A total of 121 spontaneous amelanotic melanomas of the skin were identified in 70 of 11,171 male and 51 of 10,927 female Fischer-344/N rats in 63 2-yr carcinogenicity studies conducted by the National Cancer Institute's Carcinogenicity Testing Program/National Toxicology Program. Amelanotic melanomas had characteristic anatomical locations and histologic features distinguishable from Schwann cell tumors. Of the 121 tumors, 84, 19, 10, and 8 cases occurred in the pinna, eyelid, scrotum, and perianal region (anus and tail), respectively. Amelanotic melanomas originated from the dermis and consisted of spindle cells arranged in an interlacing fascicular pattern often with a perivascular orientation; epithelioid cells were rarely seen. Only the tumors arising in the pinna metastasized to the lung and/or mandibular lymph nodes. The metastatic rate was 19% (16/84) of the tumors and was clearly increased with an increase in tumor size. Most metastasizing tumors had focal areas consisting of anaplastic spindle cells with an increased number of mitosis. The tumor cells stained positive for S-100 protein but negative for melanin. Ultrastructurally, the tumors were diagnosed as amelanotic melanomas based on the identification of numerous, intracytoplasmic pre-melanosomes without melanin formation in the tumor cells which were not enveloped by pericytoplasmic basal laminae. One localized amelanotic melanoma of the pinna was successfully transplanted to the subcutaneous tissue in the flank of 3 Fischer-344/N rats.
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