KEYWORDS: Breast, 3D image processing, Ultrasonography, System integration, Imaging systems, 3D scanning, Radiotherapy, Breast cancer, Image guided radiation therapy
Permanent breast seed implantation (PBSI) is a single visit accelerated partial breast irradiation method that uses needles inserted via a template to distribute Pd-103 radioactive seeds with two-dimensional (2D) ultrasound (US) guidance. This guidance approach is limited by its dependence on the operator and average seed placement errors greater than benchmark values established by dosimetric studies. We propose the use of a three-dimensional (3D) US imaging approach for needle guidance with integrated template tracking. We previously described the preliminary development and validation of the 3D US mechatronic system. The present work demonstrates the accuracy of the integrated system by quantifying agreement between tracking and imaging sub-systems and its use guiding a phantom procedure. Tracking error was measured by inserting a needle a known distance through the template and comparing expected tip position from tracking to observed tip position from imaging. Mean ± standard deviation differences in needle tip position and angle were 2.90 ± 0.76 mm and 1.77 ± 0.98°, respectively, validating the needle tracking accuracy of the developed system. The system was used to guide 15 needles into a patient-specific phantom according to the accompanying treatment plan and micro-CT images taken before and after to evaluate placement accuracy. Seed positions were modelled using needle positions and the resulting dosimetry compared to a procedure specific benchmark. The mean tip difference was 2.08 mm while the mean angular difference was 2.6°, resulting in acceptable dosimetric coverage. These results demonstrate 3D US as a potentially feasible technique for PBSI guidance.
KEYWORDS: Ultrasonography, Breast, 3D scanning, 3D image reconstruction, Scanners, Image guided radiation therapy, Breast cancer, 3D acquisition, 3D metrology, 3D image processing, Imaging systems
Permanent breast seed implantation (PBSI) is a single-visit technique for accelerated partial breast irradiation that uses a
template and needles to implant seeds of Pd-103 under 2D ultrasound (US) guidance. The short treatment time is
advantageous given the widely hypothesized link between treatment burden and mastectomy use. However, limitations
of 2D US contribute to high operator dependence and seed placement error that we aim to address by developing a 3D
US guidance system.
A 3D US scanner for PBSI and a mechanism for template localization have been developed and validated. The 3D US
system mechatronically moves and tracks a 2D US transducer over a 5 cm translation and 60° tilt, reconstructing the 2D
images into a 3D volume as they are acquired. Additionally, a localizing arm, tracked via encoded joints and mounted to
the scanner, determines template position by localizing divots on a modified needle template.
Volume reconstruction was validated using linear measurements of a grid phantom and volumetric measurements of two
surgical cavity phantoms. Localizing arm measurement accuracy was established using a testing jig with divots at known
positions. Imaging volume was rigidly registered to scanner geometry using a string phantom mounted to a test jig.
Lastly, volunteer scans were conducted to demonstrate clinical applicability.
Median linear and average volumetric measurements were within ±1.4% of nominal and ±4.1% of water displacement
measurements, respectively. Median measurement accuracy of the localizing arm was 0.475 mm. Imaging volume target
registration error was 0.458 mm. Volunteer scans produced clinical quality images.
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