Daily quality assurance phantom for ultrasound image guided radiation therapy

Abstract

A simple phantom was designed, constructed, tested and clinically implemented for daily quality assurance (QA) of an ultrasound image guided radiation therapy system, the Restitu Ultrasound system (Resonant Medical, Montreal, QC). The phantom consists of a high signal background gel surrounding a low signal egg shaped target. Daily QA checks involve ultrasound imaging the phantom and segmenting the embedded target using automatic tools available on the Restitu system. This process serves to confirm system hardware and software functioning and, in particular, that target position can be accurately determined. Experiments were conducted to test the stability of the phantom at room temperature, the tissue mimicking properties, the reproducibility of target position measurements and the usefulness as a daily QA device. The phantom proved stable at room temperature, exhibited no evidence of bacterial or fungal invasion in 9 months, experienced limited evaporation (approximately 0.2cm3/month) and was shown to be nearly tissue mimicking. The phantom performs well in a clinical setting as it is light weight and easy to operate and provides reproducible measures of target position even with multiple users. At our center the phantom is being used for daily QA of the Restitu system with clinically acceptable tolerances of ± 1cm3 on the volume of the target and ± 2 mm on target position. For routine daily QA this phantom is a good alternative to the calibration phantom supplied by Resonant Medical and we recommended that this larger phantom be reserved for less frequent, detailed QA checks and system calibration.