Imaging errors in localization of COMS-type plaques in choroidal melanoma brachytherapy.

PURPOSE To investigate mathematically and experimentally the sources of errors in localization of COMS-type plaques in melanoma brachytherapy when using ultrasound and transillumination and to give recommendations to avoid such errors. METHODS A computer-aided simulation tool was developed to model the errors seen in the patient images when using ultrasound imaging and transillumination during localization of the COMS-type plaque in melanoma brachytherapy. Several laboratory experiments were performed on sheep eyes to confirm the validity of the error sources and the simulator outputs. Results were compared to the intraoperative ultrasonographic patient images for validation. RESULTS Based on mathematical modeling and computer simulation results, transillumination provides acceptable accuracy for small to medium tumors with less than 7-mm height but shows poor accuracy as the tumor height increases. In ultrasound imaging, two sources of error are predicted in longitudinal scan using our computer simulation modeling, and the results are supported by the experiments on sheep eyes and patient images taken during the operation. Both errors are related to the lens. The first error is due to attenuation and refraction of the lens causing opaque areas and a change of curvature in the image of the plaque edge that is nearest to the lens. The second type of error is related to the total internal reflection from the lens. CONCLUSIONS The simulation method presented in this article allows quantitative assessment of the sources, mechanisms, and measures of errors in localization of the CMOS-type plaques by ultrasound and transillumination techniques. Errors predicted by the simulation method are validated by experiments. Error assessment by this method provides guidelines for avoiding such errors.