What is Cancer?
Normal cells within the human body receive signals from the cells surrounding them and if these signals are not received the cell is programmed to die (this programmed cell death is known as apoptosis). However, cancerous cells are lacking this program and as such they are classed as immortal in that, given enough food the cells will keep on dividing and growing.
Indeed, cancer can be classed as a group of disorders that are characterized by uncontrolled division of cells. In many cases of cancer these cells are also able to spread either by directly growing to invade adjacent tissues or by fragments of a tumour fragmenting from the main body to be troasported through the blood stream or lymphatic system to distant sites within the body (a process known as metastasis).
In effect cancer is a disorder of DNA repair and this is why cancer risk increases with age. Most cancer types are characterized by damage to DNA that results in mutations of genes which control cell division and cell death. In most cases mutations to several of these control genes are needed and this is one of the reasons why cancer incidence increases with age. However, leukaemias are more commonly characterized by larg-scale changes to chromosomes such as when a part of one chromosome is incorporated into another. In some instances (such as cervical cancer) the cancer can be induced by a virus that inserts its DNA into the human genome. Some cancers can be inherited also and a good example is early-onset breast cancer which can often be traced to inherited mutations in the BRCA1 and/or BRCA2 genes.
Classification of Cancers
Neoplasm of neoplasia is the scientific designation for all cancerous disorders. As each type of cancer is, effectively, a disease in its own right a neoplasm is the top level of the cancer hierarchy and neoplasms can either be benign or malignant.
- Cancer — this is the most widely-used term for neoplasms and it is generally understood to be synonymous with a malignant neoplasm. However, the term cancer is occasionally used instead of carcinoma (defined below).
- Tumour, when used correctly, is the medical term for any kind of swelling or lump. However, in common parlance the term tumour tends to be used synonymously with 'neoplasm'. Though this is inaccurate as many neoplasms, such as leukaemias, are not associated with a lump or a swelling.
- Paraneoplasia is a disturbance in the patient associated with a neoplasm, but which is not related to the invasion of the cancerous cells themselves. Such disturbances can be hormonal, neurological, haemotological, digesitve, biochemical or metabolic.
Classification of Cancer Types
As all neoplastic cells are derived from the cells or tissues that gave rise to them the neoplasms themselves tend to display the characteristics of the originating cells (and this is why cancer is a collection of diseases and not a single disease). Because of this cancers tend to be classified with respect to the type of cell presumed to be the origin of the neoplasm. The following are the main classes accepted today:
- Carcinoma. This is by far the commonest type of neoplasm. Carcinomas are malignant neoplasms derived from epithelial cells; those cells of the skin, the breast and the lining of the gut. This type of cancer includes breast cancer, prostate cancer, lung cancer, colon cancer and skin cancer.
- Leukaemias and Lymphomas are cancers derived from blood and bone marrow cells.
- Sarcomas are cancers derived from connective tissue (tissues involved in structure and support) or mesenchymal (embryonic tissues that later develop into blood vessels and related organs).
- Mesotheliomas are cancers derived from mesothelial (lining of body cavities) of the peritoneum and pleura.
- Gliomas are cancers derived from glia, the most common nerve types in the brain.
- Germinoma are cancers derived from germ cells and are normally testicular or ovarian cancers.
- Choriocarcinomas are cancers derived from the placenta.
A cancer is classed based on the organ in which the cancer is found and the class of the cancer. So, liver cancers are hepatocarcinomas.
How Cancers Develop
In cancer, the ordered and healthy process of normal cell replacement and proliferation which occurs in almost all tissues of the body is disturbed. In healthy tissues this process is very highly regulated so that a balance is maintained between cell proliferation and apoptosis (programmed cell death). However, the mutations in the DNA of healthy cells that lead to cancer disrupt these processes.
Malignant neoplastic cells tend to display the following abnormal properties:
- They evade programmed cell death (apoptosis)
- Cancer cells are termed immortal in that they have unlimited growth potential.
- They generate their own growth factors and are not dependent on the surrounding cells.
- They cannot be induced to commit suicide or stop gorowing by surrounding cells.
- The rate of division is significantly increased compared with normal cells.
- They show an altered ability to acquire a new cellular type (differentiate)
- They can spread to and invade neighbouring tissues
- They can spread to (metastasize) distant sites.
- They can spontaneously induce the development of blood vessels (angiogenesis).
- They are immune to contact inhibition (the process where cell division and motility is inhibited when two or more cells come into contact).
Usually the characteristics described above are not acquired all at once. Indeed, the characteristics are generally acquired as the tumour develops. This process is termed clonal evolution.
Indeed, the initial genetic mutation is just the beginning of the process by which cancer develops. It now seems increasingly likely that you need a number of changes within your cells in order to develop cancer, including:
- An initiator to cause a genetic mutation. Sometimes you're born with this initial genetic mutation. Other times a genetic mutation is caused by forces within your body, such as hormones, viruses and chronic inflammation. Genetic mutations can also be caused by forces outside of your body, such as ultraviolet (UV) light from the sun or cancer-causing chemicals (carcinogens) in your environment.
- A promoter to cause rapid cell growth. Promoters take advantage of genetic mutations created by initiators. Promotors cause cells to divide rapidly. This could lead to a tumor. Promoters could be inherited, could come from inside your body or could come from outside your body.
- A progressor to cause cancer to become aggressive and spread. Without a progressor a tumor may remain benign and localized. Progressors make cancers more aggressive and more likely to spread. Like initiators and promoters, progressors could be inherited or they could come from environmental sources.
Your personal genetic makeup and inheritance, forces within your body, your lifestyle choices and your environment can all set the stage for cancer or help complete the process once it's started. For instance, if you've inherited a genetic mutation that predisposes you to cancer, you may be more likely than other people to develop cancer when exposed to a certain cancer-causing substance. The genetic mutation begins the cancer process, and the cancer-causing substance could play a role in further cancer development. Likewise, smokers who work with asbestos are more likely to develop lung cancer than smokers who don't work with asbestos because the two carcinogens both play roles in cancer development.
What Cancer Looks Like
Cancerous tissue also looks different from normal tissue at a histopathological level. Therefore when you look at a tissue sample under a microscope it is possible to distinguish between the normal and the cancerous tissue. The sketch shown here indicates the typical differences seen where the cancerous tissue shows more cell proliferation, larger nuclei to the cells and a more disorganized structure.
The second image shows the differences between normal (left) and cancerous (right) cells in the colon.