CANCER

Cancer /ˈkænsər/ (medical term: malignant neoplasm) is a class of diseases in which a cell, or a group of cells display uncontrolled growth (division beyond the normal limits), invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood). These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, and do not invade or metastasize. Most cancers form a tumor but some, like leukemia, do not. The branch of medicine concerned with the study, diagnosis, treatment, and prevention of cancer is oncology.

Cancer affects people at all ages with the risk for most types increasing with age.^[1] It caused about 13% of all human deaths in 2007^[2] (7.6 million).^[3] Cancers are primarily an environmental disease with 90-95% of cases due to lifestyle and environmental factors and 5-10% due to genetics.^[4] Common environmental factors leading to cancer death include: tobacco (25-30%) , diet and obesity (30-35%), infections (15-20%), radiation, stress, physical activity, environmental pollutants.^[4] These environmental factors cause abnormalities in the genetic material of cells.^[5]

Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are typically activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Tumor suppressor genes are then inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system.

Definitive diagnosis requires the histologic examination of a biopsy specimen, although the initial indication of malignancy can be symptomatic or radiographic imaging abnormalities. Most cancers can be treated and some forced into remission, depending on the specific type, location, and stage. Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. As research develops, treatments are becoming more specific for different varieties of cancer. There has been significant progress in the development of targeted therapy drugs that act specifically on detectable molecular abnormalities in certain tumors, and which minimize damage to normal cells. The prognosis of cancer patients is most influenced by the type of cancer, as well as the stage, or extent of the disease. In addition, histologic grading and the presence of specific molecular markers can also be useful in establishing prognosis, as well as in determining individual treatments.

Cancer
Classification and external resources
A coronal CT scan showing malignant cancer of the lung sac. Legend: → tumor ←, ★ central pleural effusion, 1&3 lungs, 2 spine, 4 ribs, 5 aorta, 6 spleen, 7&8 kidneys, 9 liver.
DiseasesDB	28843
MedlinePlus	001289
MeSH	D009369

CHEMOPREVENTION

The concept that medications could be used to prevent cancer is an attractive one, and many high-quality clinical trials support the use of such chemoprevention in defined circumstances.

Daily use of tamoxifen, a selective estrogen receptor modulator (SERM), typically for 5 years, has been demonstrated to reduce the risk of developing breast cancer in high-risk women by about 50%. A recent^[when?] study reported that the selective estrogen receptor modulator raloxifene has similar benefits to tamoxifen in preventing breast cancer in high-risk women, with a more favorable side effect profile.^[78]

Raloxifene is a SERM like tamoxifen; it has been shown (in the STAR trial) to reduce the risk of breast cancer in high-risk women equally as well as tamoxifen. In this trial, which studied almost 20,000 women, raloxifene had fewer side effects than tamoxifen, though it did permit more DCIS to form.^[78]

Finasteride, a 5-alpha-reductase inhibitor, has been shown to lower the risk of prostate cancer, though it seems to mostly prevent low-grade tumors.^[79] The effect of COX-2 inhibitors such as rofecoxib and celecoxib upon the risk of colon polyps have been studied in familial adenomatous polyposis patients^[80] and in the general population.^[81][82] In both groups, there were significant reductions in colon polyp incidence, but this came at the price of increased cardiovascular toxicity.

CANCER SCREENING

Cancer screening is an attempt to detect unsuspected cancers in an asymptomatic population. Screening tests suitable for large numbers of healthy people must be relatively affordable, safe, noninvasive procedures with acceptably low rates of false positive results. If signs of cancer are detected, more definitive and invasive follow up tests are performed to confirm the diagnosis.

Screening for cancer can lead to earlier diagnosis in specific cases. Early diagnosis may lead to extended life, but may also falsely prolong the lead time to death through lead time bias or length time bias.

A number of different screening tests have been developed for different malignancies. Breast cancer screening can be done by breast self-examination, though this approach was discredited by a 2005 study in over 300,000 Chinese women. Screening for breast cancer with mammograms has been shown to reduce the average stage of diagnosis of breast cancer in a population. Stage of diagnosis in a country has been shown to decrease within ten years of introduction of mammographic screening programs. Colorectal cancer can be detected through fecal occult blood testing and colonoscopy, which reduces both colon cancer incidence and mortality, presumably through the detection and removal of pre-malignant polyps. Similarly, cervical cytology testing (using the Pap smear) leads to the identification and excision of precancerous lesions. Over time, such testing has been followed by a dramatic reduction of cervical cancer incidence and mortality. Testicular self-examination is recommended for men beginning at the age of 15 years to detect testicular cancer. Prostate cancer can be screened using a digital rectal exam along with prostate specific antigen (PSA) blood testing, though some authorities (such as the US Preventive Services Task Force) recommend against routinely screening all men.

Screening for cancer is controversial in cases when it is not yet known if the test actually saves lives. The controversy arises when it is not clear if the benefits of screening outweigh the risks of follow-up diagnostic tests and cancer treatments. For example: when screening for prostate cancer, the PSA test may detect small cancers that would never become life threatening, but once detected will lead to treatment. This situation, called overdiagnosis, puts men at risk for complications from unnecessary treatment such as surgery or radiation. Follow up procedures used to diagnose prostate cancer (prostate biopsy) may cause side effects, including bleeding and infection. Prostate cancer treatment may cause incontinence (inability to control urine flow) and erectile dysfunction (erections inadequate for intercourse). Similarly, for breast cancer, there have recently^[when?] been criticisms that breast screening programs in some countries cause more problems than they solve. This is because screening of women in the general population will result in a large number of women with false positive results which require extensive follow-up investigations to exclude cancer, leading to having a high number-to-treat (or number-to-screen) to prevent or catch a single case of breast cancer early.

Cervical cancer screening via the Pap smear has the best cost-benefit profile of all the forms of cancer screening from a public health perspective as, largely caused by a virus, it has clear risk factors (sexual contact), and the natural progression of cervical cancer is that it normally spreads slowly over a number of years therefore giving more time for the screening program to catch it early. Moreover, the test is easy to perform and relatively cheap.

For these reasons, it is important that the benefits and risks of diagnostic procedures and treatment be taken into account when considering whether to undertake cancer screening.

Use of medical imaging to search for cancer in people without clear symptoms is similarly marred with problems. There is a significant risk of detection of what has been recently^[when?] called an incidentaloma - a benign lesion that may be interpreted as a malignancy and be subjected to potentially dangerous investigations. Recent^[when?] studies of CT scan-based screening for lung cancer in smokers have had equivocal results, and systematic screening is not recommended as of July 2007. Randomized clinical trials of plain-film chest X-rays to screen for lung cancer in smokers have shown no benefit for this approach.

Canine cancer detection has shown promise, but is still in the early stages of research.