3 July 2011
As a prelude to writing about more specific topics, I thought it would be worthwhile to answer the following question that has indirectly come up from several sources:
What types of cancer are there?
“Cancer” is actually a broad term used to describe a class of over 200 diseases with overlapping symptoms, mechanisms, epidemiology, and therapy. The specific type of cancer is named according to the tissue and cell type of origin.
Carcinomas are cancers of epithelial cells, like skin, lining of throat, gut, and ductal tissues. Example: breast carcinoma. Sarcomas are cancers of connective tissue such as bone, cartilage, fat tissues. Example: Osteosarcoma (bone). Blastoma is a type of cancer derived from developmental precursor cells in unborn children. Not surprisingly, these most commonly become apparent in children. An example is Neuroblastoma, which arises from the neural crest, an embryonic structure in the third trimester. Leukemia is cancer of the blood. (think: white blood cells) An example is Chronic Myleogenous Leukemia. Teratoma is a type of cancer from pluripotent cells in testes or ovaries. Example: Testicular cancer.
In humans, the most common cancers, by far, are carcinomas. Breast cancer, prostate cancer, lung cancer, colon cancer, and pancreatic cancers usually derive from the epithelial cells that make up the organ, making them carcinomas. Epithelial cells usually line the outside of an organ, or line the inside of a lumen (enclosed space, like the inside of the gut). Epithelial cancers are likely common because they are the first tissues exposed to external insults. For instance, skin cells are the first to intercept UV rays. Epithelial cells in the lungs and the cells lining the throat and trachea are the first to see a cigarette’s smoke, etc…
Another reason why carcinomas are the most common type of cancer in humans is that epithelial tissues are constantly being re-made and have a relatively high turnover rate. Example: your skin cells are always flaking off. (A side note: did you know that a lot of indoor dust is actually dried up, exfoliated human skin cells?) While this is in itself a defense mechanism against cancer, epithelial tissues are also in a perpetual state of growth. Already being friendly to growing, many propose this as being one step closer to a tumor than other tissues.
Cancers are named for their tissue of origin, as are metastases. A woman battling recurrent breast cancer in her lungs is not the same as having lung cancer, though the symptoms may be very similar. The architecture of the tumors is different, and most importantly, the type of therapy that would have the most effect on the cancer is usually defined by the tumor of origin, not necessarily the tissue it’s in.
There are theoretically as many different kinds of cancer as there are cell types in the body. And of those types of cancer, scientists are beginning to discover discrete genetic pathways that can lead to further classification of subtypes of the same cancer origin. For instance, there are several types of breast carcinoma that arise from the same cell type. Some breast cancers rely on amplification of an oncogene called HER-2, some rely on a steady supply of estrogen to continue growing, etc. While that may seem somewhat esoteric, that knowledge has lead to significant breakthroughs in the way women with breast cancer are treated, reduced symptoms, and allowed for much less invasive therapy.
-Ryon
Source: Weinberg, The Biology of Cancer, 4th Edition
(See the Cancer for Dummies main page for more topics)
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