Cancer:

A cancer is an uncontrolled proliferation of cells.

·  In some the rate is fast; in others, slow; but in all cancers the cells never stop dividing.

·  This distinguishes cancers - malign tumours - from benign growths like moles where their cells eventually stop dividing.

·  Cancers are clones. No matter how many trillions of cells are present in the cancer, they are all descended from a single ancestral cell.

·  Cancers begin as a primary tumour. At some point, however, cells break away from the primary tumour and - travelling in blood and lymph - establish metastases in other locations of the body. Metastasis is what usually kills the patient.

·  Cancer cells contain mutated genes known as oncogenes. The mutations are found in genes that are involved in mitosis; that is, in genes that control the cell cycle.

What probably happen is:

·  A single cell in a tissue suffers a mutation in a gene involved in mitosis.

·  This results in giving that cell a slight growth advantage over other cells in the tissue.

·  As that cell develops into a clone, some if its descendants suffer a second mutation

·  This further deregulates the cell cycle of that cell and its descendants.

·  As the rate of mitosis in that clone increases, the chances of further DNA damage increases.

·  Eventually the growth of that clone becomes completely unregulated.

·  The result: full-blown cancer.

Colon Cancer: An Example

·  Begins with the development of polyps in the epithelium of the colon. Polyps are benign growths

·  As time passes, the polyps may get bigger.

·  At some point, nests of malignant cells may appear within the polyps

·  If the polyp is not removed, some of these malignant cells will escape from the primary tumour and metastasise throughout the body.

·  Examination of the cells at the earliest, polyp, stage, reveals that they contain oncogenes.

Cancers become more common as one gets older.

This explains why cancer has become such a common cause of death during the twentieth century. It probably has very little to do with exposure to the chemicals of modern living and everything to do with the increased longevity that has been such a remarkable feature of this century.

A population whose members increasingly survive accidents and infectious disease is a population increasingly condemned to death from such "organic" diseases as cancer.

Causes of Cancer:

·  anything that damages DNA; that is anything that is mutagenic

·  radiation that can penetrate to the nucleus and interact with DNA

·  chemicals that can penetrate to the nucleus and damage DNA. Chemicals that cause cancer are called carcinogens.

·  anything that stimulates the rate of mitosis. This is because a cell is most susceptible to mutations when it is replicating its DNA during the S phase of interphase.

·  certain hormones (e.g. hormones stimulating mitosis in the breast & prostate glands)

·  certain viruses

Viruses and Cancer

Many viruses have been studied that reliably cause cancer when lab. animals are infected with them. What about humans? The evidence is indirect but some likely culprits are:

·  the hepatitis B and hepatitis C viruses, which infect the liver and are closely associated with liver cancer.

·  herpes viruses, some may cause Burkitt's lymphoma) and some are associated with Kaposi's sarcoma (a malignancy frequently seen in the late stages of AIDS)

Note:

·  Many people are infected by these viruses and do not develop cancer.

·  When cancers do arise in infected people, they still follow our rule of clonality. Many cells have been infected, but only one (usually) develops into a tumour. So it appears that an infected cell will develop into a tumour only if it suffers one or more other types of damage

Radiation and cancer

High doses of radiation cause cancer. Various studies, including excellent ones on the survivors of Hiroshima and Nagasaki, show that a popn. exposed to a dose of 12,500 mrem will have a measurable increase (about 1%) in the incidence of cancer. Note that the measurements are made on a popn. not on individuals. We can never say that a particular individual exposed to a particular dose of radiation will develop cancer. The induction of cancer is a chance ("stochastic") event unlike the induction of radiation sickness which is completely predictable. The element of chance arises because cancer is an event that occurs in a single cell unlucky enough to suffer damage to specific genes mutating them to oncogenes. However, the energy needed to cause mutations is very low. So if you expose a sufficiently large number of cells to even tiny doses of radiation, some cell is going to be unlucky.

How can we evaluate the risk?

Chernobyl

·  It has been estimated (in this case, using a collective dose value of 5 x 106 person mrem/cancer) that the radioactive fallout from the nuclear accident at Chernobyl in 1986 will cause an ­ of 17,000 cancers over the lifetime of people living in the Northern Hemisphere.

·  Large though this estimate seems, it is dwarfed by the 513 million cancer deaths that will occur anyway in this population.

This is why I say above that the answer to the question of the dangers of low doses of radiation is unknowable.