Clinico-pathological classification

Malignant melanomas undergo two growth phases - radial and vertical. Vertical invasion is a poor prognostic sign.

Different types may be recognised - Clark (1969):

• superficial spreading (70%)
• nodular (15%)
• lentigo maligna (4-15%)
• acral lentiginous including periungual (5%)
• amelanotic melanoma (1)

Histologically and cytologically, there are several features that distinguish malignant melanomas:

• size:
  • most malignant melanomas are greater than 10mm in diameter
  • most benign tumours are less than 6mm
• symmetry: malignant lesions are usually asymmetrical with respect to cell type, extension and degree of pigmentation
• ascent of melanocytes: malignant tumours show melanocytes travelling upwards at the lateral margins of the tumour
• epidermal hyperplasia: commonly seen with malignant lesions
• dermal growth:
  • malignant melanomas vary in their cell type and architecture in different parts of the tumour
  • benign lesions are regular in appearance and change to smaller cells within the depth of the lesion
• cytology: with increasing malignancy, there is a wider variation in cell shape and amount of pigment contained withing the melanocyte

Reference:

• (1) Markovic SN et al. Malignant melanoma in the 21st century, part 1: epidemiology, risk factors, screening, prevention, and diagnosis. Mayo Clin Proc. 2007;82(3):364-80