Cancer is the name given to a cell that has lost its normal physiological or natural growth control and multiplications very quickly to produce a large new growth called tumor or neoplasm (swelling). This new growth or neoplasm could be confined to the local region or directly invade the surrounding tissues. It may also break out with tumor particles transported by blood to distant parts of the body. This is what determines the stage of the tumor.
Such cancers that invade neighboring tissues or spread to distant organs as said to be malignant. Those confined to the original tissue type are said to be benign. The level of disorganization of the tissue architecture also gives a sense of whether the tumor is benign or malignant. Benign cancers are not as dangerous as malignant cancers, although large ones depending on their locations could produce dangerous pressure effects as occurred with fibroids causing abortions.
Cancer can arrise from virtually any tissue of the body, in the presence of the right stimulus and genetic predisposition. The stimuli that initiate cancer could have radiations (x-ray, solar, etc), chemicals like carbon tetrachloride or asbestos or cigarette smoke, drugs used in early pregnancy, biochemicals like VEGF, etc. The stimuli generally alter the genetic code of the individual cells to disable the tumor suppressor genes that prevent uncontrolled cell growth or ensure programmed cell death (apoptosis)
Different tissues are sensitive to different stimuli, but those cells that naturally grow rapidly are most sensitive to cancer stimuli. Such tissues include the skin, testis, ovaries, breast, uterus, liver, spleen, gastro-intestinal tract, growing bones, blood cell, lungs, lymph nodes etc. The commonest cancers therefore involve the lungs (carcinoma), blood (leukemia), lymphoid tissues (lymphoma), bones (osteoma or steosarcoma), skin (carcinoma), liver (hepatoma), ovaries (cystadenoma or cystadenocarcinoma).
The destructive potential of a particular cancer depends on the type and location. Lung and breast cancers are known to be the two leading causes of death among women in the US. Colon cancer kills a lot of men and women. Leukemia kills a lot children in America by destroying their bone marrows and making them very anemic. Prostate cancer kills a lot men each year in the US. Sun exposure causes a lot of skin cancer in States like Florida. Tumors affecting blood vessels cause them to be fragile and bleed easily. This can be a huge problem in the eyes and brain.
The general approach to treatment of cancers is to detect them early when they are still very local and ablate them surgically, thermally (diathermy) or cryoscopically (freezing). Surgical biopsy is used to obtain some of the cancer tissue for laboratory examination and classification. Early stage cancers are generally cured by surgical excision.
Mid stage cancers, involve both wide surgical dissection, as in breast cancer, and radiation therapy to kill off residual cancer cells. Late stage cancer is mostly treated with chemotherapy and radiation. Leukemias, often involve the wiping out of patients' bone marrow cells and replacing them with donor cells. (See Part II)