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

Features:

• Classic appearance of melanoma:
  • Loosely cohesive; mix of small nests of cells, single cells.
  • Mixed of spindle and ovoid cell morphology.
  • +/-Occasional large binucleated cells.
  • Cytoplasm: 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.

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.

Breslow thickness

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

Benign lesions

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	Image?	Notes/other?
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	large atypical spindle cells, btw collagen	predominantly dermal	pleomorphic undifferentiated sarcoma (MFH), scar, dermatofibroma, DFSP, leiomyosarcoma	Image?	Notes/other?
Malignant melanoma - nevoid type	prominent nucleoli, deep mitoses - high power diagnosis	mimics nevus at low power	(benign) nevus	Image?	Notes/other?
Malignant melanoma - spitzoid type	nested pattern, nuclear atypia, no maturation (large deep cells)	NC ratio increased (vs. Spitz)	Spitz nevus	Image?	Notes/other?

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:

• S-100 +ve.
• HMB-45 +ve.
• 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 (S-100, HMB-45, MART-1) is also positive in other lesions, e.g. cellular blue nevus.

See also

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