The three most common cancers in men, women, and children

are as follows:

• Men: Prostate, lung, and colorectal

• Women: Breast, lung, and colorectal

• Children: Leukemia, brain tumors, and lymphoma

The incidence of cancer and cancer types are influenced by many factors such as age, gender, race, local environmental factors, diet, and genetics. Consequently, the incidence of cancer and cancer types varies depending on these variable factors. For example, the World Health Organization (WHO) provides the following general information about cancer worldwide:

• Cancer is a leading cause of death worldwide. It accounted for 8.2 million deaths (around 22% of all deaths not related to communicable diseases; most recent data from WHO).

• Lung, stomach, liver, colon, and breast cancer cause the most cancer deaths each year.

• Deaths from cancer worldwide are projected to continue rising, with an estimated 13.1 million deaths in 2030 (about a 70% increase).

Different areas of the world may have cancers that are either more or less predominant than those found in the U.S. One example is that stomach cancer is often found in Japan, while it is rarely found in the U.S. This usually represents a combination of environmental and genetic factors.

The objective of this article is to introduce the reader to general aspects of cancers. It is designed to be an overview of cancer and cannot cover every cancer type. This article will also attempt to help guide the reader to more detailed sources about specific cancer types.

What are risk factors and causes of cancer?

Tobacco or cigarette smoke contains at least 66 known potential carcinogenic chemicals and toxins.

Anything that may cause a normal body cell to develop abnormally potentially can cause cancer. Many things can cause cell abnormalities and have been linked to cancer development. Some cancer causes remain unknown while other cancers have environmental or lifestyle triggers or may develop from more than one known cause. Some may be developmentally influenced by a person's genetic makeup. Many patients develop cancer due to a combination of these factors. Although it is often difficult or impossible to determine the initiating event(s) that cause cancer to develop in a specific person, research has provided clinicians with a number of likely causes that alone or in concert with other causes, are the likely candidates for initiating cancer. The following is a listing of major causes and is not all-inclusive as specific causes are routinely added as research advances:

Chemical or toxic compound exposures: Benzene, asbestos, nickel, cadmium, vinyl chloride, benzidine, N-nitrosamines, tobacco or cigarette smoke (contains at least 66 known potential carcinogenic chemicals and toxins), asbestos, and aflatoxin

Ionizing radiation: Uranium, radon, ultraviolet rays from sunlight, radiation from alpha, beta, gamma, and X-ray-emitting sources

Pathogens: Human papillomavirus (HPV), EBV or Epstein-Barr virus, hepatitis viruses B and C, Kaposi's sarcoma-associated herpes virus (KSHV), Merkel cell polyomavirus, Schistosoma spp., and Helicobacter pylori; other bacteria are being researched as possible agents.

Genetics: A number of specific cancers have been linked to human genes and are as follows: breast, ovarian, colorectal, prostate, skin, and melanoma; the specific genes and other details are beyond the scope of this general article so the reader is referred to the National Cancer Institute

for more details about genetics and cancer.

It is important to point out that most everyone has risk factors for cancer and is exposed to cancer-causing substances (for example, sunlight, secondary cigarette smoke, and X-rays) during their lifetime, but many individuals do not develop cancer. In addition, many people have genes that are linked to cancer but do not develop it. Why? Although researchers may not be able to give a satisfactory answer for every individual, it is clear that the higher the amount or level of cancer-causing materials a person is exposed to, the higher the chance the person will develop cancer. In addition, people with genetic links to cancer may not develop it for similar reasons (lack of enough stimulus to make the genes function). In addition, some people may have a heightened immune response that controls or eliminates cells that are or potentially may become cancer cells. There is evidence that even certain dietary lifestyles may play a significant role in conjunction with the immune system to allow or prevent cancer cell survival. For these reasons, it is difficult to assign a specific cause of cancer to many individuals.

