HOW TO MEASURE THE LARGEST BASAL DIMENSION OF CHOROIDAL MELANOMA: A MATHEMATICAL STUDY.

Purpose: To identify the most accurate ultrasonographic technique to measure the largest basal dimension (LBD) of choroidal melanoma. Methods: B-scan ultrasound images were retrospectively reviewed in 99 eyes of 99 choroidal melanoma patients. The LBD was measured using one, two, and three straight lines along the inner and outer sclera. Theoretical arc length, calculated using trigonometry formulas based on the spherical model with the axial length as the sphere diameter, was used for comparisons with the actual measurements using straight lines. Results: For straight-line measurements in the inner sclera, the lowest error was found when using two straight-line measurements (P = 0.118). Differences in measurement using one-segment or three-segment measurements as compared with the theoretical arc length were found to be statistically significant (P < 0.001 in both cases). For tumors with LBD smaller than 12 mm, the absolute error, compared with the theoretical arc length, was smaller than 1 mm. In the outer sclera, the smallest errors were also found for measurements using two straight-line segments; however, it was statistically different than the theoretical inner arc length (P < 0.001). Conclusion: When using ultrasound to estimate LBD of ocular tumors, 2 straight-line measurements should be used when LBD is larger than 12 mm. For tumors with LBD smaller than 12 mm, measurements using 1 straight-line segment can provide accurate estimates.