An iris melanoma is part of the group of uveal tumors, which make up one of the most common eye tumors. These arise from the pigmented tissue inside the eye called the uvea. Iris melanomas are the least common among this type of tumor, comprising 4-5 per hundred uveal tumors.
The incidence of uveal tumors, put together, is 4.3 cases per million in the US. Males are at a slightly higher risk than females. The incidence rises with age and the age at which it is highest is 70 years.
The uvea consists of the iris, ciliary muscle, and choroid. The iris is composed of circular and longitudinal layers of muscle that surround a central opening called the pupil. This aperture is crucial in regulating the amount of light that enters the eye. While the circular muscle contraction tightens the opening, the longitudinal muscle layer dilates it in dim light.
The iris gets its vast range of coloration from the pigment called melanin, which is present in specialized pigment-producing cells called melanocytes. It is these cells which give rise to the tumor called melanoma.
Risk factors for an iris melanoma include fair skin, red or blond hair, and blue or green eyes. All of these indicate a relative poverty of melanocytes.
It is also seen to have an increased incidence in those who have the following conditions:
- Atypical mole syndrome
- Dysplastic nevus syndrome
- Ocular or oculodermalmelanocytosis
Early melanomas are silent, or asymptomatic. The iris may show a dark spot, or the pupillary opening may be distorted by the presence of a melanoma. In contrast to melanomas elsewhere in the uveal tract, iris tumors are slow-growing or may even appear dormant, and are small in size.
Diagnosis and Treatment
Especially with small melanomas of the iris, it is difficult to distinguish it from benign conditions. The course of management recommended in such cases may be to follow up the mole for progressive growth.
If an iris melanoma is small and has well-defined margins, it can be removed surgically, in a procedure named iridectomy or iridocyclectomy. Either the part of the iris which has the lesion, or the affected iris with part of the ciliary muscle, is excised. Local resection is suitable for small lesions, and those without extraocular or ciliary body involvement.
In more advanced cases, radioactive plaques are sutured to the sclera to destroy the tumorous cells. These consist of a gold carrier, with radioactive metal pellets or ‘seeds’. In other patients, enucleation may be required.
Among all uveal tumors, iris melanomas have the best prognosis, with the mortality being 2-3% in cases with metastasis. It depends on factors such as:
- Cell type: spindle-A cell type melanomas have the best outcome, while epithelioid cell types show the poorest prognosis. Increased vascularity, heterogeneity of pigmentation and rapid growth indicate an epithelioid type tumor.
- Tumor size > 3mm wide or 1 mm thick were suspicious for malignancy
- Ciliary body extension: when the iridocorneal angle showed the presence of tumor the ciliary body was likely to be involved.
- Tumor-related ocular signs and symptoms like increased intraocular pressure
- Satellite lesions
- Presence of lymphocytic infiltration and increased mitotic spindles on histology