Uveal melanoma most often assumes a nodular or dome-shaped configuration, but occasionally tumors can be flat or diffuse and involve extensive areas of the uvea with little elevation.
Tumor size classifications according to boundary lines used in a Collaborative Ocular Melanoma Study (COMS) are as follows:
- Small: Range from 1.0 mm to 3.0 mm in apical height and largest basal diameter of 5.0 to 16.0 mm.
- Medium: Range from 3.1 to 8.0 mm in apical height and a basal diameter of not more than 16.0 mm.
- Large: Greater than 8.0 mm in apical height or a basal diameter more than 16.0 mm, when the apical height is at least 2.0 mm.
Although most ocular melanomas have a raised configuration, about 5% grow in a diffuse pattern that also may have prognostic significance. The tumors have a horizontal, flat-growth pattern, with the thickness measuring approximately 20% or less than the greatest basal dimension. This uncommon variant of uveal melanoma seems to have a poorer prognosis, particularly when the diameter is large, and the margins are poorly defined.
In clinical practice, the tumor base may be estimated in average optic disc diameters (1 dd = 1.5 mm). The average elevation may be estimated in diopters (3 diopters = 1 mm). Other techniques, such as ultrasonography, should be used to provide more accurate measurements.
An important function of ophthalmic ultrasonography is the detection of extrascleral extension.[4,5] Extrascleral extension measuring 2 mm or more in thickness can be demonstrated provided it is located behind the equator where the intraocular tumor, sclera, and adjacent orbital fat are readily imaged. Orbital extraocular extension of choroidal melanoma may be found in eyes with medium and large tumors, but it is very rare in eyes with small melanomas.
Systemic metastases are evident in only 1% to 4% of patients at the time of diagnosis of the primary ocular melanoma. Because the uveal tract is a vascular structure without lymphatic channels, tumor spread occurs principally by local extension and by dissemination through the blood stream. Lymphatic spread is rare but may occur after local extension into the conjunctiva and its lymphatics. Given the rarity of nodal metastases, sentinel node biopsies of nonclinically involved nodes are not done as part of the staging procedure.