Feasibility of pulsed proton acoustics for 3D dosimetry
Proton therapy has the potential to deposit its energy in tissue with high conformity to the tumor and significantly reduced integral dose to normal tissue compared to conventional radiation, such as x-rays. As a result, local control can be enhanced while reducing side-effects and secondary cancers. This is due to the way charged Particles deposit their energy or dose, where protons form a Bragg peak and establish a well-defined distal edge as a function of depth (range). To date, the dose delivered to a patient from proton therapy remains uncertain, in particular the positioning of the distal edge of the Bragg peak and the lateral displacement of the beam. The need for quality assurance methods to monitor the delivered dose during proton therapy, in particular intensity modulated proton therapy (IMPT) is critical. We propose to measure the acoustic signal generated from the deposited energy from ionizing radiation, in particular a proton beam; and to investigate the feasibility of ultrasound tomographic imaging to map the three dimensional dose (3D) dose from a proton pencil beam. A pulsed proton beam in water was simulated using Monte Carlo (MC) methods, and the pressure signal resulting from the deposited dose was simulated based on the thermoacoustics wave. A cylindrical scanner design with 71 ultrasound transducers focused to a centeral point within the scanner was utilized. Finally, a 3-D filtered backprojection algorithm was developed to reconstruct computed tomographic images of the deposited dose. The MC dose profile was compared to the radioacoustic reconstructed images, and the dependency of the proton pulse sequence parameters, pulse width (tPW) and rise time ( Δ t), on sensitivity were investigated. Based on simulated data, the reconstructed radioacoustic image intensity was within 2%, on average, of the MC generated dose within the Bragg peak, and the location of the distal edge was within 0.5mm. The simulated pressure signal for different tPW and Δ t for the same number of protons (1.8x107) demonstrated that compressing the protons in a shorter period of time significantly increased the thermoacoustic signal and thus sensitivity. This study demonstrates that computed tomographic scanner based on ionizing radiation induced acoustics can be used to verify dose distribution and proton range. Realizing this technology into the clinic will have significant impact on treatment verification during particle beam therapy and image guided techniques.
Stantz, Purdue University.
Biomedical engineering|Medical imaging|Acoustics|Oncology
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