Histological Type

Malignant melanoma in situ

— intraepidermal: spread can be lentiginous (continuous basal layer) or upward (single cells, nests, "buck-shot" or Pagetoid) in this noninvasive radial growth phase.

Lentigo maligna melanoma

— face/Hutchinson's melanotic freckle: a lentiginous single and nested basal layer proliferation of melanocytes with cytological atypia (enlarged, hyperchromatic angular nuclei and cytoplasmic vacuola-tion) ± architectural atypia (expanded junctional nests) on a background of dermal solar elastosis. Expansion and spindling of junctional nests and any clinically nodular areas should raise a suspicion of invasion. The clinical term lentigo maligna encompasses any degree of proliferation that is confined to the epidermis (i.e. it includes Clark level I or melanoma in situ), while lentigo maligna melanoma implies the presence of dermal invasion (at least Clark level II).

Superficial spreading melanoma

— radial phase of spread.1

Usually an asymmetrical lateral border of atypical junctional cell nests with a central segment of epidermis showing upward melanocytic spread (single cells/nests/"buck-shot" patterns). Moderate dusty pigmentation ± a dermal component related to the growth phase.

Nodular melanoma

— vertical phase of spread.2

Often exophytic/nodular and thick ± pigmentation, with <2 or 3 rete pegs showing atypical junctional nests at the lateral border of the lesion.

Acral/mucosal/lentiginous melanoma

— sole of foot, nail bed, mucosae. Features are often a combination of lentigo maligna and superficial spreading patterns ± a nodular, vertical growth phase component.

1Radial growth phase includes melanoma in situ (i.e. intraepidermal) ± microinvasion of the papillary dermis. The radial phase may be indolent with no metastatic potential and 95-100% survival rate. The dermal component is usually <1 mm thick, i.e. the lesion is wider than it is deep and can have morphologically bland cell nests (usually <10 cells across) of uniform size and cytological appearance. This may be accompanied by signs of regression with a brisk lymphocytic response. The radial phase potentially progresses by clonal expansion to the vertical phase. 2Vertical growth phase tumour comprises expansive nests, nodules or plaques of cyto-logically atypical melanoma cells in the dermis; it implies a biological potential for metastatic spread and is the main determinant of prognosis. The cell nests are usually larger than the biggest intraepidermal nest, >10-25 cells in dimension, show variation throughout the lesion, mitoses and a variable host dermal lymphocytic response. Vertical growth phase melanomas are often at least Clark level III and thicker than 1 mm with an inconstant relationship between the width and depth of the lesion.

Breslow depth or thickness (mm) = the maximum vertical depth from the top of the granular layer or ulcerated surface to the deepest point of invasion

Figure 21.1. Malignant melanoma. |QAJ

In general, these main subtypes do not show prognostic differences but there is correlation with recurrence and metastases, e.g. nodular melanoma is often thick (with a significant Breslow depth) and ulcerated. Knowledge of the various clinicopathological subtypes also helps in their diagnostic recognition.

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