Simulation and experimental detection of radiation-induced acoustic waves from a radiotherapy linear accelerator

Abstract

Acoustic waves are generated in objects irradiated with megavoltage photon beams produced by a clinical linear accelerator. The detection of such induced acoustic waves (IAWs) has potential applications in radiation therapy dosimetry. This work developed a novel simulation platform to model IAWs by combining radiotherapy Monte Carlo and acoustic wave transport simulation techniques. Simulations and experimental measurements were performed to assess the IAWs when a lead block suspended in a water tank was irradiated with a photon beam produced by a clinical linear accelerator. Simulations correctly predicted the frequency of such IAWs, which allowed improved experimental detection through the development of a custom band-pass filter and signal amplification system. Photon beam energy, lead block depth, and lead block distance from the transducer were varied, and experimental and simulated signals showed the same trends for such set-up changes. These results demonstrate the ability of our simulation platform to accurately predict the frequency spectrum and relative amplitude trends of IAWs. The simulation platform can be applied to assess IAWs in clinically relevant radiotherapy dosimetry situations.

Photoacoustic dose monitoring in clinical high-energy photon beams

“…For lead block to transducer distances above 55 mm, signal arrival time and first pulse duration are in agreement with the simulated waves. For shorter distances, as observed by other authors (Hickling et al 2014a), unfiltered electromagnetic noise and reflected waves distort the shape of the pulses.…”

“…The proposed setup can detect acoustic waves originated from dose with µs-long pulses. As shown by other authors (Xiang et al 2013b, Hickling et al 2014a, lead amplification can increase detection limits and facilitate the developing and testing of detector setups. While the thermal noise at the transducer should not limit the accuracy of the system significantly (Ahmad et al 2015), the presence of strong electromagnetic and radiofrequency fields in a radiotherapy treatment room makes it necessary to use very low noise electronics (Diao et al 2015, Jones et al 2015, characterize thoroughly the background signals associated with the acceleration of the beam to perform a dry run for background subtraction (Hickling et al 2018a) and average a high number of samples to isolate the acoustic signal.…”

“…While the proposed setup is similar to the suggested in previous works (Diao et al 2015, Hickling et al 2014, the post-processing of the signals (background subtraction in dry run + filter by frequency response of the system) is novel and allows for a cleaner measurement of the acoustic waves. In systems where this dry run mode is not available, it could be achieved by delivering the radiation fields with the jaws closed or away from the volume of interest.…”

“…For simplicity reasons, in both cases the complex energy spectrum of the 6 MV photon beam was approximated by a monoenergetic beam of 2 MeV. This approach has been used by other authors (Hickling et al 2014a) and was validated by a previous study (Giza 2017), where no relevant differences were found on the pulse shape or arrival times of signals induced by monoenergetic photon beams in the 0.5-6 MeV energy range.…”

Simple analytic model for radiotherapeutic X-ray induced acoustic signal as a function of absorption parameters

“…On the other hand, detection by immersion ultrasound transducer with a sufficiently large bandwidth so that frequency dependence of the absorption can be ignored is an priori assumption of our model. This is a reliable assumption considering the sharp peak of the therapeutic XA signal at a central frequency 8 .…”