Investigating Suitability of Different In-House Phantom Materials for Quantification of Transrectal Ultrasound Needle Fidelity for HDR Interstitial Brachytherapy
Abstract
Purpose
To investigate the suitability of three different materials as background media for use with in-house phantoms for HDR interstitial brachytherapy. In-house phantoms are designed to supplement commercial phantom capabilities and enable interstitial needle placement for needle fidelity determinations. Material considerations included ease of use, fiducial visibility, and geometric fidelity.
Methods
3D-printed phantoms incorporating seven nylon monofilament string fiducials arranged at fixed distances (60 mm lateral, 40 mm axial separation between outer strings), and an opening for a transrectal ultrasound probe, were fabricated and filled with one of three different materials: liquid water, agar, or a synthetic medical gel (SimuGel #3, Humimic Medical). Each phantom was imaged using 2D and 3D transrectal ultrasound (TRUS) and distances between the outer fiducials were measured and compared to designed values. The agar phantom was imaged at two temperatures (19.8°C and 8.5°C), while the other phantoms were imaged at room temperature.
Results
Room temperature agar demonstrated the highest geometric fidelity, with all but the most proximal lateral 3D US measurement meeting TG-128 tolerances (3 mm lateral, 2 mm axial). Room temperature water was challenging to work with due to the need to seal the opening for the TRUS probe and exhibited poor resolution, with fiducials appearing as wide streaks. The synthetic gel provides durability advantages since it is shelf-stable and reusable, but exhibited the worst geometric fidelity with all measurements at or exceeding tolerance (2.0-9.3mm) due to the manufacturer reported 1458.85 m/s speed of sound being significantly lower than the fixed 1540 m/s setting of the clinical ultrasound.
Conclusion
Of the materials tested, room temperature agar was the most suitable for achieving geometric fidelity with a clinical ultrasound system. Although agar’s limited shelf life can be extended with refrigeration, it is important to image the phantom at higher temperatures for needle fidelity measurements.