Daily quality assurance phantom for ultrasound image guided radiation therapy

A simple phantom was designed, constructed, tested, and clinically implemented for daily quality assurance (QA) of an ultrasound-image-guided radiation therapy (US-IGRT) system, the Restitu Ultrasound system (Resonant Medical, Montreal, QC). The phantom consists of a high signal echogenic background gel surrounding a low signal hypoechoic egg-shaped target. Daily QA checks involve ultrasound imaging of the phantom and segmenting of the embedded target using the automated tools available on the US-IGRT system. This process serves to confirm system hardware and software functions and, in particular, accurate determination of the target position. Experiments were conducted to test the stability of the phantom at room temperature, its tissue-mimicking properties, the reproducibility of target position measurements, and the usefulness of the phantom as a daily QA device. The phantom proved stable at room temperature, exhibited no evidence of bacterial or fungal invasion in 9 months, and showed limited desiccation (resulting in a monthly reduction in ultrasound-measured volume of approximately 0.2 cm3). Furthermore, the phantom was shown to be nearly tissue-mimicking, with speed of sound in the phantom estimated to be 0.8% higher than that assumed by the scanner calibration. The phantom performs well in a clinical setting, owing to its light weight and ease of operation. It provides reproducible measures of target position even with multiple users. At our center, the phantom is being used for daily QA of the US-IGRT system with clinically acceptable tolerances of +/-1 cm3 on target volume and +/-2 mm on target position. For routine daily QA, this phantom is a good alternative to the manufacturer-supplied calibration phantom, and we recommended that that larger phantom be reserved for less frequent, more detailed QA checks and system calibration.