Cancer is a class of diseases in whicha group of cells display uncontrolledgrowth (division beyond the normal limits), invasion (intrusion on anddestruction of adjacent tissues), and sometimes metastasis(spread to other locations in the body via lymph or blood). These threemalignant properties of cancers differentiate them from benigntumors, which are self-limited, and do not invade or metastasize. Mostcancers form a tumorbut some, like leukemia,do not.
Cancers are caused by abnormalities in the genetic material ofthe transformed cells. These abnormalities maybe due to the effects of carcinogens, such as tobaccosmoke, radiation, chemicals, orinfectiousagents.
Cancers are classified by the type of cellthat resembles the tumor and, therefore, the tissue presumed to be the originof the tumor. These are the histology and the location, respectively. Examplesof 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 or blastoma: A tumor (usually malignant) which resembles an immature or embryonic tissue. Many of these tumors are most common in children.
Signs and symptoms
Roughly, cancer symptoms can be divided into three groups:
- Local symptoms: unusual lumps or swelling (tumor), hemorrhage (bleeding), pain and/or ulceration. Compression of surrounding tissues may cause symptoms such as jaundice (yellowing the eyes and skin).
- Symptoms of metastasis (spreading): enlarged lymph nodes, cough and hemoptysis, hepatomegaly (enlarged liver), bone pain, fracture of affected bones and neurological symptoms. Although advanced cancer may cause pain, it is often not the first symptom.
- Systemic symptoms: weight loss, poor appetite, fatigue and cachexia (wasting), excessive sweating (night sweats), anemia and specific paraneoplastic phenomena, i.e. specific conditions that are due to an active cancer, such as thrombosis or hormonal changes.