Malignant melanoma in situ of the mucous membrane has been rarely reported, although the mucous membrane including oral mucosa is the favorable predilection site of malignant melanoma among Japanese.
A 66-year-old Japanese woman visited us with a 6-year history of a pigmented macule on her lips. Physical examination evaluated that the pigmented macule continuously spread from the left oral angle to the buccal mucosa, upper and lower lips, and the inner side of nasolabial fold. The pigmented macule showed haphazard combination in color ranging from tan to brown black. Most of the sections through the brown-black macule showed an increased number of melanocytes in the basal layer in part associated with the nest formation. In addition almost all of the basal layer was composed of pleomorphic melanocytes with bizarre nuclei. Such atypical melanocytes scattered even within the spinous layer. A section of the pigmented macule on the nasolabial fold also showed a lentiginous proliferation of melanocytes associated with solar elastosis of the upper dermis. Based on the clinical and histopathologic findings we concluded a deagnosis having with malignant melanoma in situ of the mucous membrane, but not with lentigo maligna.
