Stage I melanomas are defined by two primary characteristics:

• Tumor thickness (known as Breslow depth): how deeply the tumor has penetrated the skin. Thickness is measured in millimeters (mm).
  • 1 mm = .04 inch, or less than 1/16 inch (about equal to the edge of a penny)
    
  • 2 mm = between 1/16 and 1/8 inch (about equal to the edge of a nickel)
    
  • 4 mm = between 1/8 and 1/4 inch (about equal to the edges of two nickels)
    

• Ulceration: a condition in which the epidermis that covers a portion of the primary melanoma is not intact. Ulceration is determined by microscopic evaluation of the tissue by a pathologist, not by what can be seen with the naked eye.

The designation of Clark level measures the depth of invasion according to the number of layers of skin the tumor has penetrated. There are five anatomic layers of the skin:

• Level I: epidermis
• Levels II-IV: dermis
• Level V: the subcutis

Clark level is a secondary characteristic only of Stage I melanomas where the primary tumor is no more than 1.0 mm thick.

Subclasses of Stage I Melanoma

Stage IA (T1aN0M0)

• T1a: the tumor is no more than 1.0 millimeter (mm) thick, with no ulceration and a Clark level II or III
• N0: the tumor has not spread to nearby lymph nodes
• M0: the tumor has not spread to sites distant from the primary tumor

• T1b: the tumor is no more than 1.0 mm thick, with ulceration or a Clark level IV or V
• T2a: the tumor is 1.01-2.0 mm thick, with no ulceration
• N0: the tumor has not spread to nearby lymph nodes
• M0: the tumor has not spread to sites distant from the primary tumor