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What is Cancer?

Cancer /ˈkænsə(r)/ ( listen) (medical term: malignant neoplasm) is a class of diseases in which a group of cells display uncontrolled growth (division beyond the normal limits), invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood). These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, and do not invade or metastasize. Most cancers form a tumor but some, likeleukemia, do not. The branch of medicine concerned with the study, diagnosis, treatment, and prevention of cancer is oncology.

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Cancer affects people at all ages with the risk for most types increasing with age.[1] Cancer caused about 13% of all human deaths in 2007[2] (7.6 million).[3]

Brain Cancer Cell

Cancers are caused by abnormalities in the genetic material of the transformed cells.[4] These abnormalities may be due to the effects of carcinogens, such as tobacco smokeradiationchemicals, or infectious agents. Other cancer-promoting genetic abnormalities may randomly occur through errors in DNA replication, or are inherited, and thus present in all cells from birth. The heritability of cancers is usually affected by complex interactions between carcinogens and the host’s genome.

Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are typically activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Tumor suppressor genes are then inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system.

Definitive diagnosis requires the histologic examination of a biopsy specimen, although the initial indication of malignancy can be symptomatic or radiographic imaging abnormalities. Most cancers can be treated and some cured, depending on the specific type, location, and stage. Once diagnosed, cancer is usually treated with a combination of surgerychemotherapy and radiotherapy. As research develops, treatments are becoming more specific for different varieties of cancer. There has been significant progress in the development of targeted therapy drugs that act specifically on detectable molecular abnormalities in certain tumors, and which minimize damage to normal cells. The prognosis of cancer patients is most influenced by the type of cancer, as well as the stage, or extent of the disease. In addition, histologic grading and the presence of specific molecular markers can also be useful in establishing prognosis, as well as in determining individual treatments.

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Classification

Cancers are classified by the type of cell that resembles the tumor and, therefore, the tissue presumed to be the origin of the tumor. These are the histology and the location, respectively. Examples of general categories include:

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How Cancer develop???

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Malignant tumors (cancers) are usually named using -carcinoma-sarcoma or -blastoma as a suffix, with the Latin or Greek word for the organ of origin as the root. For instance, a cancer of the liver is calledhepatocarcinoma; a cancer of the fat cells is called liposarcoma. For common cancers, the English organ name is used. For instance, the most common type of breast cancer is called ductal carcinoma of the breastor mammary ductal carcinoma. Here, the adjective ductal refers to the appearance of the cancer under the microscope, resembling normal breast ducts.

Benign tumors (which are not cancers) are named using -oma as a suffix with the organ name as the root. For instance, a benign tumor of the smooth muscle of the uterus is called leiomyoma (the common name of this frequent tumor is fibroid). Unfortunately, some cancers also use the -oma suffix, examples being melanoma and seminoma.

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Signs and symptoms

Roughly, cancer symptoms can be divided into three groups:

Every symptom in the above list can be caused by a variety of conditions (a list of which is referred to as the differential diagnosis). Cancer may be a common or uncommon cause of each item.

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Causes

Active Cancer Cell inside the body!

Cancer is a diverse class of diseases which differ widely in their causes and biology. Any organism, even plants, can acquire cancer. Nearly all known cancers arise gradually, as errors build up in the cancer cell and its progeny (see mechanisms section for common types of errors).

Anything which replicates (our cells) will probabilistically suffer from errors (mutations). Unless error correction and prevention is properly carried out, the errors will survive, and might be passed along to daughter cells. Normally, the body safeguards against cancer via numerous methods, such as: apoptosis, helper molecules (some DNA polymerases), possibly senescence, etc. However these error-correction methods often fail in small ways, especially in environments that make errors more likely to arise and propagate. For example, such environments can include the presence of disruptive substances called carcinogens, or periodic injury (physical, heat, etc.), or environments that cells did not evolve to withstand, such as hypoxia[5] (see subsections). Cancer is thus aprogressive disease, and these progressive errors slowly accumulate until a cell begins to act contrary to its function in the animal.

The errors which cause cancer are often self-amplifying, eventually compounding at an exponential rate. For example:

  • A mutation in the error-correcting machinery of a cell might cause that cell and its children to accumulate errors more rapidly
  • A mutation in signaling (endocrine) machinery of the cell can send error-causing signals to nearby cells
  • A mutation might cause cells to become neoplastic, causing them to migrate and disrupt more healthy cells
  • A mutation may cause the cell to become immortal (see telomeres), causing them to disrupt healthy cells forever

Cancer Cell being attached by the immune system!

