Instant Expert: Cancer

Cancer is a group of over one hundred diseases, all of which share the common feature of uncontrolled spreading of malignant cells in the body (Image: Science Source / Science Photo Library)

One in three of us will get cancer at some point in our lives. Once so feared its name was whispered, the disease is no longer an inevitable death sentence. Today, almost two-thirds of those who develop cancer will still be alive five years later, compared with just half in the 1970s.

Cancer has turned out to be tougher to crack than everyone hoped when US President Richard Nixon launched the War on Cancer in 1971. But death rates are falling, thanks to earlier detection of tumours and improved use of existing treatments - mainly chemotherapy and radiotherapy. The success has been biggest for children: since the mid-1970s, death rates from cancers of childhood have halved. Of those with the most common childhood cancer, acute lymphocytic leukaemia, 85% are still alive five years later, compared with just 53% in the 1970s.

Inner workings

A key reason why cancer patients are living longer, is that scientists now understand more clearly what happens when cells turn cancerous. Cancer begins when genes which normally control cell division, growth and repair are damaged through mutation. These genes can then cause cells to grow and divide uncontrollably, destroying neighbouring healthy cells. For example, agene called p53, which normally acts as a brake on cell division, turns out to be mutated or lost in about half of all tumours. Another proto-oncogene, Mycnormally helps healthy cell division, but can become an oncogene if damaged, causing cells to divide unchecked.

Mutations can arise by chance errors in DNA replication, and genes can also be damaged by carcinogens - such as tobacco chemicals, benzene, possiblyacrylamide and some food additives - or ultraviolet light from sunshine. Certainviruses can also trigger gene mutations, such as the human papilloma virus that can cause cervical cancer.

Some mutated genes are inherited: two examples are BRCA1 and BRCA2, which together account for about 5% of all breast cancer cases. Other genessuch as DBC2, EMSY and FA have been implicated in ovarian, breast and lung cancer.

Once a cell has turned cancerous, it divides until a mass of cells forms a tumour. Diagnostic tests can quickly distinguish between malignant, or cancerous, tumours and those that are benign, or harmless. As a malignant tumour progresses, cells or clumps of cells break off and spread, or metastasise, around the body via the lymphatic system and blood vessels.

The latest generation of treatments exploit our knowledge of what happens within cancerous cells. These target proteins and messenger chemicals - such as growth factors or enzymes - that the errant cells need to survive and grow.

Glivec (or Gleevec), for example, targets abnormal proteins that help cancerous cells to grow. Dubbed the first "magic bullet" for cancer, it is used to treat one type of leukaemia and a rare cancer of the gut. Another drug, 17AAG, targets cancer cells by suppressing Hsp90 - a protein vital for their growth. The drug is undergoing trials to treat melanoma and other cancers that affect the prostate, kidney and breast.

An alternative approach is to persuade the immune system to attack tumours, using vaccines, biological therapies such as alpha interferon or interleukin 2 andgenetically altered white blood cells. In the future, scientists hope to targetstem-cell-like cells within cancers that may be responsible for most of the growth of some tumours, and evade existing drugs. They also plan to use nano-drugs, nano-bullets and "smart bombs" to deliver molecules with pinpoint precision to tumour cells.

Even soil-living or flesh-eating bacteria, engineered viruses, weed extracts,microwaves, chemicals from Antarctic sea squirts and the immune cells of siblings have been recruited to destroy tumours.

Prevalence Patterns

In developed countries, the cancer most likely to afflict you is non-melanomaskin cancer, often caused by the Sun's harmful ultraviolet rays. This is usually treated and cured. Melanoma, a deadlier skin cancer, is less common but isincreasing by around 3% a year in the US. Though UV light is the cause of many skin cancers, there is some evidence that small amounts may also help prevent other cancers. In the developing world the most common cancers are linked to infectious agents, such as cervical cancer or liver cancer, caused by hepatitis B.

The biggest killer in industrialised nations remains lung cancer. For men, lung cancer mortality has been falling since 1990, mirroring a peak in popularity of smoking during the 1960s. For women, who commonly took up smoking later, lung cancer mortality is still rising. Tobacco is also rapidly becoming the leading cause of cancer in Asia.

Breast cancer accounts for almost one in three of all cancers diagnosed in women each year. For men, prostate cancer is just as common, with the highest rates in African American men and Caribbean men of African origin.

Screening for both of these diseases has improved sharply. Early breast tumours show up on a mammogram long before they can be felt as a lump. More women now survive the disease - three-quarters of women whose breast cancer was diagnosed 10 years ago are still alive today. Similarly, a prostate-specific antigen (PSA) test has revolutionised early diagnosis for this cancer in the US. Some experts argue that screening can be harmful, however.

Factoring in risk

Dozens of factors affect an individual's risk of getting cancer. Smoking is the biggest single risk factor - with tobacco linked to about a third of all cancers.

Another clearly established risk factor is exposure to ionising radiation. This may be responsible for cancers in people living around Chernobyl in Ukraine,Toikamura in Japan and for people working in nuclear power plants. Radiationmay also have led to a high incidence of cancer in those who witnessed early nuclear tests or have been in contact with depleted uranium munitions, though a recent study contests this risk.

Risk factors for developing breast cancer include: being childless or delaying childbearing until aged over 30, starting periods early, using hormone replacement therapy, being exposed to oestrogen-like chemicals, and drinking one or more units of alcohol daily. More than one risk factor is usually needed before cancer cells develop.

Risks posed by living near overhead power lines or petrol stops, or usingcellphones, are less clear.

Preventative steps

If everyone stopped smoking, cancer deaths could be cut by one-third, researchers estimate. Moves to protect people from passive smoking, in bars for example, are gaining ground in many industrialised countries.

Staying out of sunlight and using strong sunscreens could prevent hundreds of thousands of us from developing skin cancer worldwide annually. Foods rich inantioxidants and beneficial fatty acids such as omega-3 and oleic acid found inolive oil - a key ingredient of the healthy Mediterranean diet - seem to protect against some cancers, although the findings are mixed.

Doctors can increasingly intervene directly to prevent cancers. For example,vaccines against hepatitis B could soon cut deaths from liver cancer. There are also preventive therapies - such as tamoxifen or the trial drug anastrazole - thatinterfere with the production of the hormone oestrogen, implicated in many breast cancers. Doctors believe that it could halve the rates of breast cancer in women with a family history of the disease.

All this means that, while hopes of total cure for cancer are still unrealistic, the disease is increasingly under control.

Src: http://www.newscientist.com/article/dn9957-instant-expert-cancer.html?full=true