Cancer, human definition

Figure 5.31 Possible effects that animal viruses may have on cells they infect, humans has, in most cases, been uncertain. It is difficult to prove the viral origin of a human cancer because of the difficulties of carrying out the necessary experimentation. However, it is now well established that certain specific kinds of human tumors do have a viral origin. A summary of some of the human cancers with definite viral origins is given in Table.

As mentioned earlier, epidemiological studies have revealed that the incidence of cancer increases exponentially with age. From these studies, it has been suggested that three to seven mutations or hits are necessary for cancer development. Definitive proof that these hits occur in oncogenes and tumor suppressor genes and that these genes cooperate in tumorigenesis comes from molecular studies in human cancer and mouse cancer models. 

There are many definitions of the word risk. It is a combination of uncertainty and damage a ratio of Itazards to safeguards a triplet combination of event, probability, and consequences or even a measure of economic loss or human injury in terms of both the incident likelihood and tlie magnitude of the loss or injuiy (AICliE, 1989). People face all kinds of risks eveiyday, some voluntarily and otliers involuntarily. Tlierefore, risk plays a very important role in today s world. Studies on cancer caused a turning point in tlie world of risk because it opened tlie eyes of risk scientists and healtli professionals to tlie world of risk assessments. 

Hazard identification is defined as tlie process of determining whetlier human exposure to an agent could cause an increase in the incidence of a health condition (cancer, birtli defect, etc.) or whetlier exposure to nonliumans, such as fish, birds, and otlier fonns of wildlife, could cause adverse effects. Hazard identification cliaracterizes tlie liazard in terms of tlie agent and dose of the agent. Since tliere are few hazardous chemicals or hazardous agents for wliich definitive exposure data in humans exists, tlie identification of health hazards is often characterized by the effects of health hazards on laboratory test animals or other test systems. ... 

Tea flavonoids, or tea extracts, have been linked to benefits in reducing the risk of certain cancers and cardiovascular diseases in experimental animals. However, epidemiological studies have produced inconsistent evidence in the relationship between tea drinking and cancer (Blot et a/., 1997 Goldbohm etal, 1996 Hertog eta/., 1997 Yang eta/., 1996). Therefore, further research is needed before definitive conclusions on the impact of tea consumption upon the cancer risk in humans can be reached. The metabolites of catechins and flavonols after consumption of tea infusions have scarcely been investigated, and thus more research is needed as to the role of those compounds in the reported health benefits of tea consumption. 

Laboratory studies with mice and rats have conclusively demonstrated that the injection of cadmium metal or salts causes malignancies (sarcoma) at the site of injection and testicular tumors. However, the simultaneous administration of zinc is protective against sarcoma and interstitial cell tumor development (USEPA 1980). In rats, no dose-related increases in tumors were found at maximum oral daily doses of 4.4 mg Cd/kg BW (USPHS 1993). Among humans, the available epidemiological evidence is not sufficient to conclude that cadmium is definitely implicated as a carcinogen (USEPA 1980 Nomiyama 1982), although cadmium exposure is associated with lung cancer in humans (Shimada et al. 1998). 

Subsequent studies in experimental animals have yielded provocative results. Resveratrol is known to extend the lifespan of a number of organisms from yeast to vertebrates. Resveratrol is also known to prevent or slow the progression of cancer, cardiovascular diseases, diabetes, inflammation, and ischemic injuries in experimental animals. In short, the suggestion that resveratrol in red wine may be responsible for favorable outcomes in human health is supported by a number of studies in experimental animals. However, the support is suggestive but certainly not definitive. Carefully controlled clinical trials in people will be required to establish the role, if any, of resveratrol or related small molecules in human health. Such clinical trials are currently underway. 

We do not definitely know if 1,4-dichlorobenzene plays a role in the development of cancer. The Department of Health and Human Services (DHHS) has determined that 1,4-di chlorobenzene may reasonably be anticipated to be a carcinogen in humans. The International Agency for Research on Cancer (lARC) has determined that 1,4-dichlorobenzene is possibly carcinogenic to humans. The EPA has determined that 1,4-di chlorobenzene is a possible human carcinogen. 

Early hope that animal, and especially bacterial, tests would produce cheap and definitive information about the causes of cancer in humans has now faded. Disputes over tests are even more controversial than disputes over human evidence, and many scientists consider them unreliable. 

In the absence of definitive human data, risk assessment may have to depend on the results of cancer bioassays in laboratory animals, short-term tests, or other experimental methods. Hence the following issues must be addressed under such circumstances the ability of the test system to predict risks for man (quantitatively as well as qualitatively) the reproducibility of test results the influence of species differences in pharmacokinetics, metabolism, homeostasis, repair rates, life span, organ sensitivity, and baseline cancer rates extrapolation across dose and dose rates, and routes of exposure the significance of benign tumors fitting models to the data in order to characterize dose-incidence relationships and the significance of negative results. 

The fundamental question of risk assessment for potential human carcinogens requires definition of substances that exceed an evidentiary threshold. Once the scientific evidence establishes a substantial basis for conclusion of known or potential human cancer, it is then in order to determine a procedure for risk quantification. Quantitative risk assessments must always be read with the qualitative evidence of the likelihood of carcinogenicity. 

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