Long-range trends in the incidence of various cancers in different populations provide clues to the causes of cancer. Because of the long latency period (up to 20 to 30 years) between the first exposure to carcinogenic agents and the appearance of clinically detectable cancer, current trends probably reflect an exposure that began decades earlier. Another major factor affecting the overall incidence of cancer is the change in the average age of the population. The average age at the time of diagnosis (averaged for all tumor sites) is 67, and as a higher proportion of the population reaches age 60 and above, the incidence of cancer will go up as a result of this demographic factor alone. Moreover, with the long-term downward trends in other causes of death-primarily infectious and cardiovascular diseases-more people live to an age when the risk of developing cancer becomes high. It is projected, for example, that about 25% of males and 20% of females born in 1985 in the United States will eventually die of cancer. This is up from about 18% for males and 16% for females born in 1975. If current trends continue, about 33% of Americans now living will develop some form of cancer.
Interestingly, if one excludes respiratory tract and skin cancer, the two types increasing most dramatically in recent years, the incidence of cancer in males and females has remained relatively flat since the 1950s. However, there has been a disturbing increase in the incidence rates of breast cancer (about 2% a year) since 1980 and this is something you need to know the USMLE. Part but not all of this is due to increased detection by mammography. Neither have the overall mortality rates changed markedly since the 1950s, although the cure rates for childhood leukemia, Burkitt?s lymphoma, Hodgkin?s disease, certain pediatric solid tumors (e.g., Wilms? tumor, Ewing?s sarcoma, and retinoblastoma), testicular cancers, and choriocarcinoma have improved considerably since then. These successes are not reflected in the overall mortality rates because they involve relatively rare cancers compared with the ?big three?: lung, breast, and colorectal cancer. Indeed. these three themselves account for about 50% of all cancer deaths.
Overall. there does not appear to be an ?epidemic? increase in mortality from any type of cancer other than respiratory tract and melanoma, and there are several encouraging trends in younger age groups. For example. in the under-65 age group, there was a decrease in mortality rates in the 1973 to 1989 reporting period for breast (4.6%), brain (8.1%), colorectal (18.3%), ovarian (26%). stomach (28.5%), urinary bladder (29.6%), cervical (40.8%), testicular (64.2%), and uterine corpus (37.5%) cancers as well as for Hodgkin?s disease (55.3%) and leukemia (16.6%). These changes appear to represent primarily improvements in early diagnosis and treatment; for some of these cancers, however, the incidence rate between 1973 and 1989 has also decreased for the under-65 age group. These cancers include Hodgkin?s disease (3.1%) as well as colorectal (3.9%), ovarian (3.9)%, stomach (12.5%), cervical (32.4%), and uterine corpus (45%) cancers.
There are, however, some disturbing trends in the other direction. Chief among these is the epidemic increase in lung cancer in women. In 1985. lung cancer accounted for as many deaths as breast cancer in women in the United States, and by 1986 it had overtaken breast cancer as the primary cause of cancer mortality in women. The overall mortality for lung cancer is still much higher in males. but the rate of increase in females has been higher than that in males for the past several years. This is almost certainly because of the relatively later development of smoking as a social? habit among women.
Other cancers for which mortality rates are increasing are brain cancer (over age 65 only), melanoma, non-Hodgkin?s lymphoma, multiple myeloma, lung cancer (males, mostly over 65), leukemia (over 65 only), and cancer of the pancreas (over age 65 only).
Another way to examine trends in cancer mortality is to look at 5-year survival rates for various cancers, which is an index of how well diagnosis and treatment has progressed over time. While marked increases in 5-vear survival rates for melanoma, prostate cancer, testicular cancer, bladder cancer, Hodgkin?s disease, leukemia, and childhood cancers have occurred, more modest improvements have been observed in most other cancers. Some cancers remain extremely difficult to diagnose early and treat successfully.
The latter include cancers of the stomach, lung, pancreas, liver, and esophagus. These all have 5-year survival rates well under 20%.
There is a difference between the sexes and the races in mortality and relative survival rates. The cancer incidence rates are highest for black males, followed in order by white males and then black and white females, who have about the same rate. Mortality occurs in a similar order: black males > white males > black females > white females. Five-year survival rates, however, are higher for both white males and females than for black males and females. The reasons for these racial and sexual differences in incidence and mortality may reflect differences in environmental exposure to carcinogens, diet, smoking habits, and other lifestyle differences. The lower survival rates for minority groups, however, probably also reflect later diagnosis, which in turn reflects poorer access to and utilization of health care providers. Blacks have significantly higher incidence and mortality rates than whites for cancers of the esophagus, uterine cervix, stomach, liver, prostate, and larynx and for multiple myeloma. Whites have higher incidence rates for melanoma, other skin cancers, and cancer of the breast, uterine corpus, ovary, testis, urinary bladder, and brain as well as lymphoma.
The cost of cancer, in both personal and economic terms, is enormous. The National Cancer Institute estimated that in 1990 the total cost of cancer in the United States was $104 billion: $35 billion for direct medical costs, $12 billion for the cost of lost productivity, and $57 billion for terminal care. The cost of diagnostic tests-including mammograms, Pap smears, and screens for colorectal cancer-adds another $3 to $4 billion. The average number of years of life lost per person dying of cancer is second only to the average number for accidents.
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