Conversion Factor for B-Scan Ultrasound Measurement of Intraocular Tumors With and Without the Sclera

To investigate the relationship between choroidal nevus and melanoma thickness measured with or without the sclera included by B-scan ultrasound and to present a simple conversion formula.

Medical records were retrospectively reviewed for choroidal nevus or melanoma evaluated at the Mayo Clinic in Rochester, Minnesota, with B-scan ultrasound between February 4, 2004, and April 23, 2020. Charts were retrospectively reviewed for high-quality B-scan images in which the ultrasound transducer was perpendicular to the lesion, measuring the tumor thickness without the sclera included. Measurements were repeated with the sclera included for each patient. Univariate and multiple linear regression analyses were performed to identify factors correlated with scleral thickness.

There were 201 tumors included in the study, with a mean patient age ± SD of 61 ± 14 years, largest tumor basal diameter of 11.8 ± 4.8 mm, tumor thickness without the sclera included of 3.72 ± 2.7 mm, and thickness with the sclera included of 4.54 ± SD 2.8 mm. On the univariate analysis, factors associated with perceived scleral thickness by B-scan ultrasound included age (P < .001), tumor thickness (P < .001), and basal diameter (P = .06). On the multivariate analysis, factors associated with perceived scleral thickness included age and tumor thickness (P < .001) for all tumors and for the subset of 141 tumors with a thickness of 2 mm or greater (P < .001). For tumors of 2 mm or greater in thickness, perceived scleral thickness by ultrasound can be estimated by the formula 0.00495(patient age) + 0.02451(tumor thickness without the sclera) + 0.42549.

We present a simple formula for converting between B-scan ultrasound measurements of choroidal nevus and melanoma measuring 2 mm or greater in thickness with and without the sclera included.