Melanoma develops from cells called melanocytes. Melanocytes produce melanin, the substance that gives colour to the eyes, skin and hair. Intraocular melanoma is a cancer that develops from melanocytes in the eye. Because intraocular melanoma usually develops in the uvea part of the eye, it is also called uveal melanoma.
Intraocular melanoma and melanoma of the skin both develop from melanocytes, but they behave differently. For example, intraocular melanomas often spread to the liver first, whereas skin melanomas often spread to nearby lymph nodes. There are no lymphatic channels within the eye and orbit, so intraocular melanoma does not spread to nearby lymph nodes like melanoma of the skin. Intraocular melanomas spread to the liver when cancer cells enter the bloodstream and travel to the liver (which filters blood).
Intraocular melanoma accounts for about 5% of all melanomas. Although intraocular melanoma is rare, it is the most common type of primary cancer within the eye in adults. The incidence of intraocular melanoma increases with age. It is more common in people in their 60s and 70s and is slightly more common in men.
Intraocular melanomas can develop sporadically with no known cause (referred to as de novo) or develop from a pre-existing mole (nevusnevusSee mole.) in the eye.
Intraocular melanoma usually affects only one eye. There are 4 tissues in the eye area where melanoma can occur:
- uvea (choroid, ciliary body and iris) – most common location
- orbit – least common location
The uvea is the coloured (pigmented) layer that forms part of the wall of the eye. It lies between the sclera and the retina. The uvea is made up of the choroid, ciliary body and iris. Intraocular melanomas can develop anywhere in the uvea and can be classified as posterior or anterior uveal melanomas.
Posterior uveal melanoma
PosteriorPosteriorReferring to or having to do with the back of the body or a structure. uveal melanoma begins in the choroid or ciliary body further back in the eye.
- They are the most common of the uveal melanomas.
- These tumours make up about 95% of all cases of intraocular melanoma.
- Most tumours (about 90%) develop in the choroid.
- Most choroidal melanomas are believed to start from an existing eye mole (uveal nevus).
- Ciliary body melanomas can grow to quite a large size before they are found. Small ciliary body melanomas may not need to be treated right away, but are followed regularly.
- Tumours are usually detected when vision changes occur because the tumour grows into nearby ocular structures.
- Posterior uveal melanomas can cause increased pressure in the eye (secondary glaucoma) and spread outside the eye (extraocular extension) into the orbit.
- They spread (metastasize) more often than melanoma that begins in the iris.
Anterior uveal melanoma
AnteriorAnteriorReferring to or having to do with the front of the body or a structure. uveal melanoma begins in the iris near the front of the eye.
- These tumours are rare and account for about 5% of all cases of intraocular melanoma.
- Melanoma is the most common primary malignant tumour of the iris.
- Most iris melanomas seem to start from an existing eye mole (iris nevus).
- They are easy to see and are therefore often detected early.
- Anterior uveal melanoma can cause clouding of the lens of the eye (cataract) and secondary glaucoma.
- These tumours tend to be slow growing and have a lower rate of metastasizing compared to choroid or ciliary body tumours.
- Metastatic disease can occur if there is cancer in the ciliary body.
Uveal melanomas have distinct cell types within the tumour. These cancer cells are classified based on their size, shape and characteristics. The cell types found in intraocular melanoma are:
- spindle cell melanomas (type A, type B)
- The cancer cells are round or shaped like a spindle.
- Type A spindle cells have slender nucleinucleiThe part of the cell that holds the chromosomes, which contain DNA (genetic information). and lack visible nucleoli (structure within the nucleus of the cell made of protein and RNARNAThe molecules inside the cell that help transmit the genetic information that controls many cell functions, including the type of proteins a cell makes.). Type B spindle cells have larger nuclei and distinct nucleoli.
- Spindle cell tumours tend to grow in a close interconnected (cohesive) fashion.
- Small intraocular tumours tend to contain spindle cells.
- The cancer cells are larger and resemble skin cells.
- Epithelioid cell tumours grow less cohesively than spindle cell tumours.
- Larger sized intraocular melanomas tend to contain more epithelioid cells than small tumours.
- The tumour contains a mixture of both spindle and epithelioid cells.
- Most tumours are mixed.
Mixed and epithelioid cell types are more common and more aggressive than spindle cell types.
Uveal melanoma often grows in the shape of a dome or a nodule (a small lump or growth made up of cells or tissues). Occasionally, the tumours are flat or spread out and involve extensive areas of the uvea. The size of the tumour is used to help classify and stage the melanoma and predict the chance of metastatic disease.
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