Clark level of invasion: A method for determining the prognosis (outlook) with melanoma. The method was devised by the pathologist Wallace Clark and measures the depth of penetration of a melanoma into the skin according to anatomic layer.
There are five Clark levels of invasion:

• Level I: Melanomas confined to the outermost layer of the skin, the epidermis. Also called "melanoma in-situ."
• Level II: Penetration by melanomas into the second layer of the skin, the dermis.
• Levels III-IV: Melanomas invade deeper through the dermis, but are still contained completely within the skin.
• Level V: Penetration of melanoma into the fat of the skin beneath the dermis, penetration into the third layer of the skin, the subcutis.
  
  The Clark levels provide a system to relate the degree of penetration of melanoma into the skin to the 5-year survival rate after surgical removal of the melanoma.
  
  Another melanoma measurement system, that of Breslow thickness, named after its originator Alexander Breslow, is in wider use today. In it, invasive tumor thickness is used to predict 5-year survival. For example, a thickness of the melanoma of less than 0.76 millimeters is associated with a 5-year survival of 97% of patients whereas a tumor thickness of more than 8.0 millimeters is associated with 5-year survival of 32%.