A case of multiple trichilemmal cysts is presented. After trauma, a large tumor developed on one of the cysts. The histologic features of this tumor were similar to those of proliferating trichilemmal cyst, but some of the tumor cells showed definite nuclear atypicality. The malignant potential of this tumor was confirmed by later development of metastases to the regional lymph nodes.
Frequency of malignant transformation arising in giant congenital nevi is considered to be 4%-5%. More than a half of the patients in which malignant melanoma developed in giant congenital nevi were under the age of 10. It may be hypothesized that dermabrasion of giant congenital nevus may provoke malignant transformation. Some of the cell groups in giant congenital nevus are potentially malignant. Some groups of nevus cells were larger in size than those of other portions of nevus. Electron microscopic observation revealed that nuclei of these larger nevus cells were significantly indented, and melanization of melanosomes was irregular. Coexistence of alpha-like actin with beta- and gamma-actins in giant congenital nevus cells and disappearance of alpha-like actin in malignant melanoma cells were noted.
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