Cancer classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Classification

Nomenclature

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

  • Tumor: 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-malignancy, pre-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, atypia, dysplasia 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 a the site of the original tumor after surgery.
  • Metastasis: new tumors that appear far from the original tumor.
  • 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.

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

  • Carcinoma: Malignant tumors derived from epithelial cells. This group represents the most common cancers, including the common forms of breast, prostate, lung and colon cancer.
  • Sarcoma: Malignant tumors derived from connective tissue, or mesenchymal cells.
  • Lymphoma and leukemia: Malignancies derived from hematopoietic (blood-forming) cells
  • Germ cell tumor: Tumors derived from totipotent cells. In adults most often found in the testicle and ovary; in fetuses, babies, and young children most often found on the body midline, particularly at the tip of the tailbone; in horses most often found at the poll (base of the skull).
  • Blastic tumor: A tumor (usually malignant) which resembles an immature or embryonic tissue. Many of these tumors are most common in children.

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 called hepatocarcinoma; 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 breast or mammary ductal carcinoma. Here, the adjective ductal refers to the appearance of the cancer under the microscope, resembling normal breast ducts.

Benign tumors 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.

Adult Cancers

In the U.S. and other developed countries, cancer is presently responsible for about 25% of all deaths.[1] On a yearly basis, 0.5% of the population is diagnosed with cancer. The statistics below are for adults in the United States, and may vary substantially in other countries:

Male Female
most common (by occurrence) most common (by mortality) [1] most common (by occurrence) most common (by mortality) [1]
prostate cancer (33%) lung cancer (31%) breast cancer (32%) lung cancer (27%)
lung cancer (13%) prostate cancer (10%) lung cancer (12%) breast cancer (15%)
colorectal cancer (10%) colorectal cancer (10%) colorectal cancer (11%) colorectal cancer (10%)
bladder cancer (7%) pancreatic cancer (5%) endometrial cancer (6%) ovarian cancer (6%)
cutaneous melanoma (5%) leukemia (4%) non-Hodgkin lymphoma (4%) pancreatic cancer (6%)

Childhood Cancers

Cancer can also occur in young children and adolescents, but it is rare (about 150 cases per million yearly in the US). Statistics from the SEER program of the US NCI demonstrate that childhood cancers increased 19% between 1975 and 1990, mainly due to an increased incidence in acute leukemia. Since 1990, incidence rates have decreased [2]

The age of peak incidence of cancer in children occurs during the first year of life. Leukemia (usually ALL) is the most common infant malignancy (30%), followed by the central nervous system cancers and neuroblastoma. The remainder consists of Wilms' tumor, lymphomas, rhabdomyosarcoma (arising from muscle), retinoblastoma, osteosarcoma and Ewing's sarcoma.[1] Teratoma is the most common tumor in this age group, but most teratomas are surgically removed while still benign, hence not necessarily cancer.

Female and male infants have essentially the same overall cancer incidence rates, but white infants have substantially higher cancer rates than black infants for most cancer types. Relative survival for infants is very good for neuroblastoma, Wilms' tumor and retinoblastoma, and fairly good (80%) for leukemia, but not for most other types of cancer.

References

  1. 1.0 1.1 1.2 1.3 Jemal A, Murray T, Ward E, Samuels A, Tiwari RC, Ghafoor A, Feuer EJ, Thun MJ (2005). "Cancer statistics, 2005". CA Cancer J Clin. 55 (1): 10–30. PMID 15661684.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. Cancer Incidence and Survival among Children and Adolescents, United States SEER program 1975-1995, available online from the SEER web site


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