TH‐E‐218‐03: On the Significance of Density‐Induced Speed of Sound Variations on Ultrasound‐Guided Radiotherapy

Purpose: US systems assume that speed of sound (SOS) is constant in human soft tissues (at a value of 1540 m/s), while its actual non homogeneous distribution produces small but systematic errors of up to a few millimeters in the positions of scanned structures. This work aims at showing the effect of SOS aberration on ultrasound guided radiotherapy (US‐gRT) as a function of implemented workflow. Methods: Four scenarios were considered of possible changes between simulation (SIM) and treatment (TX) stage: no changes; patient rigid rotation‐translation; probe position change; patient tissues thicknesses change. The US‐gRT workflows investigated were inter‐modality (INTER), where TX US scans are compared to SIM CT scans; intra‐modality (INTRA), where TX US scans are compared to SIM US scans; and INTERc and INTRAc where US images are corrected for SOS aberration using a SIM CT based algorithm. To simulate the effect of tissues thicknesses change experimentally, a dual layered phantom was filled with 2 cm of water (SOS 1490 m/s) first, and of 20% saline solution (SOS 1600 m/s) after, on top of 6 cm of sunflower oil (SOS 1450 m/s); the phantom was CT and US scanned, the layers thicknesses were reduced to 1 cm and 5 cm respectively, a second US scan was performed in the new configuration and the errors resulting from the different workflows were compared. Results: INTER produces in all scenarios an error proportional to SOS aberration; while INTRA and corrected workflows produce at worst errors proportional to the SOS distribution variation (ΔSOS). The dual layered phantom proved experimentally that the ΔSOS error, even when a substantial change in thicknesses is applied, is a second order effect. Conclusions: Our work suggests that a correction as the one used for this study is reasonably advantageous for all workflows and most of the clinical cases.