Cancer
A malignant tumor, cancer comprises a broad spectrum of malignant neoplasms classified as either carcinomas or sarcomas. Carcinomas originate in the epithelial tissues, while sarcomas originate from connective tissues and structures that have their origin in mesodermal tissue. Cancer is an invasive disease that spreads to various parts of the body. It spreads directly to those tissues immediately surrounding the primary site of the cancer and may spread to remote parts of the body through the lymphatic and circulatory systems.
Cancer occurs in most, if not all, multicellular animals. Evidence from fossil records reveal bone cancer in dinosaurs, and sarcomas have been found in the bones of Egyptian mummies. Hippocrates is credited with coining the term carcinoma, the Greek word for crab. Why the word for crab was chosen enjoys much speculation, but may have had to do with the sharp, biting pain and invasive, spreading nature of the disease.
A carcinogen is any substance or agent that produces or induces the development of cancer. Carcinogens are known to affect and initiate metabolic processes at the level of DNA (the information-storing molecules in cells). DNA damage (mutation)is the development of cancer after exposure to a carcinogen. This kind of mutation is actually reversible; our bodies continually experience DNA damage, which is continually being corrected. It is only when promoter cells intervene during cell proliferation that tumors begin to develop. Although several agents can induce cell division, only promoters induce tumor development.
An example of this process would be what happens in an epidermal cell, when its DNA undergoes rapid, irreversible alteration or mutation after exposure to a carcinogen. The cell undergoes proliferation, producing altered progeny, and it is at this point that the cell may proceed on one of two pathways. The cell may undergo interrupted exposure to promoters and experience early reversible precancerous lesions. Or it may experience continuous exposure to the promoters, thereby causing malignant cell changes. During the late phase of promotion, the primary epidermal cell becomes tumorous and begins to invade normal cells; then it begins to spread. It is at this stage that tumors are identified as malignant.
The spread of tumors throughout the body is believed to be governed by several processes. One possible mechanism is direct invasion of contiguous organs. This mechanism is poorly understood, but it involves multiplication, mechanical pressure, release of lytic enzymes, and increased motility of individual tumor cells. A second process is metastasis. This is the spread of cancer cells from a primary site of origin to a distant site, and it is the life-threatening aspect of malignancy. At present there are many procedures available to surgeons for successfully eradicating primary tumors; however, the real challenge in reducing cancer mortality is finding ways to control metastasis.
Clinical manifestations of cancer take on many forms. Usually little or no pain is associated with the early stages of malignant disease, but pain does affect 60–80% of those terminally ill with cancer. General mechanisms causing pain associated with cancer include pressure, obstruction, invasion of a sensitive structure, stretching of visceral surfaces, tissue destruction, and inflammation. Abdominal pain is often caused by severe stretching from the tumor invasion of the hollow viscus, as well as tumors that obstruct and distend the bowel. Tumor compression of nerve endings against a firm surface also creates pain. Brain tumors have very little space to grow without compressing blood vessels and nerve endings between the tumor and the skull. Tissue destruction from infection and necrosis can also cause pain. Frequently infection occurs in the oral area, in which a common cause of pain is ulcerative lesions of the mouth and esophagus.
Cancer treatments involve chemotherapy, radiotherapy, surgery, immunotherapy, and combinations of these
| Frequency of Cancer-Related Death |
| Cancer Site | Number of Deaths Per Year |
| Lung | 160,100 |
| Colon and rectum | 56,500 |
| Breast | 43,900 |
| Prostate | 39,200 |
| Pancreas | 28,900 |
| Lymphoma | 26,300 |
| Leukemia | 21,600 |
| Brain | 17,400 |
| Stomach | 13,700 |
| Liver | 13,000 |
| Esophagus | 11,900 |
| Bladder | 12,500 |
| Kidney | 11,600 |
| Multiple myeloma | 11,300 |
modalities. Chemotherapy and its efficacy is related to how the drug enters the cell cycle; the design of the therapy is to destroy enough malignant cells so that the body's own immune system can destroy the remaining cells naturally. Smaller tumors with rapid growth rates seem to be most responsive to chemotherapy. Radiation therapy is commonly used to eradicate tumors without excessive damage to surrounding tissues. Radiation therapy attacks the malignant cell at the DNA level, disrupting its ability to reproduce. Surgery is the treatment of choice when it has been determined that the tumor is intact and has not metastasized beyond the limits of surgical excision. Surgery is also indicated for benign tumors that could progress into malignant tumors. Premalignant and in situ tumors of epithelial tissues, such as skin, mouth, and cervix, can be removed.
Chemotherapy and radiation treatments are the most commonly used therapies for cancer. Unfortunately, both methods produce unpleasant side effects; they often suppress the immune system, making it difficult for the body to destroy the remaining cancer even after the treatment has been successful. In this regard, immunotherapy holds great promise as an alternative treatment, because it makes use of the unique properties of the immune system.
Immunotherapies for the treatment of cancer are generally referred to as biological response modifiers (BRMs). BRMs are defined as mammalian gene products, agents, and clinical protocols that affect biologic responses in host-tumor interactions. Immunotherapies have a direct cytotoxic effect on cancer cells, initiation or augmentation of the host's tumor-immune rejection response, and modification of cancer cell susceptibility to the lytic or tumor static effects of the immune system. As with other cancer therapies immunotherapies are not without their own side effects. Most common are flu-like symptoms, skin rashes, and vascular-leak syndrome. At their worst, these symptoms are usually less severe than those of current chemotherapy and radiation treatments.
Hazardous Material; Hazardous Waste; Leukemia; Radiation Sickness
Resources
Books
Aldrich, T., and J. Griffith. Environmental Epidemiology. New York: Van Nostrad Reinhold, 1993.
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