In humans, cancer may be caused by genetics and environmental exposures; however, in the majority of instances the identification of the critical time window of exposure is problematic. The evidence for exposures occurring during the preconceptional period that have an association with childhood or adulthood cancers is equivocal.
Agents definitely related to cancer in children, and adulthood if exposure occurs in utero, include: maternal exposure to ionizing radiation during pregnancy and childhood leukemia and certain other cancers, and maternal use of diethylstilbestrol during pregnancy and clear-cell adenocarcinoma of the vagina of their daughters. The list of environmental exposures that occur during the perinatal/postnatal period with potential to increase the risk of cancer is lengthening, but evidence available to date is inconsistent and inconclusive.
Critical windows of exposure for children’s health: cancer in human epidemiological studies and neoplasms in experimental animal models, Environ Health Perspectives, NCBI PubMed PMID: 10852857, 2000 Jun.
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In animal models, preconceptional carcinogenesis has been demonstrated for a variety of types of radiation and chemicals, with demonstrated sensitivity for all stages from fetal gonocytes to postmeiotic germ cells. Transplacental and neonatal carcinogenesis show marked ontogenetic stage specificity in some cases. Mechanistic factors include the number of cells at risk, the rate of cell division, the development of differentiated characteristics including the ability to activate and detoxify carcinogens, the presence of stem cells, and possibly others. Usefulness for human risk estimation would be strengthened by the study of these factors in more than one species, and by a focus on specific human risk issues.
Unlike the situation for preconceptional exposures, there is good evidence that exposure of the human fetus to certain potentially harmful agents can increase the risk of cancer during childhood and possibly during early adulthood. Nonetheless, although numerous potentially harmful agents are suspected including infections, drugs, and maternal lifestyle characteristics the only two generally accepted carcinogenic in utero exposures are ionizing radiation and DES: the former acting directly on the fetus and the latter acting via the placenta.
The strong associations for DES have led researchers to postulate in utero effects for other endogenous and exogenous hormones, particularly for cancers with a suspected hormonal component to their etiology such as breast and testicular cancers. Further, since the birth of the first test-tube baby in 1978 there has been concern about the health of offspring resulting from assisted reproductive technology (ART). Multiple pregnancies often result from ART, which is one of the main determinants of the health of the child at birth. The importance of follow-up studies of these children to assess adverse health outcomes diagnosed after birth, even in adulthood, has been recognized, but few comprehensive and powerful epidemiological studies have been done. Two case reports have highlighted possible increases in cancer incidence in children born as a result of in vitro fertilization, raising concerns about the role of prenatal exposure (before and after conception) to high levels of estrogen and related compounds used for ovarian stimulation. To date, there are limited epidemiological data on this topic; a study of U.K. births after ART failed to find an excess incidence of childhood cancer, but, as noted by the authors, the study was too small to be able to detect a reasonable excess, even if it existed.
With respect to mechanisms and the timing of exposure, it is thought that the carcinogenic effects of both ionizing radiation and DES may be mediated via teratogenesis. This has been documented for DES, which causes various genital tract abnormalities in males as well as in females. In addition, it has been suggested that the exposure of pregnant women to substances that inhibit the function of the topoisomerase II enzymes could be related to the development of acute leukemia in their offspring.
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