Thus cancer often explodes in something akin to a chain reaction caused by a few errors, which compound into more severe errors. Errors which produce more errors are effectively the root cause of cancer, and also the reason that cancer is so hard to treat: even if there were 10,000,000,000 cancerous cells and one killed all but 10 of those cells, those cells (and other error-prone precancerous cells) could still self-replicate or send error-causing signals to other cells, starting the process over again. This rebellion-like scenario is an undesirable survival of the fittest, where the driving forces of evolution itself work against the body’s design and enforcement of order. In fact, once cancer has begun to develop, this same force continues to drive the progression of cancer towards more invasive stages, and is called clonal evolution.[6]

Research about cancer causes often falls into the following categories:

  • Agents (e.g. viruses) and events (e.g. mutations) which cause or facilitate genetic changes in cells destined to become cancer.
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  • The precise nature of the genetic damage, and the genes which are affected by it.
  • The consequences of those genetic changes on the biology of the cell, both in generating the defining properties of a cancer cell, and in facilitating additional genetic events which lead to further progression of the cancer.

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Pathophysiology

Cancer is fundamentally a disease of regulation of tissue growth. In order for a normal cell to transform into a cancer cell, genes which regulate cell growth and differentiation must be altered.[29] Genetic changes can occur at many levels, from gain or loss of entire chromosomes to a mutation affecting a single DNA nucleotide. There are two broad categories of genes which are affected by these changes. Oncogenes may be normal genes which are expressed at inappropriately high levels, or altered genes which have novel properties. In either case, expression of these genes promotes the malignant phenotype of cancer cells. Tumor suppressor genes are genes which inhibit cell division, survival, or other properties of cancer cells. Tumor suppressor genes are often disabled by cancer-promoting genetic changes. Typically, changes in many genes are required to transform a normal cell into a cancer cell.[30]

There is a diverse classification scheme for the various genomic changes which may contribute to the generation of cancer cells. Most of these changes are mutations, or changes in the nucleotide sequence of genomic DNA. Aneuploidy, the presence of an abnormal number of chromosomes, is one genomic change which is not a mutation, and may involve either gain or loss of one or more chromosomes through errors in mitosis.

Large-scale mutations involve the deletion or gain of a portion of a chromosome. Genomic amplification occurs when a cell gains many copies (often 20 or more) of a small chromosomal locus, usually containing one or more oncogenes and adjacent genetic material. Translocation occurs when two separate chromosomal regions become abnormally fused, often at a characteristic location. A well-known example of this is the Philadelphia chromosome, or translocation of chromosomes 9 and 22, which occurs in chronic myelogenous leukemia, and results in production of the BCRabl fusion protein, an oncogenic tyrosine kinase.

Small-scale mutations include point mutations, deletions, and insertions, which may occur in the promoter of a gene and affect its expression, or may occur in the gene’s coding sequence and alter the function or stability of its protein product. Disruption of a single gene may also result from integration of genomic material from a DNA virus or retrovirus, and such an event may also result in the expression of viral oncogenes in the affected cell and its descendants.

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Glossary

The following closely related terms may be used to designate abnormal growths:

  • Tumor or tumour: originally, it meant any abnormal swelling, lump or mass. In current English, however, the word tumor has become synonymous with neoplasm, specifically solid neoplasm. Note that some neoplasms, such as leukemia, do not form tumors.
  • Neoplasm: the scientific term to describe an abnormal proliferation of genetically altered cells. Neoplasms can be benign or malignant:
    • Malignant neoplasm or malignant tumor: synonymous with cancer.
    • Benign neoplasm or benign tumor: a tumor (solid neoplasm) that stops growing by itself, does not invade other tissues and does not form metastases.
  • Invasive tumor is another synonym of cancer. The name refers to invasion of surrounding tissues.
  • Pre-malignancypre-cancer or non-invasive tumor: A neoplasm that is not invasive but has the potential to progress to cancer (become invasive) if left untreated. These lesions are, in order of increasing potential for cancer, atypiadysplasia and carcinoma in situ.

