In multicellular organisms, the cell divisions occur under the control of strict control of internal and external signals. However, cancer involves an uncontrolled growth of cells. The branch of medicine that deals with the study of cancer and tumors is known as oncology. A tumor is an abnormal mass of cells that may be benign or malignant. Benign tumors do not spread to other parts of the body while, malignant tumors can spread from the original site known as the primary site to other organs establishing secondary tumors.
According to their tissue of origin, cancers are classified as carcinoma that is derived from epithelial tissue while sarcoma is derived from the tissue of mesodermal origin. Leukemia is the tumor of the bone marrow while, lymphoma is the tumor which arises from the lymphoid tissue.
In 2002, there were 276700 new cases of malignant cancer registered in England. Individual cancers are not evenly distributed among the two sexes. Examples are the cervical cancer in women and prostate cancer in men. The incidence of cancers increases with age due to a number of factors like prolonged exposure to carcinogenic agents and an increase in the rate of mutations.
Molecular biology of cancers
It was suggested that the cancer was caused by mutations in the DNA of cells. It was proposed that the cancer develops by a multistep theory. The mutated forms of normal genes i.e., proto-oncogenes are known as oncogenes. They encode for factors like growth factors and receptors such as MYS and the RAS family of oncogenes. This causes increased proliferation of abnormal cells leading to cancer. In contrast to proto-oncogenes, tumor suppressor genes encode for proteins that inhibit the proliferation of cells that contain deleterious mutations. Examples include the TP53 gene and the Wilms’ tumor gene, WT1.
Causes of cancer
A number of factors are associated with the cancer caused due to mutations. Inherited cancers may be direct or indirect. Examples of cancer associated genes with a direct effect include the genes BRCA1 and BRCA2 that are associated with a greater risk of developing breast cancer. Some cancers are also associated with chromosomal abnormalities. An example is chronic myelogenous leukemia (CML) where the patients have Philadelphia chromosome that is formed due to translocations in chromosome 9 and 22. Chromosomal abnormalities are also seen in breast cancer and prostate cancer. Chemical carcinogens are another reason for cancer. This act in three steps namely: tumor initiation, progression and promotion. Proto-oncogenes and tumor suppressor genes are the targets for chemical carcinogens. The potential carcinogens can be detected by a number of tests. An example is the Ames test that uses a mutant of Salmonella typhimurium that cannot grow on the media in the absence of histidine. Diet also plays an important role in causing cancer. Obesity has been associated with a number of cancers like those of GIT, breast, uterus and prostate. Irradiation may also cause cancer and may induce gene amplification and increase chromosomal instability. Xeroderma pigmentosum is an example of radiation induced carcinogenesis. Viruses are also associated with cancers. Examples are hepatitis B and C virus that cause hepatocellular cancer and human papilloma viruses that cause cervical cancer.
General diagnosis of cancer
A number of clinical tests are available to detect the presence of cancer in an individual showing the symptoms. These tests can be divided into different categories like those that detect and estimate the tumor associated molecules in the blood. The detection and quantification of ‘tumor associated antigens’ is done by using monoclonal antibodies. However, these tests only give an indication of the cancer. Another category is to detect and localize the tumors within the body. This involves the use of X rays, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound and mammography in case of breast cancer. Histological examination of biopsies is another option to determine the nature of the tumor. These samples may be obtained by endoscopy or during surgical operations. The histological diagnosis of cancerous cells can be supported by molecular diagnostic techniques and may act as a prognostic test. Fluorescence in situ hybridization (FISH) is also used in the diagnosis.
General treatment of cancer
‘Staging’ of the cancer is important to determine the treatment the patient receives. Chemotherapy involves the use of cytotoxic drugs. Many drugs like folic acid antagonists and alkylating agents target cell division. However, chemotherapy may cause anemia, nausea, hair loss. Chemotherapeutic drugs are administered intravenously by infusion or intrathecally. Radiation therapy involves the use of high energy electromagnetic waves. The common side effects include fatigue, nausea and external burns to the skin. Immunotherapy involves different processes that manipulate the immune system and thus improve the defense of the body against the tumor. Examples include interferon α to treat multiple myeloma, CML and malignant melanoma, Interleukin 2 (IL-2) to treat renal cancer and malignant melanoma and BiovaxIDTM vaccine for treating follicular lymphoma.
Specific types of cancer
It is important to decide the type of cancer to determine the treatment to be administered. Lung cancer is a fatal disease and is caused mainly due to smoking and passive smoking. Symptoms include persistent and nagging cough, shortness of breath, chest pain and an unexplained loss of weight. Treatment involves removing the tumor by surgery, radiotherapy or chemotherapy using drugs like paclitaxel depending on the stage of the cancer. The second leading cause of cancer deaths in women is the breast cancer. The symptoms are a new painless lump occurring in the breast, a discharge from the nipple and an eczematous rash around the nipple. Screening is done by mammography or by Color Doppler ultrasound. It is treated by removing the tumor.
Prostate cancer mainly affects men over 50 years of age. Its symptoms are difficulty and pain on passing urine, frequent need to urinate and with a possibility of blood in the urine. There is a range of treatments possible like prostatectomy, radiation therapy and hormonal therapy. Leukemia is the tumor of the bone marrow. The characteristic symptoms include excessive bleeding of the gums and nose, bruising, fatigue and breathlessness. Treatment involves chemotherapy, radiation therapy and stem cell transplantation.