As the second-leading cause of death in the United States, exceeded only by heart disease (Tables 1 and 2), cancer is responsible for one of every four deaths. According to the National Centers for Disease Control & Prevention (CDC), about 1.2 million new cancer cases are expected to be diagnosed in the United States in 1999, and this estimate does not include the vast majority of noninvasive cancers or skin cancers. It is also estimated that this year, about 563,100 Americans will die of cancer, translating to an average of more than 1,500 people a day losing their lives to this disease.
Interestingly, the incidence and death rates for all cancers combined, and for most of the top 10 cancer sites, declined between 1990 and 1995. This reverses an almost 20-year trend of increasing cancer cases and deaths in the United States, according to a recent cancer report from the American Cancer Society (ACS), National Cancer Institute (NCI), and the CDC.
Although five-year relative survival rates are based on patients diagnosed and treated at least eight years in the past and do not completely reflect the most recent advances in treatment, these statistics are invaluable in monitoring the progress of early detection and treatment of cancer. Currently, the five- year relative survival rate for all cancers combined is 60%.
Who or what is to blame?
Many cancers can be attributed to modifiable risk factors. For example, tobacco use still accounts for at least 35% of all cancer deaths annually in the United States, despite the billions of dollars invested in programs designed to reduce tobacco use among both children and adults. The origins of many other cancers are not as easily defined. For example, fewer than half of all breast cancer cases are attributable to well-established risk factors, and many of those risk factors are not modifiable.
Until recently, our most effective weapons in the war against cancer have been early-detection methods coupled with effective treatments. Physicians and scientists debate whether the recent declines in cancer incidence and mortality are attributable to better detection and treatment methods or to the minimization of risk, and many are calling for a shift in the focus of cancer research from detection and treatment to prevention. It has been estimated that 3% to 35% of all cancer deaths could be avoided using relatively simple cancer- preventive or early-detection procedures.
What causes human cancer?
The basic abnormality that defines cancer is the continuous unregulated proliferation of nondifferentiated cells-virtually any of the various types of cells found in the human body. For this reason, there are more than 100 different and specific types of cancer, each one unique in its metabolic behavior and hence its response to drug therapies.
Most cancers are classified into one of three groups: carcinomas, sarcomas, and leukemias or lymphomas. Carcinomas, which are cancerous epithelial cells, account for approximately 90% of all human cancers. Sarcomas, the solid tumors of connective tissues such as bone, muscle, and cartilage, are somewhat rare in humans. Leukemias and lymphomas, while forms of cancer, do not usually produce a tumorlike solid mass but instead arise from blood-forming cells or cells of the immune system. These cancer cells invade the blood and blood-forming organs, including the spleen, lymphatic system, and bone marrow and account for approximately 8% of human cancers.
Carcinogens and cancer development
A fundamental feature of cancer is the development of tumors from abnormally proliferating single cells. This clonality allows a cell containing a single cancer-initiating mutation to replicate itself while retaining the mutation. Cancer development is a multistep process in which a variety of mutations lead to the natural selection of cells with progressively increasing capacities for proliferation, survival, invasion, and metastasis. …