The Clark levels refer to the depth of invasion of the dermis by melanoma.
Level 1: Melanoma confined to the epidermis (melanoma-in-situ)
Level 2: Invasion of the papillary dermis
Level 3: Invasion to the junction of papillary and reticular dermis
Level 4: Invasion of reticular dermis
Level 5: Invasion of subcutaneous fat
The interface between papillary and reticular dermis can be identified by the presence of superficial capillary plexus. In addition, it is the zone where horizontally oriented elastic fibers of reticular dermis are replaced by vertically oriented fibers of papillary dermis. The Clark levels provide independent prognostic information for thin melanomas (1mm or less) but not for thicker tumors. The distinction between Clark level 2 and 3 as well as between level 3 and 4 is difficult to apply in practice and lacks reproducibility when compared to Breslow thickness. It has also been found that if the mitotic rate can be assessed in thin melanomas, then Clark level does not provide any additional independent prognostic information. AJCC Staging System (7th Edition, 2009) no longer recommends the use of Clark levels, even for thin melanomas, if mitotic activity can be adequately examined in the dermal (invasive) component. The image shows infiltration of the subcutaneous tissues by melanoma (Clark level 5).