The following terms can be used to describe a cancer:

  • Screening: a test done on healthy people to detect tumors before they become apparent. A mammogram is a screening test.
  • Diagnosis: the confirmation of the cancerous nature of a lump. This usually requires a biopsy or removal of the tumor by surgery, followed by examination by a pathologist.
  • Surgical excision: the removal of a tumor by a surgeon.
    • Surgical margins: the evaluation by a pathologist of the edges of the tissue removed by the surgeon to determine if the tumor was removed completely (“negative margins”) or if tumor was left behind (“positive margins”).
  • Grade: a number (usually on a scale of 3) established by a pathologist to describe the degree of resemblance of the tumor to the surrounding benign tissue.
  • Stage: a number (usually on a scale of 4) established by the oncologist to describe the degree of invasion of the body by the tumor.
  • Recurrence: new tumors that appear at the site of the original tumor after surgery.
  • Metastasis: new tumors that appear far from the original tumor.
  • Median survival time: a period of time, often measured in months or years, over which 50% of the cancer patients are expected to be alive.[93]
  • Transformation: the concept that a low-grade tumor transforms to a high-grade tumor over time. Example: Richter’s transformation.
  • Chemotherapy: treatment with drugs.
  • Radiation therapy: treatment with radiations.
  • Adjuvant therapy: treatment, either chemotherapy or radiation therapy, given after surgery to kill the remaining cancer cells.
  • Prognosis: the probability of cure after the therapy. It is usually expressed as a probability of survival five years after diagnosis. Alternatively, it can be expressed as the number of years when 50% of the patients are still alive. Both numbers are derived from statistics accumulated with hundreds of similar patients to give a Kaplan-Meier curve.
  • Cure: A cancer patient is “cured” if they live past the time by which 95% of treated patients live after the date of their diagnosis of cancer. This period varies among different types of cancer; for example, in the case of Hodgkin’s disease this period of time is 10 years, whereas for Burkitt’s lymphoma this period would be 1 year.[94] The phrase “cure” used in oncology is based upon the statistical concept of a median survival time and disease-free median survival time.[95]

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Notes

  1. ^ Cancer Research UK (January 2007). “UK cancer incidence statistics by age”. Retrieved 2007-06-25.
  2. ^ WHO (February 2006). “Cancer”. World Health Organization. Retrieved 2007-06-25.
  3. ^ American Cancer Society (December 2007). “Report sees 7.6 million global 2007 cancer deaths”. Reuters. Retrieved 2008-08-07.
  4. ^ Kinzler, Kenneth W.; Vogelstein, Bert (2002). “Introduction”The genetic basis of human cancer (2nd, illustrated, revised ed.). New York: McGraw-Hill, Medical Pub. Division. p. 5. ISBN 978-0-07-137050-9.
  5. ^ Nelson DA, Tan TT, Rabson AB, Anderson D, Degenhardt K, White E (September 2004). “Hypoxia and defective apoptosis drive genomic instability and tumorigenesis”Genes & Development 18 (17): 2095–107. doi:10.1101/gad.1204904PMID15314031.
  6. ^ Merlo LM, Pepper JW, Reid BJ, Maley CC (December 2006). “Cancer as an evolutionary and ecological process”. Nat. Rev. Cancer 6 (12): 924–35. doi:10.1038/nrc2013PMID 17109012.
  7. a b Sasco AJ, Secretan MB, Straif K (August 2004). “Tobacco smoking and cancer: a brief review of recent epidemiological evidence”. Lung cancer (Amsterdam, Netherlands) 45 Suppl 2: S3–9. doi:10.1016/j.lungcan.2004.07.998PMID15552776.
  8. ^ Biesalski HK, Bueno de Mesquita B, Chesson A, et al. (1998). “European Consensus Statement on Lung Cancer: risk factors and prevention. Lung Cancer Panel”CA: a cancer journal for clinicians 48 (3): 167–76; discussion 164–6.doi:10.3322/canjclin.48.3.167PMID 9594919.
  9. ^ O’Reilly KM, Mclaughlin AM, Beckett WS, Sime PJ (March 2007). “Asbestos-related lung disease”American family physician 75 (5): 683–8. PMID 17375514.
  10. ^ Seitz HK, Pöschl G, Simanowski UA (1998). “Alcohol and cancer”. Recent developments in alcoholism : an official publication of the American Medical Society on Alcoholism, the Research Society on Alcoholism, and the National Council on Alcoholism 14: 67–95. PMID 9751943.

January 27, 2010 - Posted by | Blood, Cells, Liver, Lungs

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