Malignant melanoma with paradoxical maturation.

Area of Science:

• Dermatopathology
• Oncology
• Histology

Background:

• Melanocytic nevi typically mature with depth, showing smaller cells.
• Most conventional malignant melanomas (MMs) lack maturation and display large, pleomorphic cells.
• A subset of melanomas exhibits paradoxical maturation, mimicking benign nevi.

Purpose of the Study:

• To describe and differentiate invasive melanomas with paradoxical maturation (IMPs) from conventional MMs and compound nevi (CN).
• To establish diagnostic criteria for IMPs to avoid misdiagnosis.

Main Methods:

• Histologic, cytomorphometric, and immunohistochemical analysis of 17 IMPs, 2 epidermotropic metastatic melanomas with maturation (EMMMs), 13 CN, and 14 conventional MMs.
• Evaluation of nuclear/cellular areas, Ki-67, glycoprotein (gp)100, and tyrosinase expression at different dermal depths.

Main Results:

• IMPs showed decreased nuclear/cellular areas and reduced Ki-67, gp100, and tyrosinase expression with depth, similar to CN.
• IMPs differed from conventional MMs by having smaller deep areas and reduced superficial/deep Ki-67, deep gp100, and deep tyrosinase.
• IMPs differed from CN by exhibiting larger areas, more confluence, increased mitotic figures, higher Ki-67 and gp100 expression, and more melanin.

Conclusions:

• Invasive melanomas with paradoxical maturation (IMPs) can mimic benign nevi, posing a diagnostic hazard.
• Histologic, morphometric, and immunohistochemical criteria are proposed to aid in the accurate diagnosis of IMPs.
• Epidermotropic metastatic melanomas with maturation (EMMMs) share features with primary IMPs.