The Clark scale is used to determine how deeply the melanoma has penetrated into the skin layers, according to About.com. The levels range from level 1 to level 5.
In level 1, the cancer is confined only to the top layer of the skin, which is called the epidermis, states About.com. At this point, the melanoma is 100-percent curable. In level 2, the cancer has penetrated the upper dermis, and in level 3 it has filled the upper dermis but hasn't yet reached the lower dermis.
In Clark level 4, the lower dermis has been penetrated by the melanoma, says About.com, and in level 5 it has invaded subcutaneous tissue. This is the third, deepest layer of skin, and holds fat, connective tissue, blood vessels and nerves. It is important in regulating body and skin temperature.
Clark levels were developed in 1966 by Dr. W.H. Clark, according to About.com. However, it is not used as extensively as it was in the past because it is not as sophisticated as the Breslow depth, also known as the Breslow thickness. The Clark levels also don't predict the outcome as accurately, and medical professionals find it hard to tell the difference between level 2 and level 3. The Clark levels also cannot be used on the palms of the hands or the soles of the feet.