7. WHERE DOES MELANOMA DEVELOP?

Melanoma arises as a result of the mutation of Melanocytes, 70% of which are located in the skin.  Consequently, the most common place for melanoma to begin or its ‘primary' location is in the skin.  These melanomas are called cutaneous melanomas.  Cutaneous melanomas can develop from a normal area of skin (skin with no moles or prior abnormal colouration) or from an existing mole or freckle.

Cutaneous melanomas present in locations with differing frequencies, dependent on gender.  For males melanoma most commonly presents as a primary on their back, while for females, melanoma most commonly presents as a primary on their legs.  These differences can most likely be explained by differing patterns of sun exposure.  However, it should be noted that melanomas can appear anywhere on the body, even locations not exposed to the sun, therefore care should be taken when trying to detect melanomas.      

To a lesser extent melanocytes are also found in other locations in the body namely; the eyes, the ears, the mucous lining of the mouth, nose, stomach, rectum and genitals and in the inner tissue of the brain and spinal cord. Therefore it is possible, but less common, for melanoma to originate from these locations.  Generally the prognosis for melanoma originating from the above mentioned locations is poorer then that of cutaneous melanoma. This is due to non-cutaneous melanoma usually being diagnosed at a later stage.

Given the ability for melanoma to develop in numerous locations and metastasise with speed, it is possible though uncommon for melanoma to be diagnosed after it has metastasized to distant organs.  In such cases the location of the primary melanoma can remain unknown, a concept called an occult primary malignancy.   Occult primary melanoma accounts for 4-12% of clinical presentations at major centres.