Clark's index (level of invasion of malignant melanoma)

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According to Clark (1,2), there are five levels of invasion for malignant melanoma which are another factor used in assessing prognosis:

I: confined to epidermis, in situ
II: invasion of the papillary dermis
III: filling of the papillary dermis, but no extension into the reticular dermis
IV: invasion of the reticular dermis
V: invasion of the subcutaneous tissue

In the American Joint Committee on Cancer (AJCC) 2002 Staging system for malignant melanoma there is less significance with respect to the use of Clark's levels in determining prognosis. Analysis of histopathologic features of melanoma comparing Clark’s level and Breslow’s depth have shown that the actual tumour thickness is more predictive of outcome and that there are no natural break-points of survival, as believed previously. However there is an instance in which Clark’s level invasion continues to carry prognostic information: In patients with “thin”(<1.0 mm) melanoma, Clark’s level IV or V lesion portends a worse prognosis. For this reason, pathologists need to continue to include Clark’s level along with tumour thickness and ulceration in the histology report of the primary melanoma (3). Also ulceration of a primary tumour was associated with a worse prognosis than a non-ulcerated lesion (3).

Reference:

  1. Clark, W.H., Jr.: A classifiation of malignant melanoma in man correlated with histogenesis and biologic behavior. In Montagna W, Hu F (eds): Advances in Biology and Skin, Vol 8, The Pigmentary System, Pergamon Press, New York, 1966: 612-647.
  2. Clark, W.H., Jr., From, L., Bernadino, E.A., et. al.: Histogenesis and biologic behavior of primary human malignant melanoma of the skin. Cancer Res 1969; 29:705-727.
  3. Cancer Control. 2002 Jan-Feb;9(1):9-15.

Last reviewed 01/2018

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