Collision of squamous-cell carcinoma with melanoma in situ in a child with xeroderma pigmentosum

Coexistence (collision) of two different neoplasms in the same lesion has previously been documented by several authors. In this report, we describe a 13-year-old boy with xeroderma pigmentosum presenting with squamous-cell carcinoma and melanoma arising at the same site on the nose. Histopathologically, the melanoma component of the lesion was located mainly eccentrically to the squamous-cell carcinoma component. Immunohistochemical stains confirmed the histopathologic findings. Mutations for p53 assessed using single-strand conformation polymorphism, and sequencing analysis revealed a CC-to-TT transition at codon 159 of the p53 gene in the squamous-cell component but not in the melanoma component. This finding suggests a possible role for UV in the pathogenesis of at least the squamous-cell component of the tumor. To the best of our knowledge, this is the first report of a collision tumor comprising squamous-cell carcinoma and melanoma arising in childhood.