Recently, other risk factors have been added to the list of items that may increase cancer risk. Specifically, red meat (such as beef, lamb, and pork) was classified by the International Agency for Research on Cancer as a high-risk agent for potentially causing cancers; in addition, processed meats (salted, smoked, preserved, and/or cured meats) were placed on the carcinogenic list. Individuals that eat a lot of barbecued meat may also increase risk due to compounds formed at high temperatures. Other less-defined situations that may increase the risk of certain cancers include obesity, lack of exercise, chronic inflammation, and hormones, especially those hormones used for replacement therapy. Other items such as cell phones have been heavily studied. In 2011, the World Health Organization classified cell phone low-energy radiation as "possibly carcinogenic," but this is a very low-risk level that puts cell phones at the same risk as caffeine and pickled vegetables.

Proving that a substance does not cause or is not related to increased cancer risk is difficult. For example, antiperspirants are considered to possibly be related to breast cancer by some investigators and not by others, and there are conflicting reports that have been described in the literature. However, there is no firm or definitive scientific evidence that links antiperspirant use to breast cancer development.

What are cancer symptoms?

Symptoms and signs of cancer depend on the type of cancer, where it is located, and/or where the cancer cells have spread. For example, breast cancer may present as a lump in the breast or as nipple discharge while metastatic breast cancer may present with symptoms of pain (if spread to bones), extreme fatigue (lungs), or seizures (brain). A few patients show no signs or symptoms until the cancer is far advanced.

The American Cancer Society describes seven warning signs and/or symptoms that cancer may be present, and which should prompt a person to seek medical attention. The word CAUTION can help you remember these.

• Change in bowel or bladder habits

• A sore throat that does not heal

• Unusual bleeding or discharge (for example, nipple secretions or a "sore" that will not heal that oozes material)

• Thickening or lump in the breast, testicles, or elsewhere

• Indigestion (usually chronic) or difficulty swallowing

• Obvious change in the size, color, shape, or thickness of a wart or mole

• Nagging cough or hoarseness

Other signs or symptoms may also alert you or your doctor to the possibility of your having some form of cancer. These include the following:

• Unexplained loss of weight or loss of appetite

• A new type of pain in the bones or other parts of the body that may be steadily worsening, or come and go, but is unlike previous pains one has had before

• Persistent fatigue, nausea, or vomiting

• Unexplained low-grade fevers that may be either persistent or come and go

• Recurring infections which will not clear with usual treatment

Anyone with these signs and symptoms should consult their doctor; these symptoms may also arise from noncancerous conditions.

Many cancers will present with some of the above general symptoms but often have one or more symptoms that are more specific for the cancer type. For example, lung cancer may present with common symptoms of pain, but usually, the pain is located in the chest. The patient may have unusual bleeding, but the bleeding usually occurs when the patient coughs. Lung cancer patients often become short of breath and then become very fatigued.

Because there are so many cancer types (see next section) with so many nonspecific and sometimes more specific symptoms, the best way to learn about the signs and symptoms of specific cancer types is to spend a few moments researching symptoms of a specific body area in question. Conversely, a specific body area can be searched to discover what signs and symptoms a person should look for in that area that is suspected of having cancer. The following examples are two ways to proceed to get information on symptoms:

• Use a search engine (Google, Bing) to find links to cancer by listing the symptom followed by the term "cancer" or if you know the type you want information about, (lung, brain, breast) use MedicineNet's search option. For example, listing "blood in urine and cancer" will bring a person to websites that list possible organs and body systems where cancer may produce the listed symptoms.

• Use a search engine as above and list the suspected body area and cancer (for example, bladder, and cancer), and the person will see sites that list the signs and symptoms of cancer in that area (blood in urine is one of several symptoms listed).

• Be aware that many websites are not necessarily reviewed by a health care professional and could contain information that is not accurate. Your health care professional is ultimately the best resource if you have concerns.

Besides, if the cancer type is known (diagnosed), then even more specific searches can be done listing the diagnosed cancer type and whatever may be questioned about cancer (symptoms, tumor grades, treatments, prognosis, and many other items).

One's research should not replace consulting a health care provider if someone is concerned about cancer.