A patient-specific three-dimensional couplant pad for ultrasound image-guided radiation therapy: A feasibility study

Background: A wide application of ultrasound for radiation therapy has been hindered by a few issues such as skin and target deformations due to probe pressure, optical tracking disabilities caused by irregular surfaces and inter-user variations. The purpose of this study was to overcome these barriers by using a patient-specific three-dimensional (3D) couplant pad (CP). Methods: A patient skin mold was designed using a skin contour of simulation CT images and fabricated by a 3D printer. A CP was then casted by pouring gelatin solution into a container accommodating the mold. To validate the use of the CP in positioning accuracy and imaging quality, phantom tests were carried out in our ultrasound-based localization system and then daily ultrasound images of four patients were acquired with and without the CP before treatment. Results: In the phantom study, the use of CP increased a contrast-to-noise ratio from 2.4 to 4.0. The positioning accuracies in the US scans with and without the CP were less than 1 mm in all directions. In the patient study, the use of CP decreased the centroid offset of the target volume after target position alignment from 4.4 mm to 2.9 mm. One patient with a small volume of target showed a substantial increase in the inter-fractional target contour agreement (from 0.07 (poor agreement) to 0.31 (fair agreement) in Kappa values) by using the CP. Conclusions: Our patient-specific 3D CP based on a 3D mold printing technique not only maintained the tracking accuracy but also reduced the inter-user variation, as well as that could potentially improve detectability of optical markers and target visibility for ultrasound image-guided radiotherapy.