This brain has been sliced through the middle in a plane parallel to the ground. The front of the brain is at the top and back is at the bottom. The two cerebral hemispheres are clearly seen divided by the longitudinal fissure running vertically through the centre. Fifteen or so small, black, roundish secondary tumours can be seen growing scattered throughout the brain. These originate from cells that have escaped from a primary melanoma in the choroid of the eye and that have taken hold in the brain and have grown into individual secondary tumours or metastases. This is clearly demonstrated in Figure 2, where the black melanoma cells can be seen in the substance of the brain in a microscopic section.
The choroid is a layer of blood vessels that lies under and supports the retina, the light–sensitive layer of cells that covers the inner surface of the eye. Melanin-producing cells called melanocytes in the choroid produce the black pigment melanin which reduces reflections within the eye which would otherwise bounce around inside the eye and confuse the light sensitive cells. This type of melanoma occurs predominantly in light–skinned people, with Caucasians being 19 times more likely to have choroidal melanoma than African Americans and 16 times more likely than Asians. The molecular changes involved in development of choroidal melanoma are well understood, with multiple alterations in chromosome number and changes in genes that regulate cell division being part of the cancer development and metastatic processes. Melanoma of the eye is rare and incidence is not increasing, in contrast to melanoma of the skin where case numbers are increasing rapidly. There is some evidence to suggest that exposure to UV light in the form of sunbathing or tanning beds, may increase the risk of eye melanoma, but this remains controversial. Brain metastases such as these are difficult to treat surgically and at present the best hope of treatment lies in localised forms of radiotherapy, like the gamma ‘knife’.