A case report : Pigmented nevus of the lower gingiva

Objectives: Pigmented nevus of the oral cavity is relatively rare in Japan. A number of pigmented lesions are difficult to classify and pose challenges in a melanoma diagnosis. It is important to exclude non-melanocytic lesions and benign melanocytic entities, both of which can mimic melanoma histologically. Some pigmented oral lesions will be more difficult to classify when there are unusual features present such as asymmetry of the silhouette, excessive junctional activity, deep extension into the subcuits, cellular pleomorphism, or mitotic activity. We report this case that proved difficult to diagnose with malignant melanoma.
Methods: A 39-year-old Japanese woman was assigned to our department because of gingival swelling and pigmentation. Intraoral examination revealed an elastically hard, pigmented, dome-shaped swelling, measuring 7×6 mm, on the labial–side gingiva, extending from the right lower incisor to the lateral incisor. The coal-black, pigmented, partly white, spotted lesion had a rough surface and slight tenderness on palpation. The swelling had been present for about 5 years with no change in size or symptoms. Radiographically, there was no abnormality in the mandible. Laboratory examinations did not find abnormality.
Results: An incisional biopsy was performed, and H&E-stained histological sections showed mucosa lined by parakeratinized squamous epithelium with a dense diffuse infiltrate of small monomorphous melanocytes (nevus cells), which are occasionally presenting melanin, but without cellular atypia or mitosis. To better characterize this case, immunohistochemistry was performed. The melanocytic cells were negative for HMB-45 and positive for Melan-A, S-100 and vimentin. The positivity of the melanocytic cells for Ki-67 was 0.3% (proliferation index).
Conclusions: Lesion was removed surgically by taking surrounding normal mucosa along with the biopsy of the lesion. The patient has been under follow –up.