Melanotic oncocytic metaplasia of the nasopharynx: a case report with a focus on immunohistochemical analyses and literature review

Abstract

Melanotic oncocytic metaplasia (MOM) of the nasopharynx is an extremely rare lesion, with only 21 cases reported in English literature to date. MOM typically occurs near the Eustachian tube opening in Asian men in their 60 s to 70 s. Here, we present a case of MOM in a 57-year-old Japanese man who is a heavy smoker. The patient did not have complaints; MOM was diagnosed incidentally as 4 flat elevated lesions with brown to black discoloration, ranging from 2 to 3 mm in maximal diameter, were found in the right torus tubarius. On suspecting melanoma, the largest lesion was biopsied. Microscopic examination identified both oncocytic metaplasia and melanin pigmentation of the epithelium in the same gland. Upon immunohistochemical examination, melanocytes displayed reactivity for 3 out of 4 melanocytic markers; immunopositivity for S-100 protein, Melan-A, and MITF and immunonegativity for HMB-45 was observed. Normal melanocytes in the nearby surface respiratory epithelium displayed the same pattern of immunoreactivity. Immunopositivity for S-100 protein and immunonegativity for HMB-45 have been previously reported in MOM. Reduction of stimulation of melanocytes in a longstanding lesion like MOM may explain the immunonegativity for HMB-45. S-100 protein, in conjunction with more specific marker for melanocytes, Melan-A or MITF, could prove the definite presence of melanocytes in this case of MOM. As it has been shown by previous reports that MOM pursues a benign course, it will be sufficient to follow up the patients regularly for the remaining 3 lesions.

Keywords: Melanotic oncocytic metaplasia; immunohistochemistry; nasopharynx.

Figure 1

Laryngoscopical examination revealed lesions in the right torus tubarius. (A) One of the 4 flat elevated lesions found in the right torus tubarius. It displayed brown to black discoloration and ranged from 2 to 3 mm in maximal diameter. (B) (A) in higher magnification.

Figure 2

Histopathological findings. A. The surface of the lesion was covered by respiratory epithelium with goblet cells. It was composed of pre-existing seromucous glands with diffuse oncocytic metaplasia (× 20). B. Oncocytes presented abundant eosinophilc granular cytoplasm. Brown pigments were also observed in the cytoplasm of many oncocytes (× 200). C. Upon closer inspection, melanocytes were hardly discernible. Mitotic figures or atypia of oncocytes were not observed. The pigments stained positive with Fontana-Masson staining (inset) (× 400).

Figure 3

Immunohistochemistry of the lesion. Giemsa staining used as a counterstain revealed metachromasia for melanin pigment. Arrows indicate melanocytes. A. Melanocytes stained positive for S-100 protein (× 400). B. Melanocytes stained negative for HMB-45 (× 400). C. Melanocytes were immunoreactive for Melan-A (× 400). D. Melanocytes were immunopositive for MITF (× 400).

Figure 4

Immunohistochemistry of the surface respiratory epithelium. Giemsa staining revealed metachromasia for melanin pigment. A. Melanocytes stained positive for S-100 protein (× 400). B. Melanocytes were negative for HMB-45 staining (× 400). C. Melanocytes were positive for Melan-A staining (× 400). D. Melanocytes were positive for MITF staining (× 400).