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 reticular dermis by a nodular melanoma (Clark level 4). The reticular dermis can be recognized by the horizontally oriented elastic fibers.