New primary malignant melanoma, epidermotropism and Indian-file arrangement of metastatic tumor cells in a case with intransit metastases of acral type of malignant melanoma.

Multiple malignant melanocytic lesions developed on a localized area of the thigh in an 80-year-old woman 3 years after excision of a primary malignant melanoma on the left second toe and dissection of left inguinal lymph nodes. Biopsy specimens obtained from 2 small pigmented macules revealed the histologic features of new primary lesions, simulating that of the primary lesion on the toe; no tumor cells were found within vessel lumina. A brown-black papule neighboring one of the macules histologically resembled compound type of melanocytic nevus, but the tumor cells compressed thinned epidermis with focal epidermotropism. Other papules, whose color ranged from light brown to reddish brown, showed metastatic tumor cells only in the dermis. In 3 of them, individual tumor cells were lined up along the collagen bundles, showing an 'Indian-file' arrangement. Such a combination of various histologic features in a case of intransit metastases of acral type of melanoma has not been reported in the literature.