Background: Melanocytoma (magnocellular nevus) is a jet-black benign lesion histologically composed of polygonal tumor cells with small, inconspicuous nuclei and abundant cytoplasm. Melanocytomas in general are rare. Most cases occur in the optic nerve head. Conjunctival melanocytoma (magnocellular nevus) is extremely rare, and only 3 lesions of the ocular surface have been reported. Objectives: To describe the clinical and histological spectrum of conjunctival melanocytoma and discuss differential diagnoses of this rare lesion. Method: Four heavily pigmented conjunctival lesions were excised for slight tumor growth and histologically processed. The specimens were routinely stained with hematoxylin and eosin and periodic acid-Schiff. Sections were bleached and immunohistochemical stains were performed for CD68, HMB-45, S100, melanin, and Ki-67. Results: Histological examination revealed findings of a conjunctival melanocytoma in 3 cases. The fourth case was diagnosed histologically as a combined melanocytic lesion with a compound nevus and an inverted type A nevus. None of the lesions exhibited transition towards malignancy. The differential diagnoses included conjunctival melanoma, granular cell nevus, compound nevus with reactive changes, and blue nevus. Conclusions: Conjunctival melanocytic lesions suspicious for melanocytoma should be bleached to evaluate their cytologic features. CD68 can be helpful in identifying heavily pigmented melanomacrophages which may mimic a melanocytoma. As conjunctival melanocytomas are extremely rare, their pathogenesis may be different from that of other conjunctival nevi.

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