An iris nevus is a localized proliferation of melanocytic cells that generally appears as a darkly pigmented lesion of the iris stroma with minimal distortion of the iris architecture (see Chapter 17, Fig 17-1).
Figure 12-14 Iris nevus. Spindle-shaped nevus cells (arrows) form a plaque on the surface of the iris and extend downward into the iris stroma just anterior to the pigmented epithelium (asterisk).
(Courtesy of Michele Bloomer, MD.)
An iris nevus appears histologically as a combination of, or an accumulation of, any of the following:
All of these cell types usually contain melanin granules in the cytoplasm. The nuclei of these cells are typically oblong or ovoid with a bland appearance and indistinct nucleoli. Less commonly, epithelioid nevus cells may be observed. A variety of growth patterns and cytologic appearances are possible, but cellular atypia and significant mitotic activity are not present. The nevus cells aggregate within the stroma and sometimes also appear as a plaque on the surface of the iris (Fig 12-14). Occasionally, nevus cells may extend into the adjacent angle structures.
Melanomas arising in the iris tend to follow a relatively nonaggressive clinical course, in contrast to posterior (ciliochoroidal) melanomas. Most iris melanomas develop in the inferior sectors of the iris (see Chapter 17, Fig 17-3). The lesions can be quite vascularized and may occasionally cause spontaneous hyphema.
Iris melanomas can be composed of spindle melanoma cells, epithelioid melanoma cells, or a combination of these. Histologically, spindle cells possess plump, spindle-shaped nuclei that have a coarse, granular appearance and prominent nucleoli. These cells are the equivalent of spindle-B cells in posterior uveal melanoma (see the “Melanoma” section under Choroid and Ciliary Body). Epithelioid cells are polyhedral, with large, round nuclei that have a clumped chromatin pattern and prominent eosinophilic nucleoli. Both types of cells tend to have a high nuclear-to-cytoplasmic ratio. The cytoplasm of melanoma cells can range from amelanotic to heavily pigmented.
Typically, iris melanomas grow as a solid mass in the stroma, sometimes covered by a surface plaque. Occasionally, the tumor demonstrates satellite lesions or a diffuse growth pattern and replaces normal iris stroma (Fig 12-15). In some cases, iris melanoma may be seen arising from a nevus on histology. The modified Callender classification for posterior melanomas (see “Melanoma” section under Choroid and Ciliary Body) is not applicable to iris melanomas in terms of prognostic significance. Iris melanomas are classified by a separate staging system, one based on infiltration of adjacent structures and the presence or absence of coexisting glaucoma. In some cases, distinguishing between iris nevus and melanoma histologically can be challenging and requires the expertise of an experienced ophthalmic pathologist.
Figure 12-15 Iris melanoma. A, Clinical photograph. The pigmented tumor is seen between the 10:30 and 2:00 clock-hours. B, Gross appearance of a pigmented iris mass (between arrows).C, Low magnification shows the melanoma completely replacing the normal iris stroma, extending into the anterior chamber, touching the posterior cornea, and occluding the angle. D, Histologic examination shows numerous plump epithelioid melanoma cells containing prominent nucleoli (arrowheads).
(Courtesy of Hans E. Grossniklaus, MD.)
Although iris melanomas may grow in a locally aggressive fashion, they rarely metastasize. One exception occurs when melanomas grow to diffusely involve the entire iris stroma. This diffuse form of melanoma may invade the anterior chamber angle and extend posteriorly to involve the ciliary body.
Kivelä T, Simpson ER, Grossniklaus HE, et al. Uveal melanoma. In: Amin MB, Edge SB, Greene FL, eds. AJCC Cancer Staging Manual. 8th ed. Springer; 2017:805–817.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.