Primary melanocytic orbital tumors include melanoma, melanocytic hamartoma, and melanotic neuroectodermal tumor (MNET) of infancy (retinal anlage tumor). Primary orbital melanoma usually originates from congenital ocular melanocytosis or hypercellular blue nevus that affects the orbital tissue (1,2,3,4,5,6,7,8,9,10). Rarely, it can arise from the optic nerve (9) or after orbital irradiation for rhabdomyosarcoma (10). Melanoma from melanocytosis and from blue nevus are similar and are grouped together here. In the authors’ review of personal cases, the 10 primary orbital melanomas accounted for 1% of all orbital tumors (2).

The underlying congenital melanocytic lesion may be evident anteriorly as ocular melanocytosis or blue nevus, but it is often subclinical in the orbit more posteriorly, until it spawns a melanoma later in life. The melanoma that arises from blue nevus or congenital melanocytosis is generally circumscribed, even though the underlying congenital pigmentation is diffuse or patchy. Proptosis in a patient with either congenital ocular melanocytosis or episcleral blue nevus should arouse suspicion for a primary orbital melanoma or orbital extension of uveal melanoma.

Computed tomography (CT) and magnetic resonance imaging (MRI) show a circumscribed, enhancing mass in orbital soft tissue, usually in the extraconal space, sometimes in an extraocular muscle. MRI may help to detect the melanin content in the lesion. With time, the tumor can breach its pseudocapsule and diffusely invade the orbit. As part of a diagnostic workup for proptosis, the clinician should inspect the eyelid skin and episclera to look for a blue nevus and perform ophthalmoscopy to rule out uveal melanoma.

Grossly and at surgery, orbital melanoma is generally a brown or black circumscribed mass. Microscopically, it is composed of spindle or epithelioid melanoma cells. In many cases, residual areas of cellular blue nevus can be identified. Extensive tumor necrosis is common. Immunohistochemistry demonstrates a positive reaction to melanoma-specific antigens.

Because orbital melanoma is usually well circumscribed, an attempt should be made to remove the entire tumor intact. An incisional biopsy of a circumscribed orbital mass in the setting of congenital ocular melanocytosis is contraindicated if there is a chance of removing the tumor intact. We believe that orbital melanoma is more likely to recur locally or metastasize to distant organs if it is not removed intact. The surrounding flat congenital pigment should be examined at surgery, biopsied, and treated with heavy cryotherapy. Residual or recurrent orbital melanoma should usually be managed by eyelid-sparing orbital exenteration.

Wilkes SR, Uthman EO, Thornton CN, et al. Malignant melanoma of the orbit in a black patient with ocular melanocytosis. Arch Ophthalmol 1984;102:904-906.