What are carcinomas?

Related conditions:
Dysplasia, metaplasia

Definition:
Carcinoma refers to any type of cancer that arises from epithelial cells. Carcinomas are distinguished from cancers of connective tissue (sarcomas), from cancers of blood-forming cells (leukemias and lymphomas), and from germ-cell tumors (teratomas). Carcinomas arising from stratified squamous epithelium are termed squamous cell carcinomas; those arising from glandular epithelium are termed adenocarcinomas.

Risk factors: Risk factors fall into two categories: environmental and hereditary. Environmental risk factors include exposure to carcinogenic chemicals or ionizing radiation, or infection with oncogenic viruses. Thousands of carcinogenic chemicals are known, and many are subject to strict regulations regarding exposure. Exposure to ionizing radiation, the most important source of which is the ultraviolet portion of sunlight, is another risk factor. Other important risk factors are inborn genetic variations that predispose individuals to particular cancers and immune system dysfunction.

Etiology and the disease process: Epithelial cells are more prone to malignant transformation than other types of cells because they are routinely exposed to the outside environment. Therefore, carcinoma is far more prevalent than sarcoma, leukemia, or other forms of cancer. The etiology of carcinoma hinges on cellular alterations that confer several key traits on the cell: unregulated growth, stimulation of new blood vessel formation, tissue invasion, and metastatic potential. Infection by oncogenic viruses causes carcinoma by introduction of foreign genes into the cell; these oncogenic genes (oncogenes) can confer the attributes of malignancy to the infected cells. Gene disruption can cause carcinoma via abnormal activation of oncogenes or, alternatively, by abnormal inactivation of tumor-suppressor genes.

Incidence: Carcinoma is by far the most prevalent form of cancer, accounting for more than 90 percent of new cancer cases and cancer deaths. In 2005, there were approximately 1.2 million new cases and 500,000 deaths from carcinoma in the United States.

Symptoms: Symptoms of early carcinoma vary widely according to the epithelial cells affected and their growth pattern. Advanced or terminal carcinoma symptoms include cachexia, pain, fatigue, and depression.

Screening and diagnosis: Carcinoma screening consists of careful inspection of epithelial cells at risk as well as blood tests for circulating tumor markers. Inspection for skin cancer may be as straightforward as looking for suspicious changes on the skin, whereas screening for cervical cancer involves scraping or brushing cells from the uterine cervix followed by staining and microscopic examination (a Pap smear). Blood tests can also be used to screen for carcinoma based on shedding of tumor-specific molecules into the circulation. A blood test for prostate-specific antigen (PSA) is used to screen men at risk for prostate cancer; those with high levels are then candidates for diagnostic biopsy. Diagnosis of carcinoma is usually done by pathologists based on microscopic examination of biopsied tissue. Features of carcinoma include cellular traits such as abnormally large nuclei, loss of multicellular organization, and disruption of the basement membrane. Staging of carcinoma is based on searches for malignant cells in adjacent structures and lymph nodes. Carcinoma staging uses the International Union Against Cancer (UICC) and American Joint Committee on Cancer (AJCC) system that describes the tumor itself, lymph node involvement, and metastatic lesions; however, other staging systems are also used, such as Dukes’ classification of colon cancer.

Treatment and therapy: Treatment options for carcinoma include surgery, chemotherapy, radiotherapy, immunotherapy, monoclonal antibody therapy, and other, less common approaches. Treatment plans are made based on the disease grade and stage and the functional status of the patient. Surgical therapy (mechanical removal of the tumor) reliably reduces the tumor burden and offers complete resection as the best outcome. Examples include surgical removal of the breast (mastectomy) or prostate (prostatectomy). chemotherapy (systemic administration of anticarcinoma drugs) can damage tumor cells anywhere in the body, usually by interfering with deoxyribonucleic acid (DNA) replication. Many chemotherapy regimens harm healthy tissue as a dose-limiting toxicity. Radiotherapy is the use of high-energy electromagnetic radiation (X rays or gamma rays) to damage or kill cancer cells; radiation can be delivered from the outside or by implantation of radioactive pellets (brachytherapy). Immunotherapy refers to attempts to attack carcinoma cells with the patient’s own immune cells. Monoclonal antibody therapy depends on knowledge of signal transduction pathways and uses antibodies to block or disrupt the malignant cells’ growth. An example includes the use of monoclonal antibodies against human epidermal growth factor receptor 2 (HER2) such as trastuzumab to interfere with signals generated by the HER2 receptor in breast cancer.

Prognosis, prevention, and outcomes: Although some types of carcinoma are quite deadly, many forms are indolent and manageable. Prognosis depends mostly on the type of carcinoma, the histologic grade, and the anatomic stage. Prevention efforts can reduce the incidence of carcinomas within populations and take the form of avoidance of or reduction of exposure to carcinogens. Examples include smoking-cessation campaigns to prevent some cases of lung cancer and promotion of sunscreen use to prevent some cases of melanoma. Individuals may further reduce their risk of cancer by losing weight and being physically active. Outcomes continue to improve because of the integration of treatments and supportive care.

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