Desmoplastic malignant melanoma of the oral mucosa. An underrecognized diagnostic pitfall

Although cutaneous desmoplastic malignant melanoma (DMM) has been well characterized, those arising from the oral mucosa have been less well defined.

We evaluated the clinical and pathologic features of three patients with biopsy-proven DMM examined at the North Carolina Baptist Hospital. Routine hematoxylin and eosin-stained slides were reviewed in all three cases. Sections from all cases were examined immunohistochemically using the avidin-biotin-peroxidase (ABC) technique and employing commercially available antibodies to the following antigens: S-100 protein, HMB-45, NK1C3, and cytokeratin (AE1/AE3). Appropriate positive and negative controls were utilized throughout these procedures. Clinical data were obtained from the patients' medical records.

Age at diagnosis for the three male patients were 42, 64, and 75 years. Anatomically, these neoplasms arose from the left maxillary oral mucosa (two patients) and the vermilion border of the lower lip (one patient). Initial incisional biopsies of all three patients were misinterpreted as desmoid tumor (one patient) and squamous cell carcinoma (two patients), respectively. Histologically, all tumors were poorly circumscribed and composed of fascicles and bundles of spindle-shaped cells with hyperchromatic nuclei and associated with extensive stromal collagenization. Perineural invasion was noted in two tumors. An overlying atypical, intraepidermal melanocytic proliferation was observed in two tumors. Immunohistochemically, all three tumors showed positive immunoreactivity with S-100 protein and vimentin. One tumor also expressed HMB-45. Wide surgical incision was the primary mode of therapy in all patients. One patient also received multiple courses of radiation therapy and chemotherapy. One patient died of widespread metastatic disease.

DMM of the oral mucosa is a rare, often unrecognized, form of malignant melanoma associated with a fibrosarcoma-like morphologic appearance and abundant collagenization. Meticulous attention to histomorphology and judicious use of immunohistochemical stains will help prevent misdiagnosis.