroys normal adjacent tissues. The term tumor refers to an abnormal growth or mass. Tumors can be cancerous or noncancerous. Cancerous cells from the primary (initial) site can spread throughout the body (metastasize).

Types of Cancer:

Cancerous tissues (malignancies) can be divided into those of the blood and blood-forming tissues (leukemias and lymphomas) and “solid” tumors, often termed cancer. Cancers can be carcinomas or sarcomas.

Leukemias and lymphomas:

Leukemias and lymphomas are cancers of the blood and blood-forming tissues and cells of the immune system. They often harm the body by crowding out normal blood cells in the bone marrow and bloodstream, so that normal functioning cells are gradually replaced by cancerous blood cells. They expand lymph nodes, producing large masses in the arm pit, groin, abdomen, or chest.

Carcinomas:

Carcinomas are cancers of epithelial cells, which are cells that cover the surface of the body, produce hormones, and make up glands. Examples of carcinomas are cancer of the skin, lung, colon, stomach, breast, prostate, and thyroid gland. Typically, carcinomas occur more often in older than in younger people.

Sarcomas:

Sarcomas are cancers of mesodermal cells, which are the cells that form muscles and connective tissue. Examples of sarcomas are leiomyosarcoma (cancer of smooth muscle that is found in the wall of digestive organs) and osteosarcoma (bone cancer). Typically, sarcomas occur more often in younger than in older people.

Development and Spread:

Cancerous cells develop from healthy cells in a complex process called malignant transformation.

Initiation:

The first step in cancer development is initiation, in which a change in the cell's genetic material primes the cell to become cancerous. The change in the cell's genetic material may occur spontaneously or be brought on by an agent that causes cancer (a carcinogen). Carcinogens include many chemicals, tobacco, viruses, radiation, and sunlight. However, not all cells are equally susceptible to carcinogens. A genetic flaw in a cell may make it more susceptible. Even chronic physical irritation may make a cell more susceptible to carcinogens.

Promotion:

The second and final step in the development of cancer is promotion. Agents that cause promotion, or promoters, may be substances in the environment or even some drugs (such as barbiturates). Unlike carcinogens, promoters do not cause cancer by themselves. Instead, promoters allow a cell that has undergone initiation to become cancerous. Promotion has no effect on cells that have not undergone initiation. Thus, several factors, often the combination of a susceptible cell and a carcinogen, are needed to cause cancer.

Some carcinogens are sufficiently powerful to be able to cause cancer without the need for promotion. For example, ionizing radiation (which is used in x-rays and is produced in nuclear power plants and atomic bomb explosions) can cause various cancers, particularly sarcomas, leukemia, thyroid cancer, and breast cancer.

Spread:

Cancer can grow directly into surrounding tissue or spread to tissues or organs, nearby or distant. Cancer can spread through the lymphatic system. This type of spread is typical of carcinomas. For example, breast cancer usually spreads first to the nearby lymph nodes, and only later does it spread to distant sites. Cancer can also spread via the bloodstream. This type of spread is typical of sarcomas.

These events invariably occur naturally over time as we are exposed to man-made environmental contaminants and toxins that occur naturally in our food, in minerals, and in our bodies as oxidative by-products of metabolism.

Examples of naturally occurring carcinogens include aflatoxin, a by-product of mold found in peanut butter; isocyanate, found in brown mustard; estragole from basil;nitrosamines from cooked bacon; and benzo(a)pyrenes found in charred or barbecued foods.

Excess consumption of fat or salt, tobacco use, and alcohol consumption can raise susceptibility to chemical insult resulting in disease.

Vision Mechanism of Eye

The eye works on the same principle as that of the camera.

The light rays from the object pass through the conjuctiva, cornea, aqueous humour, lens and vitreous humour in that order. All these structures refract the light such that it falls on the retina. This is called focussing. Maximum focussing is done by the cornea and the lens. The light then falls on the retina.

This light is received by the photoreceptors - rods and cones, on the retina. The absorbed light activates the pigments present in the rods and cones. The pigments are present on the membranes of the vesicles. Thus, the light is then converted into action potentials in the membranes of the vesicles. These travel as nervous impulses through the rod or the cone cell and reach the synaptic knobs. From here the impulses are transmitted to the bipolar nerve cells, then to the ganglions and then to the optic nerves. Thus the nervous impulses generated in the retina are carried to the brain by about a million neurons of the optic nerve.

The vision is controlled by the occipital lobe at the back of the brain. The information received is processed and we are able to see the image. The image formed on the retina is inverted. However, the brain makes us see the image erect. So, though the eyes are essential for vision, any damage to the optic nerves also results in impairment of vision.

Accommodation:

Accommodation is a reflex action of the eye to focus the light from an object on the retina.

