Malignant melanoma arising in a giant congenital melanocytic nevus. A case report with cytogenetic and histopathologic analyses.

A malignant melanoma developed in a 2-year-old Hispanic girl with a giant congenital melanocytic nevus (bathing-trunk type). Histopathologic evaluation showed a deep-seated tumor arising from a nonepidermal origin. Cytogenetic analysis demonstrated multiple chromosomal abnormalities in hyperdiploid cells (chromosome range, 56 to 61). No two karyotypes were identical, but many abnormalities were common to all analyzed cells, suggesting both karyotypic instability and evolution. The metaphases were monosomic for 3, 12, and 16, trisomic for 1, 2, 8, 19, 20, and 21, and included structural aberrations deletion 1 and derivatives 3 and 16. Eight markers were identified, including one ring. The extra 19 was possibly an isochromosome. No abnormalities of 6 or 10 were identified.