Predominantly composed of atypical squamous cells, with scattered melanocytes
– Melanocytes
Of low density
Single, not nested
Dendritic shaped
Nuclei not atypical
• Pseudoepitheliomatous hyperplasia associated with malignant melanoma
Keratinocytes in hyperplastic areas not atypical and connect to epidermal surface
Mitoses not prominent
Dyskeratosis absent
• Melanocytic matrixoma
Dendritic melanocytes intermingled with basaloid epithelial cells with foci of matrical differentiation
• Atypical solid-cystic hidradenoma
Lacks population of spindled cells with clear cytoplasm and fine melanin pigment
Squamomelanocytic Tumor Squamomelanocytic tumor is a dermal-based tumor composed of 2 populations of cells: Atypical epithelioid melanocytic cells and larger squamoid cells . (Courtesy P. Amerio, MD, PhD.)
Squamomelanocytic Tumor at High Magnification Squamomelanocytic tumor shows squamoid cells with pink cytoplasm in islands, forming keratin pearls , surrounded by smaller melanocytic cells with less distinctive cytoplasm. (Courtesy P. Amerio, MD, PhD.)
Cytokeratin Staining in Squamomelanocytic Tumor Cytokeratin AE1/AE3 strongly highlights the islands of squamoid cells forming keratin. (Courtesy P. Amerio, MD, PhD.)
Melan-A Staining in Squamomelanocytic Tumor Melan-A staining is positive in the melanocytic cells surrounding the negative islands of squamoid cells . (Courtesy P. Amerio, MD, PhD.)
TERMINOLOGY
Definitions
• Admixed histopathologic features of both atypical squamoid and melanocytic cells
2 cell types generally within < 1 mm of each other
• Original descriptions were of dermal-based tumors
ETIOLOGY/PATHOGENESIS
Field Cancerization Theory
• Common carcinogen (i.e., ultraviolet light) may lead to proliferation of 2 different cell types
Tumor Collision Theory
• By chance, 2 different cell types have mutations and show clonal proliferation
• Arguing against this theory is presence of identical molecular alterations and immunohistochemical staining patterns in both squamous and melanocytic components of some squamomelanocytic tumors
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and larger squamoid cells 
. (Courtesy P. Amerio, MD, PhD.)
, surrounded by smaller melanocytic cells 
with less distinctive cytoplasm. (Courtesy P. Amerio, MD, PhD.)
. (Courtesy P. Amerio, MD, PhD.)
