Iris melanomas account for 3%–5% of all uveal melanomas (Fig 17-2; see p. 307). Small melanomas of the iris may be challenging to clinically differentiate from benign iris nevi.
Iris melanomas range in appearance from amelanotic (off-white) to dark-brown lesions, and three-quarters of them involve the inferior iris (see Fig 17-2A, C, D). In rare cases, their growth pattern is diffuse, resulting in unilateral acquired hyperchromic heterochromia (darker iris) and secondary glaucoma. One subtype grows in a pattern that resembles tapioca pudding (see Fig 17-2D, F).
Clinical evaluation of iris melanoma is identical to that for iris nevi. Signs suggesting malignancy include large size, prominent ectropion uveae and intratumoral vascularity (see Fig 17-2D), sectoral cataract, secondary glaucoma, seeding of the iris stroma (see Fig 17-2B) and peripheral angle structures, extrascleral extension, and documented progressive growth. The differential diagnosis of iris melanoma is listed in Table 17-1 and pictured in Figure 17-3.
Advances in high-frequency ultrasonography (echography) enable excellent characterization of iris tumor size and anatomical relationship to normal ocular structures (see Figs 17-1G, H and 17-2G, H). Anterior segment optical coherence tomography (OCT) may be helpful, but commercially available instrumentation often does not penetrate to the level of the posterior iris. Iris fluorescein angiography may document intrinsic vascularity; however, this finding is of limited value in narrowing the differential diagnosis. Biopsy may be considered when the diagnosis is not clear.
When there is documented growth or secondary glaucoma occurs, diagnostic/therapeutic excision of an iris melanoma is indicated. Alternatively, brachytherapy (radioactive plaque) or proton beam therapy may be used, particularly for larger lesions.
The prognosis for most patients with iris melanoma is excellent; the mortality rate is low (1%–4%), possibly because the tumor is usually small and likely because its biological behavior is distinct from that of ciliary body and choroidal melanomas. The main risk factor for metastatic death is invasion of the anterior chamber angle, which may present as poorly controlled glaucoma, mimicking pigmentary glaucoma.
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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.