Melanoma Arising in a Medium-Sized Congenital Melanocytic Nevus: A Case Report.

Congenital melanocytic nevus (CMN) is a melanocytic lesion present at birth or in early infancy, and most cases follow a benign clinical course. While the risk of malignant melanoma (MM) is known to be increased in giant CMN, the risk in small- to medium-sized CMN is considered low, and routine prophylactic excision is generally not recommended. We report a case of MM arising in a medium-sized CMN in a 23-year-old woman. The lesion had remained stable for many years but showed rapid enlargement and bleeding over a two-month period. Histopathological examination revealed nodular proliferation of atypical melanocytic cells with marked nuclear pleomorphism, high mitotic activity, and vascular invasion. Immunohistochemical analysis demonstrated diffuse Preferentially expressed Antigen in MElanoma positivity, a high Ki-67 labeling index, and loss of p16 expression. Based on the integrated evaluation of morphological and immunohistochemical findings, a diagnosis of MM arising in CMN was made. The tumor was classified as Stage IIc (T4bN0M0) according to the American Joint Committee on Cancer Eighth edition. This case highlights that even in small- to medium-sized CMN, rapid clinical changes such as enlargement and bleeding may represent important warning signs of malignant transformation. Careful clinical monitoring and appropriate patient education are essential for the early detection of MM in such lesions.