Technical Note: The design, construction, and evaluation of a liquid-based single phantom solution for TG128 brachytherapy ultrasound QA

Purpose: Since the publication of the AAPM TG128 report for the quality assurance (QA) of prostate brachytherapy ultrasound systems, no commercially available phantoms have been developed which satisfy all of the task group recommendations. Current solid phantoms require a separate user-implemented setup using a container with liquid medium to evaluate the alignment between the needle template and the electronic grid, a test of geometric accuracy with critical implications in dosimetric quality. Utilizing a 3D printer, we constructed a cost-effective, liquid-based phantom that provides a complete TG128 solution which improves the efficiency of brachytherapy ultrasound QA. Methods: The TG128 report was used to guide the design process of the liquid-based phantom. The needle template and electronic grid alignment setup served as the foundation with specific components developed to integrate all remaining tests. Water was chosen as the liquid medium, with speed of sound adjusted to 1,540 m/s via salinity per the task group recommendations. The proof of concept was evaluated by comparing the time stamps labeled on QA images between the liquid-based phantom and a commercially available one for both a new and experienced user. Results: A TG128 QA trial run demonstrated that all recommended tests can be completed with the single phantom setup. Evaluation of the time data revealed a total QA duration of 45 min (average of two trials) with the liquid-based phantom, compared to 70 and 90 min with the commercial phantom for a new and experienced user. Conclusions: The liquid-based phantom is specifically designed to satisfy the recommendations of the TG128 report. The incorporation of 3D printing allows simple design modifications to adapt the phantom on-the-fly if needed. The resulting product improves the efficiency of brachytherapy ultrasound QA by eliminating the need for multiple phantom setups.