This information is provided by the National Organization for Rare Disorders (NORD).
Malignant Melanoma is a common skin cancer that arises from the melanin cells within the upper layer of the skin (epidermis) or from similar cells that may be found in moles (nevi). This type of skin cancer may send down roots into deeper layers of the skin. Some of these microscopic roots may spread (metastasize) causing new tumor growths in vital organs of the body.
Mesothelioma is a rare form of cancer that affects the cells that make up the mesothelium. The mesothelium is the lining or membrane that covers and protects various internal organs of the body. The mesothelium is composed of two layers of specialized cells known as mesothelial cells. One layer directly surrounds the internal organs; the other forms a protective sac around thoracic and peritoneal organs. The most common form of mesothelioma affects the pleura, which is the membrane or sac that lines the lungs and chest cavity. Other common sites include the peritoneum, which is the membrane lining the abdominal cavity, and the pericardium, which is the membrane lining the heart (heart sac). Mesothelial tissue is also found in other areas of the body including the membrane covering the testicles (tunica vaginalis). The disorder most often occurs in older adults. Mesothelioma is often an aggressive form of cancer with a poor prognosis, with pleural mesothelioma patients having a median survival of only about 12-14 months with current therapies. Treatments are available, but are not effective for everyone.
Ocular melanoma is an extremely rare form of cancer that affects the eye with an incidence of 5 per million adults. Although rare, it is the most common primary cancer of the eye in adults. Primary means that the cancer began at that site (in this case the eye) and did not spread there from another part of the body. In most people, this cancer arises in a part of the eye known as the uveal tract. The uveal tract is the colored (pigmented) layer of tissue that is found beneath the white of the eye (sclera) and is composed of normally pigmented cells and blood vessels. In the front of the eye, the uvea is made up of the colored part of the eye (iris) and a circle of muscle tissue (ciliary body) that releases a transparent fluid (aqueous humor) into the eye and helps to control the shape of the lens. The largest area of the uveal tract is in the back part of the eye (choroid) which is located beneath the retina, the vision sensing portion of the eye. In most instances, ocular melanomas arise within the choroid. Ocular melanoma arises from cells called melanocytes, which are the cells of the body that produce pigment. Ocular melanoma is a cancerous (malignant) tumor that can potentially spread (metastasize) to other parts of the body, most often to the liver. The exact cause of this disorder is unknown, but several risk factors have been identified.
Although these choroidal melanocytes are similar to those cells found in the skin that produce skin pigment, when choroidal melanocytes transform into cancerous cells it is called choroidal (or uveal) melanoma. However, cutaneous (skin) melanoma and uveal (ocular) melanoma are distinct conditions which share the same name but are biologically and genetically very different diseases. It is extremely rare for skin melanoma to spread into the eye and nearly unheard of for ocular melanoma to spread to the skin.