the center, surrounded by normal, yellow fatty tissue. some hormones play a role in the development of cancer by promoting cell proliferation. insulin-like growth factors and their binding proteins play a key role in cancer cell growth, differentiation and apoptosis, suggesting possible involvement in carcinogenesis. hormones are important agents in sex-related cancers such as cancer of the breast, endometrium, prostate, ovary, and testis, and also of thyroid cancer and bone cancer. for example, the daughters of women who have breast cancer have significantly higher levels of estrogen and progesterone than the daughters of women without breast cancer. these higher hormone levels may explain why these women have higher risk of breast cancer, even in the absence of a breast-cancer gene. similarly, men of african ancestry have significantly higher levels of testosterone than men of european ancestry, and have a correspondingly much higher level of prostate cancer. men of asian ancestry, with the lowest levels of testosterone-activating androstanediol glucuronide, have the lowest levels of prostate cancer. other factors are also relevant: obese people have higher levels of some hormones associated with cancer and a higher rate of those cancers. women who take hormone replacement therapy have a higher risk of developing cancers associated with those hormones. on the other hand, people who exercise far more than average have lower levels of these hormones, and lower risk of cancer. osteosarcoma may be promoted by growth hormones. some treatments and prevention approaches leverage this cause by artificially reducing hormone levels, and thus discouraging hormone-sensitive cancers. because steroid hormones are powerful drivers of gene expression in certain cancer cells, changing the levels or activity of certain hormones can cause certain cancers to cease growing or even undergo cell death. perhaps the most familiar example of hormonal therapy in oncology is the use of the selective estrogen-receptor modulator tamoxifen for the treatment of breast cancer. another class of hormonal agents, aromatase inhibitors, now have an expanding role in the treatment of breast cancer. infection and inflammation main article: infectious causes of cancer worldwide, approximately 18% of cancer cases are related to infectious diseases. this proportion varies in different regions of the world from a high of 25% in africa to less than 10% in the developed world. viruses are the usual infectious agents that cause cancer but bacteria and parasites also contribute. infectious organisms that increase the risk of cancer are frequently a source of dna damage or genomic instability. viruses hpv is the most common virus that infects the reproductive tract. infection can lead to the development of cervical cancer in women. viral infection is a major risk factor for cervical and liver cancer. a virus that can cause cancer is called an oncovirus. these include human papillomavirus (cervical carcinoma), epstein–barr virus (b-cell lymphoproliferative disease and nasopharyngeal carcinoma), kaposi's sarcoma herpesvirus (kaposi's sarcoma and primary effusion lymphomas), hepatitis b and hepatitis c viruses (hepatocellular carcinoma), and human t-cell leukemia virus-1 (t-cell leukemias). in western developed countries, human papillomavirus (hpv), hepatitis b virus (hbv) and hepatitis c virus (hcv) are the most common oncoviruses. in the united states, hpv causes most cervical cancers, as well as some cancers of the vagina, vulva, penis, anus, rectum, throat, tongue and tonsils. among high-risk hpv viruses, the hpv e6 and e7 oncoproteins inactivate tumor suppressor genes when infecting cells. in addition, the oncoproteins independently induce genomic instability in normal human cells, leading to an increased risk of cancer development. individuals with chronic hepatitis b virus infection are more than 200 times more likely to develop liver cancer than uninfected individuals. liver cirrhosis, whether from chronic viral hepatitis infection or excessive alcohol use, is independently associated with the development of liver cancer, but the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. bacteria and parasites histopathology of schistosoma haematobium eggs within the lining of the bladder.certain bacterial infections also increase the risk of cancer, as seen in helicobacter pylori-induced gastric carcinoma. the mechanism by which h. pylori causes cancer may involve chronic inflammation or the direct action of some of the bacteria's virulence factors. parasitic infections strongly associated with cancer include schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, opisthorchis viverrini and clonorchis sinensis (cholangiocarcinoma). inflammation triggered by the worm's eggs appears to be the cancer-causing mechanism. certain parasitic infections can also increase the presence of carcinogenic compounds in the body, leading to the development of cancers. tuberculosis infection, caused by the mycobacterium m. tuberculosis, has also been linked with the development of lung cancer. inflammation there is evidence that inflammation itself plays an important role in the development and progression of