Skin melanocytic tumor

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Contributed by Rachel Kowal, M.D. and Ahmed Alomari, M.D.

• Malignant melanoma:
  • Can evolve from preexisting nevi, especially giant congenital nevi (Br J Plast Surg 2003;56:164)
  • Genomic hybridization can show complex chromosomal aberrations, such as copy number changes, specifically partial copy number gains or losses (J Cutan Pathol 2021;48:1410)
  • In a study comparing proliferative nodules and melanomas arising from congenital nevi, melanomas (Am J Surg Pathol 2015;39:405)
    • Had an average mitotic count of 12.5 mitoses/mm² compared to proliferative nodules average of 1.67 mitoses/mm²
    • Are more likely to have ulceration of the overlying epidermis
    • Are far less common and less likely to present as multiple lesions than proliferative nodules
• Combined melanocytic nevi:
  • Composed of 2 distinct populations of melanocytes, typically a common nevus with an associated secondary nevus
  • Blue nevi are the most common to present in this fashion but other nevi, such as Spitz nevi and BAP1 inactivated melanocytoma, can as well (Am J Surg Pathol 2011;35:1540)
  • Blue nevus:
    • Dermal proliferation of dendritic, pigmented melanocytes with small round to oval nuclei
  • Spitz nevus:
    • Dermal proliferation of large spindled to epithelioid cells, which are more nested superficially and extend deep as individual cells
    • Can have Kamino bodies
  • BAP1 inactivated melanocytoma:
    • Predominately dermal based proliferation of large epithelioid melanocytes with characteristic biallelic inactivation of the BAP1 gene (Am J Surg Pathol 2012;36:818)

Which of the following describes melanocytic proliferative nodules?

1. Dermal proliferation of pigmented melanocytes with dendritic morphology within a fibroblastic stroma
2. Epithelioid to spindled benign melanocytic proliferation with biallelic inactivation of BAP1
3. Melanocyte proliferations with cytologic atypia, invasive growth and ulceration
4. Nodular proliferation of melanocytes that is distinct from and more cellular than a surrounding larger background nevus
5. Proliferation of large epithelioid to spindle cells with Kamino bodies

D. Nodular proliferation of melanocytes that is distinct from and more cellular than a surrounding larger background nevus. Proliferative nodules are nodular proliferations of melanocytes that are distinct from and more cellular than their surrounding larger background nevi. Answer B is incorrect because it describes BAP1 inactivated melanocytoma. Answer E is incorrect because it describes Spitz nevi. Answer C is incorrect because it describes invasive melanoma. Answer A is incorrect because it describes blue nevi.

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Reference: Proliferative nodule

Which of the following would more likely be seen in a benign melanocytic proliferative nodule as compared to an atypical proliferative nodule?

1. Blended transition from the proliferative nodule to background nevus
2. Epidermal effacement
3. Expansile growth
4. Many mitotic figures
5. Striking nuclear pleomorphism

A. Blended transition from the proliferative nodule to background nevus. Benign proliferative nodules tend to have a more subtle blend into their background nevi, while atypical proliferative nodules often have a sharp demarcation. Answers B - E are incorrect because they describe atypical proliferative nodules. Benign proliferative nodules tend to have minimal mitotic activity and minimal nuclear pleomorphism, while atypical proliferative nodules can have many mitotic figures, striking nuclear pleomorphism and expansile growth.

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Reference: Proliferative nodule