Malignant melanoma

Malignant melanoma, also melanoma, is an aggressive type of skin cancer that can be diagnostically challenging for pathologists.

It fits into the larger category of melanocytic lesions which includes many benign entities, a number of which can be difficult to distinguish from melanoma.

General

• AKA Malignant melanoma.
• Main DDx: melanocytic lesions - especially if pigmented.
• Known as the great mimicker in pathology; it may look like many things.

Clinical

• ABCD = asymmetric, borders (irregular), colour (black), diameter (large).

Microscopic

Metatstatic/non-skin

Features (non-skin):

• Classic appearance of melanoma:
  • Loosely cohesive; mix of small nests of cells, single cells.
    • Nests often have clefting with surrounding tissue.
  • Mix of spindle cells and epithelioid cells:
    • +/-Occasional large binucleated cells.
    • Cytoplasm with brown pigment (melanin).
    • Prominent (large) red nucleoli (like in serous carcinoma of the ovary).
    • Marked nuclear pleomorphism - variation in cell size, shape & staining (like in serous carcinoma of the ovary).
    • Nuclear pseudoinclusions (like in papillary thyroid carcinoma).

Notes:

• Can look almost like anything.
  • Like it is said that sarcoidosis is in every internal medicine DDx... melanoma is every pathologic DDx
• May have no nuclear atypia.
  • Diagnosis is based on architecture (upward spread in the epidermis, single cells, asymmetry).

DDx

• Carcinoma.
  • Serous carcinoma - both serous carcinoma and melanoma have a large nucleolus.
• Sarcoma - as may have spindle cells.
  • Clear cell sarcoma.
  • Metaplastic carcinoma.
  • Epithelioid angiosarcoma.
• Lymphoma.
• Other melanocytic lesions.

Images:

• Desmoplastic melanoma - several images (upmc.edu).

Skin

Features (skin):

1. Melanocytic differentiation:
   • Pigmentation (melanin).
   • Nuclear pseudoinclusion.
   • Gray cytoplasm.
   • Clear (artefactual) halo around cells.
2. Architecture:
   • Sheeting - diagnostic.
   • Asymmetry - as judged from low power magnification.
3. Lack of maturation - see below.
4. +/-Nuclear atypia - esp. nucleoli.
   • May be seen in a Spitz nevus.
5. +/-Upward scatter of melanocytes AKA intraepidermal ascent - "cannonball" appearance.
   • No diagnostic significance in the following cases:
     • Acral sites - see: Acral nevus.
     • Histologic evidence of trauma.
       • Thick dense stratum corneum.

Maturation - with depth:

• Nests get smaller.
• Cells get smaller.
• Mitoses decrease.
• Pigmentation decrease.

DDx:

• Epithelioid cell forms:
  • Spitz nevus - especially difficult.
    • Key differences: maturation and symmetry.
  • Melanocytic nevus.
• Spindle cell forms:
  • Spindle cell squamous carcinoma.
  • Atypical fibroxanthoma.
  • Leiomyosarcoma.

Images:

• Malignant melanoma - several images (upmc.edu).

Regression of melanoma

General

• Complete regression without metastases estimated to be 10-20%.
• Common ~25% of cases.
  • Poor prognostic feature.

Microscopic

Features - all required:

• No melanocytes.
• Melanophages.
• Fibrosis.
• Thinned epidermis.
• Telangiectatic vessels.
• Lymphocytes.

Metastatic versus primary

Primary lesions should have:

• Epidermal involvement.

Metastatic lesions classically have:

• Tumour angiotropism (tumours cells cluster around vessels).
• Intravascular invasion.
• No epidermal component.

Note:

• Histology is not definitive for metastatic melanoma vs. primary melanoma; epidermal involvement may be seen in mets.
  • History/clinical is important for differentiation.

Breslow thickness

• AKA maximum tumour thickness.
• Depth measured from stratum granulosum to deepest intradermal tumour cell - predictive of survival.

Clark level

• AKA anatomic level.
• Not as reproducible as Breslow thickness - not used.

Anatomic level - definition:

• I = epidermis only (AKA melanoma in situ).
• II = extends into papillary dermis but does not fill or expand.
• III = fills and expands papillary dermis.
• IV = extends into reticular dermis.
• V = extends into subdermis.

Subtypes

Subtype name	Key feature	Microscopic additional	DDx	Image	Notes/other
Melanoma in situ	confined to epidermis, nuclear atypia	melanocyte enlargement, nuclear hyperchromasia, +/- melanocytes above suprapapillary plate (above basal layer) = "Pagetoid spread"	melanocytic hyperplasia, pagetoid Spitz nevus	(upmc.edu)	lentigo maligna is melanoma in situ on sun damaged skin
Malignant melanoma - superficial spreading type	atypical melanocytes at all levels of epidermis + dermis	atypical dermal melanocytes single, in cluster or sheets	compound melanocytic nevus	Image?	Notes/other?
Malignant melanoma - lentiginous type	atypical melanocytes prominent along basal keratinocytes + in dermis	nuclear atypia	DDx?	Image?	Notes/other?
Malignant melanoma - nodular type	dermal large nodule/sheet	nuclear atypia; may not be prominent in epidermis	metastatic melanoma	Image?	Notes/other?
Malignant melanoma - desmoplastic-neurotropic type AKA desmoplastic melanoma	large atypical spindle cells, btw collagen	predominantly dermal	pleomorphic undifferentiated sarcoma (MFH), scar, dermatofibroma, DFSP, leiomyosarcoma, desmoplastic Spitz nevus, sclerosing blue nevus	(upmc.edu)	IHC: rarely S100-, generally Melan A- & HMB-45-; subdivided into mixed desmoplastic melanoma and pure desmoplastic melanoma
Malignant melanoma - nevoid type	prominent nucleoli, deep mitoses - high power diagnosis	mimics nevus at low power; "push" elastic fibers downward (unlike benign nevi)	(benign) nevus	Image?	deep HMB-45+
Malignant melanoma - spitzoid type	nested pattern, nuclear atypia, no maturation (large deep cells)	NC ratio increased (vs. Spitz)	Spitz nevus	Image?	Notes/other?

Subtypes in short

Subtype name	Key feature
in situ	confined to epidermis, unlike all others
superficial spreading	above basal layer
lentiginous	along basal keratinocytes
nodular	nodular dermal lesion
desmoplastic-neurotropic	atypical dermal spindle cells
nevoid	nevus-like at low power
spitzoid	mimics Spitz nevus (at DE junction)

Electron microscopy

• Melanosomes.

Image(s):

• Melanosomes (nature.com).

Stains

• Fontana-Masson stain, stains melanin.
  • May be useful to differentiate melanin from other brown stuff (e.g. lipofuscin, hemosiderin).

IHC

Standard panel:

1. S100 +ve.
   • Negative staining pretty much excludes the diagnosis.
2. HMB-45 +ve -- esp. deep.
3. Melan A (MART-1) +ve.

Others:

• SOX10 +ve -- useful for diff. from excision scar.
  • SOX-10 = pan-schwannian and melanocytic marker.

Notes:

• The standard panel above (S100, HMB-45, MART-1) is also positive in other lesions, e.g. cellular blue nevus.

See also

• Dermatopathology.
• Dermatologic neoplasms.
• Cytopathology.
• Melanocytic lesions.