Since development of tumor is possible in any region or organ of the body with dividing cell, tumors were broadly classified into 3 basic types based upon the embryonic origin of the tissues involved in the formation of tumor.
1. Carcinomas are the tumors developed from cells of ectodermal or endodermal origin. They invade surrounding tissues and organs and may metastasize, or spread, to lymph nodes and other sites. Based upon the histopathological appearance they can be differentiated into “Adenocarcinoma” and “squamous cell carcinoma” to reflect the involvement of glandular or squamous cells in tumor formation. Approximately 90%of human cancers are malignancies of epithelial cells.
2. Sarcomas (from the Greek ‘sarx’ meaning “flesh”) are the tumors developed from connective tissue or mesoderm. Canaers of breast, colon, pancreas bones etc. come under this type. Sarcomas are rare in humans; about 2% of human cancers come under this category.
3. Leukemia (Greek leukos “white”; aima “blood”) is the tumour developed from bone marrow tissue or in blood. It is characterized by an abnormal proliferation of blood cells, usually white blood cells (leukocytes). Leukemia tumor is subdivided into lymphoblastic or lymphocytic leukemias and myeloid or myelogenous leukemias according to the blood cells affected.
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To make it understandable to a common man, tumors were generally named after the organ or tissue from which it has been developed. For example, cancer developed in breast tissue is referred as breast cancer. Lung cancer, colon cancer, gastric cancer, skin cancer etc. are some more examples.