Melanoma in situ

Author: Mikael Häggström
Melanoma of the skin generally presents as a dark skin focality and/or a suspected malignant skin excision.

Fixation

Generally 10% neutral buffered formalin.

  See also: General notes on fixation

Gross processing

Gross examination

Note:

• Color
• Well-defined or diffuse border
• Size
• Any elevation

Tissue selection

Tissue selection from suspected malignant skin lesions, by lesion size:

<4 mm	4 - 8 mm	9 - 15 mm

In table above, each top image shows recommended lines for cutting out slices to be submitted for further processing. Bottom image shows which side of the slice that should be put to microtomy. Dashed lines here mean that either side could be used. Further information: Gross processing of skin excisions

Microscopic evaluation

Differential diagnoses

Distinguish mainly from dysplastic nevus and invasive melanoma of the skin:

Dysplastic nevus

Comparison of congenital pattern nevus, dysplastic nevus and suspected melanoma  

Parameter		Non-atypical congenital pattern	Low-grade dysplastic nevus		High-grade dysplastic nevus	Suspected melanoma in situ
			Mild dysplasia	Moderate dysplasia	Severe dysplasia
Macroscopic	Lateral circumscription	Sharp	Slightly diminished	Moderate		Poor
	Symmetry	Good			Often broken	Rare
Structural (Low mag.)	Delimitation		Rarely diffuse	Sometimes diffuse	Often diffuse
	Lentiginous proliferation		Yes, along with rete pegs	Yes, along with and focally between rete pegs	Yes, along with and focally between rete pegs	Yes partially continuous, multilayered
	Bridging		Rarely	Often
	Confluent nests		Rarely	Sometimes	Often	Often widespread
	Pigment distribution		Regular	Irregular
	Suprabasal presence (less than most superficial third of subcorneal epidermis)	Occasionally centrally	No or rarely	Occasionally centrally		Yes, multifocal
	Pagetoid migration including superficial third of subcorneal epidermis		No	No	Yes, in a maximum of 2 HPF centrally, but not peripherally	Yes, multifocal and/or in periphery Peripheral.
	Extended rete pegs	Ocassional	Yes, regular	Yes, varying	Yes, often irregular	Varying, flattened
	Concentric fibrosis	Regressive	Yes			Occasional
	Lamellar fibrosis		Rarely	Often	Often pronounced	Occasional
	Lymphocytic infiltrate		Mild, perivascular	Mild or moderate, perivascular	Varying	Varying
	Suprapapillary plate involvement	No		Usually no	Often	Yes
Cellular (high mag.)	Image
	Junctional extension	Unusual	Usual			Extensive
	Nuclear size	Age-related	Small	Medium	Large	Medium or large. Pleomorphic
	Nuclear pleomorphism		Slight	Prominent
	Chromatin pattern	Uniform	Condensed	Partically expanded	Expanded, coarse in some cells	Expanded, hyperchromatic, coarse. Usually granular.
	Nucleoli	Age-related	Small	Medium	Large	Usually large
	Mitoses	Few superficial				Superficial and deep
	Histological regression		Usually		Usually not
	Percentage of atypical melanocytes		<10%	About 10 - 50%	about 50-90%	Usually> 90%
	Intradermal melanocytic atypia		No	Rarely, in superficial part	Can be detected in superficial part
	Intradermal melanocyte maturation		Yes	Yes, can be partial	Yes, can be partial	Variable

In suspected but not certain nevus or melanoma in situ, generally perform immunohistochemistry with SOX10, whereby melanocyte proliferation and nuclear pleomorphism is easier to see.

• 

  SOX10 immunohistochemistry of a junctional nevus, with atypical melanocytic proliferation, seen mainly in hair follicles.

• 

  SOX10 immunohistochemistry of lentigo maligna, showing an increased number of melanocytes along stratum basale, and nuclear pleumorphism.

Further information: Evaluation of suspected malignancies

Invasive melanoma of the skin

Invasive melanoma of the skin has features melanoma in situ, but also has dermal involvement of atypical melanocytes with cytologic atypia and no maturation.

Further workup

Upon a diagnosis of melanoma in situ, evaluate its margins.
Optionally, attempt to determine the histopathologic type and amount of cytoplasmic pigmentation:

Margins

If melanoma, determine if the distance to any margin is greater or lesser than 2-3 mm.

• 2 mm is used as a cutoff for sharply demarcated, small, superficially spreading or nevoid melanomas.
• 3 mm is used for ill-defined lentigo maligna melanoma in situ.

If lesser, quantify the distance.

If margins are difficult to determine, consider immunohistochemistry with SOX10 to better visualize melanoma nests.

Histopathologic type

Main types of melanoma in situ are:

Type	Features	Micrograph
Superficial spreading melanoma in situ	Melanoma cells with nest formation along the dermo-epidermal junction.
Lentigo maligna	Linear spread of atypical epidermal melanocytes along stratum basale.
Acral lentiginous melanoma in situ	Continuous proliferation of atypical melanocytes at the dermoepidermal junction.

Report

Most important entries:

• Melanoma area dimensions (width x width)
• Radicality, mainly into either of the following:

• >2 or 3 mm (as per Further workup above): "Clear margins" (or: "Clear margins at over __ mm")((or the exact distance thereof)).))
• <2 or 3 mm but not continuous with edge: "Close margins at __ mm at (location). [[Locations are mainly the deep edge, or the (superior/inferior/medial/lateral) radial edge.]]." Numbers are generally given at an exactness of 0.1 mm.
• Continuous with margin: "Not radically excised at (location)."

  See also: General notes on reporting

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