Iris melanoma is a rare cancer originating in the pigmented cells (melanocytes) of the iris, the colored part of the eye that regulates light entering the pupil. It is the least common form of uveal melanoma, a cancer of the middle layer of the eye known as the uvea, accounting for only about 4 to 5% of all uveal tumors. While uveal melanoma as a whole has an incidence of about 4.3 cases per million in the United States, the subset involving the iris is far more infrequent.
Identifying the Visible Signs of Iris Melanoma
Iris melanomas often present subtly and can be asymptomatic in their early stages, making routine eye examinations important for detection. The most frequent sign is the appearance of a new, distinct pigmented spot or nodule on the iris surface. This spot may be mistaken for a benign iris nevus, commonly known as an iris freckle, but unlike a freckle, a melanoma often grows or changes shape over time.
A noticeable change in the color of one iris, called acquired heterochromia, can also be a sign, particularly if the change is a darkening or the development of new pigmented areas. As the tumor enlarges, it can mechanically distort the pupil’s shape, known as ectropion uveae. In more advanced cases, the tumor may cause secondary effects, such as increased pressure within the eye, leading to glaucoma, or a small amount of blood in the anterior chamber, known as hyphema. The presence of enlarged blood vessels on the white part of the eye, referred to as sentinel vessels, in the area of the tumor can also raise suspicion.
Understanding the Underlying Causes and Risk Factors
The exact cause of iris melanoma is not fully understood, but its development is linked to genetic changes within the eye’s melanocytes. Several factors increase an individual’s susceptibility to this rare cancer.
Individuals with light-colored eyes, such as blue or green, have a higher risk compared to those with brown eyes, likely due to a lower density of protective pigment. A fair complexion and a tendency to freckle or sunburn easily also correlate with elevated risk. Although the link is not as strong as it is for skin melanoma, high levels of lifetime sun exposure and ultraviolet (UV) radiation may contribute to the risk. The presence of numerous pigmented spots on the iris (iris nevi) is another factor, as these benign lesions are considered potential precursor lesions that can transform into melanoma over time. Genetic predisposition is also implicated, with rare inherited conditions like the BAP1 cancer syndrome increasing the risk for uveal melanoma and other cancers.
Confirmation Through Diagnostic Procedures
Diagnosis begins with a comprehensive eye examination performed by an ophthalmologist, often an ocular oncologist, who uses specialized tools. Slit lamp biomicroscopy provides a highly magnified view of the iris, allowing the specialist to assess the lesion’s size, elevation, color, and border characteristics. To determine the true extent and nature of the lesion, advanced imaging is routinely employed.
Ocular ultrasound, particularly high-frequency ultrasound biomicroscopy (UBM), is critical for measuring tumor thickness and determining if it extends into the ciliary body, which is hidden behind the iris. Gonioscopy uses a specialized lens to examine the drainage angle where the iris meets the cornea, checking for tumor extension or seeding. Angiography, which involves injecting a fluorescent dye, visualizes blood vessels within the iris; the presence of irregular intrinsic vascularity within the lesion is highly suggestive of malignancy.
Serial photography is crucial to establish a baseline and meticulously track the tumor for documented growth over time, which is considered the most important finding to confirm malignancy.
Treatment Options and Long-Term Monitoring
The management of iris melanoma is determined by the tumor’s size, location, and documented evidence of growth. For very small lesions that are stable and not causing any secondary complications like glaucoma, the initial approach may be careful observation with periodic follow-up examinations and photography. The decision to observe is based on the fact that iris melanomas generally carry a lower risk of spreading compared to melanomas in other parts of the eye.
Active treatment is necessary once growth is confirmed or if the tumor is large enough to cause vision problems or elevated eye pressure. Surgical removal is a common option for smaller, well-defined tumors, often performed as an iridectomy, which removes only the affected part of the iris. If the tumor is more extensive, the surgeon may perform an iridocyclectomy, which involves removing a portion of the iris along with the adjacent ciliary body.
Radiation therapy is a highly successful alternative to surgery, with plaque brachytherapy being the most common technique. This involves temporarily placing a small, gold plaque containing radioactive seeds, typically Iodine-125, directly onto the outer wall of the eye over the tumor site to deliver a concentrated dose of radiation. Following treatment, rigorous long-term monitoring is essential to watch for any local recurrence or the rare possibility of the cancer spreading to other parts of the body. This follow-up care often includes total body imaging, such as a PET/CT scan, to screen for metastasis, most commonly to the liver.