Patterns of tumor initiation in choroidal melanoma.

This study attempts to document the occurrence of tumors with respect to clock hour location and distance from the macula and to evaluate tumor location in relation to retinal topography and light dose distribution on the retinal sphere. Analysis of patterns of tumor initiation may provide new evidence to clarify the controversy regarding the possible light-related etiology of choroidal melanoma. Incident cases of choroidal and ciliary body melanoma in Massachusetts residents diagnosed between 1984 and 1993 were the basis for analysis. Conventional fundus drawings and photos were used to assess the initiation site of each tumor. The initiation site was defined as the intersect between the largest tumor diameter and the largest perpendicular diameter of the tumor. Initiation sites were recorded using spherical coordinates. The retinal sphere was divided into 61 mutually exclusive sectors defined according to clock hour and anteroposterior distance from the macula. Rates of initiation were computed for each sector, overall, and according to gender and other clinical factors. Results were similar in left and right eyes; therefore, these were combined in analysis. Tumor initiation had a predilection for the macula (P < 0.0001). Overall, no significant clock hour preference was observed (P = 0.63). However, the parafoveal zone showed a strong circular trend (P < 0.01), with highest rates occurring in the temporal region, and the lowest rates occurring in the nasal region. Rates of occurrence in six progressively more anterior concentric zones (designated as the foveal, parafoveal, posterior, peripheral, anterior, and ciliary body zones) were 21.4, 14.2, 12.1, 8.9, 4.5, and 4.3 counts per spherical unit per 1000 eyes, respectively. Concentric zone location did not vary by gender (P = 0.93) or laterality (P = 0.78). However, posterior location was associated with light iris color (P = 0.01). Tumor diameters were largest in the peripheral region of the fundus and smallest in the macular and ciliary body zone (P < 0.001). Clock hour location was not influenced by gender (P = 0.74), laterality (P = 0.53), iris color (P = 0.84), or tumor diameter (P = 0.73). Results suggest that tumor initiation is not uniformly distributed, with rates of occurrence concentrated in the macular area and decreasing monotonically with distance from the macula to the ciliary body. This pattern is consistent with the retinal topography and correlates positively with the dose distribution of solar light on the retinal sphere.