The adjustment to the distance of the object is done by the ciliary muscles. The ciliary muscles contract and expand to make the lens thin and thick, respectively. This changes the focal length of the lens. If the object is far, then the focal length is increased and if the object is near, then the focal length is decreased. The optimal focal length is 6 metres or 20 ft.

Muscular control of the eyes:

There are three types of movements associated with the eye that are due to the action of muscles. They are the movement of the eye, the change in size of the pupil and the change in thickness of the lens. The movement of the eye is controlled by six eye muscles that attach the sclera to the bones lining the optic cavity. These six muscles are superior and inferior rectus, internal and external rectus and superior and inferior oblique muscles.

Binocular and stereoscopic vision:

In man, the eyes are frontal and thus the vision is binocular.

This means that the image finally perceived is the result of the information received from both the eyes. Binocular vision results in stereoscopic vision that gives a 3-dimensional image. Thus, this capability of stereoscopic vision gives more information about the object. This vision is, therefore, present in predatory animals like the hawks, eagles and the members of the cat family.

The other animals like the herbivores (horse, cow, etc.) have laterally place eyes. These animals have limited stereoscopic vision. The region of the environment from which each eye collects light is called the visual field.

Eye Physiology and Anatomy

Eye is the organ of sight situated in the orbital cavity. It is almost spherical in shape and is about 2.5 cm in diameter. The volume of an eyeball is approximately 7cc. The space between the eye and the orbital cavity is occupied by fatty tissue. The bony wall of the orbit and the fat helps to protect the eye from injury.

Structurally the two eyes are separate but they function as a pair. It is possible to see with only one eye, but three- dimensional vision is impaired when only one eye is used specially in relation to the judgment of distance.

Structure of the Eye:

The eyeball has three layers namely:

• The outer fibrous layer - Sclera and cornea
• The middle vascular layer – Iris, ciliary body & choroids
• The inner nervous tissue layer – Retina

Interior of the Eyeball:

The structures inside the eyeball are:

• Aqueous humor
• Lens
• Vitreous

Accessory Structures of the Eye:

• Eyebrows
• Eyelids & eyelashes
• Lacrimal apparatus
• Extraocular muscles of the eye

Accessory Structures Of The Eye

The eye is a delicate organ which is protected by several structures such as eyebrows, eyelids, eyelashes and extraocular muscles.

Eyebrows:

Eyebrows are two arched ridges of the supraorbital margins of the frontal bone. Numerous hair projects obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.

Eyelids and Eyelashes:

The eyelids are two movable folds of tissue situated above and below the front of each eye. There is short curved hair, the eyelashes situated on their free edges.

The eyelid consists of,

• A thin covering of skin
• Three muscles – the orbicularis oculi, levator palpebrae superioris and
• Muller’s muscles
• A sheet of dense connective tissue, the tatsal plate
• A lining of the conjunctiva

Lacrimal Apparatus:

Lacrimal apparatus consists of,

• Lacrimal gland and its ducts
• Accessory lacrimal glands
• Lacrimal canaliculi
• Lacrimal sac
• Nasolacrimal duct

The tears are secreted by the lacrimal gland and accessory lacrimal glands. They drain into the conjunctival sac by small ducts. The tears then pass into the lacrimal sac (via the two canaliculi), nasolacrimal duct and finally into the nasal cavity (inferior meatus).

The lacrymal glands are present one on the outer upper border of each eye. The lacrymal secretion is watery, alkaline and carries out the following functions

• cleans the eyes
• keeps the eyes moist
• keeps the eyes free of bateria as it contains bacteriolytic lysozyme
• provides nutrition to the cornea

Extraocular Muscles of The Eye:

The eyeballs are moved by six extrinsic muscles, attached at one end to the eyeball and at the other to the walls of the orbital cavity. There are four straight and two oblique muscles.

They consist of striated muscle fibres. Movement of the eyes to look in a particular direction is under voluntary control but co-ordination of movement needed for convergence and accommodation to near or distant vision is under autonomic control.

• The medical rectus rotates the eyeball inwards.
• The lateral rectus rotates the eyeball outwards.
• The superior rectus rotates the eyeball upwards.
• The infertor rectus rotates the eyeball downwards.
• The superior oblique rotates the eyball so that the cornea turns in a downwards and outward directions.
• The inferior oblique rotates the eyeball so that cornea turns ouwards and outwards.

Arterial Supply:

The eye is supplied by the short (about 20 in number) and long ciliary (2 in number) arteries and the central retinal artery. These are branches of the ophthalmic artery, one of the branches of the internal carotid artery.

Venous Drainage:

Venous drainage is done by the short ciliary veins, anterior ciliary veins, 4 vortex veins and the central retinal vein. These eventually empty into the cavernous sinus.

Nerve Supply to The Eye:

The eye is supplied by three types of nerves

• The motor nerves
• The sensory nerves
• The autonomic nerves