Cancer Is Uncontrolled Cell Division

1. In a healthy tissue the cell cycle is tightly controlled, and cells divide only when a new cell is needed.
2. Cancer begins when changes inside a cell damage the genes that control this cycle.
3. The cell then divides repeatedly and uncontrollably, ignoring the signals that would normally stop it.
4. The damaged control is passed on to every daughter cell, so the population of faulty cells doubles again and again.

How a Tumour Forms

1. The dividing cells build up into a mass of cells called a tumour.
2. Cells inside a growing tumour need a supply of oxygen and glucose, so a tumour triggers the growth of new blood vessels towards itself.
3. A tumour can press on nearby organs, blood vessels or nerves, which is often what produces the first symptoms.
4. Tumours are classified as benign or malignant, and only malignant tumours are described as cancerous.

Benign Tumours

1. A benign tumour grows in one place and is usually contained within a membrane.
2. Its cells do not invade surrounding tissue and do not spread to other parts of the body.
3. Benign tumours are not classed as cancer and can often be removed by surgery.
4. They can still be dangerous if they grow in a confined space, which is why a benign brain tumour can press on the brain and cause serious harm.

Malignant Tumours

1. A malignant tumour is cancerous, and its cells invade the healthy tissue around it.
2. Cells break off and are carried in the blood or the lymph to other parts of the body.
3. Where they settle they divide again and form secondary tumours, and this spread is called metastasis.
4. Secondary tumours are what make malignant cancers difficult to treat, because removing the original tumour no longer removes the disease.

What Causes the Changes in the Cells

1. The changes are mutations in the genes that control cell division.
2. Mutations happen at random during DNA replication, and most are harmless or are repaired.
3. A mutation that damages a control gene may allow the cell cycle to run without a brake.
4. Cancer usually requires several mutations to build up in the same cell, which is why the risk rises with age.

Carcinogens and Risk Factors

1. A carcinogen is any agent that increases the rate of mutation and therefore the risk of cancer.
2. Ionising radiation, including X-rays, gamma rays and the ultraviolet radiation in sunlight, damages DNA directly.
3. Chemical carcinogens include the tar in tobacco smoke, asbestos fibres and some industrial solvents.
4. Some viruses raise the risk, and human papillomavirus is linked to cervical cancer.
5. Inherited alleles also matter, and a person who inherits a faulty BRCA1 allele has a higher risk of breast cancer.
6. Lifestyle factors such as obesity and high alcohol intake raise the risk of several cancers.

Interpreting Cancer Data

1. Exam data on cancer is usually given as an incidence rate, meaning new cases per 100 000 people per year, so populations of different sizes can be compared.
2. A correlation between a factor and cancer incidence does not on its own prove that the factor causes the cancer.
3. Stronger evidence comes from a dose response, where a larger exposure is matched by a larger increase in risk.
4. Percentage change is a common calculation in these questions.
5. $$percentage\; change=\frac{final\; value-initial\; value}{initial\; value}\times 100$$