cancer. chronic inflammation can lead to dna damage over time and the accumulation of random genetic alterations in cancer cells. inflammation can contribute to proliferation, survival, angiogenesis and migration of cancer cells by influencing tumor microenvironment. individuals with inflammatory bowel disease are at increased risk of developing colorectal cancers. radiation main article: radiation-induced cancer up to 10% of invasive cancers are related to radiation exposure, including both non-ionizing radiation and ionizing radiation. unlike chemical or physical triggers for cancer, ionizing radiation hits molecules within cells randomly. if it happens to strike a chromosome, it can break the chromosome, result in an abnormal number of chromosomes, inactivate one or more genes in the part of the chromosome that it hit, delete parts of the dna sequence, cause chromosome translocations, or cause other types of chromosome abnormalities. major damage normally results in the cell dying, but smaller damage may leave a stable, partly functional cell that may be capable of proliferating and developing into cancer, especially if tumor suppressor genes were damaged by the radiation. three independent stages appear to be involved in the creation of cancer with ionizing radiation: morphological changes to the cell, acquiring cellular immortality (losing normal, life-limiting cell regulatory processes), and adaptations that favor formation of a tumor. even if the radiation particle does not strike the dna directly, it triggers responses from cells that indirectly increase the likelihood of mutations. non-ionizing radiation squamous cell carcinoma on the sun-exposed skin of the nose.not all types of electromagnetic radiation are carcinogenic. low-energy waves on the electromagnetic spectrum including radio waves, microwaves, infrared radiation and visible light are thought not to be because they have insufficient energy to break chemical bonds. non-ionizing radio frequency radiation from mobile phones, electric power transmission, and other similar sources have been described as a possible carcinogen by the world health organization's international agency for research on cancer. however, studies have not found a consistent link between cell phone radiation and cancer risk. higher-energy radiation, including ultraviolet radiation (present in sunlight), x-rays, and gamma radiation, generally is carcinogenic, if received in sufficient doses. prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies. the vast majority of non-invasive cancers are non-melanoma skin cancers caused by non-ionizing ultraviolet radiation. clear evidence establishes ultraviolet radiation, especially the non-ionizing medium wave uvb, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world. ionizing radiation cross section of a meningioma displacing the underlying brain.sources of ionizing radiation include medical imaging, and radon gas. ionizing radiation is not a particularly strong mutagen. medical use of ionizing radiation is a growing source of radiation-induced cancers. ionizing radiation may be used to treat other cancers, but this may, in some cases, induce a second form of cancer. radiation can cause cancer in most parts of the body, in all animals, and at any age, although radiation-induced solid tumors usually take 10–15 years, and can take up to 40 years, to become clinically manifest, and radiation-induced leukemias typically require 2–10 years to appear. radiation-induced meningiomas are an uncommon complication of cranial irradiation. some people, such as those with nevoid basal cell carcinoma syndrome or retinoblastoma, are more susceptible than average to developing cancer from radiation exposure. children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect. ionizing radiation is also used in some kinds of medical imaging. in industrialized countries, medical imaging contributes almost as much radiation dose to the public as natural background radiation. nuclear medicine techniques involve the injection of radioactive pharmaceuticals directly into the bloodstream. radiotherapy deliberately deliver high doses of radiation to tumors and surrounding tissues as a form of disease treatment. it is estimated that 0.4% of cancers in 2007 in the united states are due to cts performed in the past and that this may increase to as high as 1.5–2% with rates of ct usage during this same time period. residential exposure to radon gas has similar cancer risks as passive smoking. low-dose exposures, such as living near a nuclear power plant, are generally believed to have no or very little effect on cancer development. radiation is a more potent source of cancer when it is combined with other cancer-causing agents, such as radon gas exposure plus smoking tobacco. rare causes organ transplantation malignant melanoma metastases in a heart. the development of donor-derived tumors from organ transplants is exceedingly rare. the main cause of organ transplant associated tumors seems to be malignant melanoma, that was undetected at the time of organ harvest. there have also been reports of kaposi's sarcoma occurring after transplantation due to tumorous outgrowth of virus-infected donor cells. trauma