Exposure To Carcinogens And The Occurrence Of Cancer – The Cancer Dormancy Time
Cancer is one of the leading killer diseases in the developed countries today. In United States alone at the present rate as much as 40% of the population could develop some type of cancer in their lifetime. It is a deadly disease causing severe damage and drain on one's finances. The loss of human productivity and the loss of life associated with cancer is truly unbearable. But the cause and origin of this deadly disease is only poorly understood. One reason why it has been difficult to recognize causes of cancer in humans is the long delay that characteristically occurs between the start of exposure to a carcinogen and the appearance of the clinical disease. This 'latent period', as it is commonly, but rather misleadingly, called is often several decades, although it may be as short as 1 year or as long as 60. The disease will remain dormant for years and even decades and then suddenly burst out.
The exact relationship between the date of exposure to carcinogens and the date of the appearance of different cancers is still uncertain, partly because the interval is subject to random factors, partly because few cancers are induced by a single, brief exposure, and partly because there are still very few sets of quantitative data with detailed information about the dates when exposure began and ended.
When cancer is induced by short but intense exposure to ionizing radiation, as following the explosions of the atomic bombs in Hiroshima and Nagasaki or in patients treated by radiotherapy, the excess incidence of solid cancer tumors increases for 15 to 20 years and then may continue to rise, level off, or decline. In the case of acute leukemia, however, a peak incidence occurs much earlier (about 5 years after irradiation) and relatively few cases appear after more than 30 years.
Short, intensive exposure to a carcinogen is, however, exceptional. The more usual situation is for sporadic or continuous exposure to a carcinogen to be prolonged for years-a decade or two in the case of occupational exposure, several decades in the case of tobacco smoking, and a lifetime in the case of ultraviolet sunlight. In this situation the incidence of cancer increases progressively with the length of exposure. In the last two cases cited, the incidence appears to increase approximately in proportion to the fourth power of the duration of exposure so that the effect after (say) 40 years is more than 10 times as great as that after 20 years, and more than 100 times as great as that after 10 years. Whether the same holds for occupational exposure is not known; but it has been shown to hold in some experiments in which carcinogenic chemicals were reappliedly applied to the skin of genetically similar mice and it may prove to be a general biological rule for many types of carcinoma and many carcinogens.