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Monte Carlo Modeling of Ultrasound Probes for Real-Time Ultrasound Image-Guided Radiotherapy

D Hristov

D Hristov1*, J Schlosser1 , M , J Chen2 ,Bazalova1, (1) Stanford Universtiy, Stanford, CA, (2) UCSF Comprehensive Cancer Center, Lafayette, CA


SU-E-J-205 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantify the effect of ultrasound (US) probe beam attenuation for radiation therapy delivered under real-time US image guidance by means of Monte Carlo (MC) simulations.

Methods: MC models of two Philips US probes, an X6-1 matrix-array transducer and a C5-2 curved-array transducer, were built based on their CT images in the EGSnrc BEAMnrc and DOSXYZnrc codes. Due to the metal parts, the probes were scanned in a Tomotherapy machine with a 3.5 MV beam. Mass densities in the probes were assigned based on an electron density calibration phantom consisting of cylinders with mass densities between 0.2-8.0 g/cm³. Beam attenuation due to the probes was measured in a solid water phantom for a 6 MV and 15 MV 15x15 cm² beam delivered on a Varian Trilogy linear accelerator. The dose was measured with the PTW-729 ionization chamber array at two depths and compared to MC simulations. The extreme case beam attenuation expected in robotic US image guided radiotherapy for probes in upright position was quantified by means of MC simulations.

Results: The 3.5 MV CT number to mass density calibration curve was found to be linear with R² > 0.99. The maximum mass densities were 4.6 and 4.2 g/cm³ in the C5-2 and X6-1 probe, respectively. Gamma analysis of the simulated and measured doses revealed that over 98% of measurement points passed the 3%/3mm criteria for both probes and measurement depths. The extreme attenuation for probes in upright position was found to be 25% and 31% for the C5-2 and X6-1 probe, respectively, for both 6 and 15 MV beams at 10 cm depth.

Conclusion: MC models of two US probes used for real-time image guidance during radiotherapy have been built. As a result, radiotherapy treatment planning with the imaging probes in place can now be performed.

Funding Support, Disclosures, and Conflict of Interest: J Schlosser is an employee of SoniTrack Systems, Inc. D Hristov has financial interest in SoniTrack Systems, Inc.

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