Technical note: Application of an optical hydrophone to ionoacoustic range detection in a tissue-mimicking agar phantom.

Background: Ionoacoustics is a promising approach to reduce the range uncertainty in proton therapy. A miniature-sized optical hydrophone (OH) was used as a measuring device to detect weak ionoacoustic signals with a high signal-to-noise ratio in water. However, further development is necessary to prevent wave distortion because of nearby acoustic impedance discontinuities while detection is conducted on the patient's skin.

Ionoacoustic application of an optical hydrophone to detect proton beam range in water.

Background: Proton range uncertainty has been the main factor limiting the ability of proton therapy to concentrate doses to tumors to their full potential. Ionoacoustic (IA) range verification is an approach to reducing this uncertainty by detecting thermoacoustic waves emitted from an irradiated volume immediately following a pulsed proton beam delivery; however, the signal weakness has been an obstacle to its clinical application. To increase the signal-to-noise ratio (SNR) with the conventional piezoelectric hydrophone (PH), the detector-sensitive volume needs to be large, but it could narrow the range of available beam angles and disturb real-time images obtained during beam delivery.

Technical Note: Range verification of pulsed proton beams from fixed-field alternating gradient accelerator by means of time-of-flight measurement of ionoacoustic waves.

Purpose: Ionoacoustics is one of the promising approaches to verify the beam range in proton therapy. However, the weakness of the wave signal remains a main hindrance to its application in clinics. Here we studied the potential use of a fixed-field alternating gradient accelerator (FFA), one of the accelerator candidates for future proton therapy.

On-line range verification for proton beam therapy using spherical ionoacoustic waves with resonant frequency.

In contrast to conventional X-ray therapy, proton beam therapy (PBT) can confine radiation doses to tumours because of the presence of the Bragg peak. However, the precision of the treatment is currently limited by the uncertainty in the beam range. Recently, a unique range verification methodology has been proposed based on simulation studies that exploit spherical ionoacoustic waves with resonant frequency (SPIREs).

A novel range-verification method using ionoacoustic wave generated from spherical gold markers for particle-beam therapy: a simulation study.

This study proposes a novel alternative range-verification method for proton beam with acoustic waves generated from spherical metal markers. When proton beam is incident on metal markers, most of the resulting pressure waves are confined in the markers because of the large difference in acoustic impedance between the metal and tissue. However, acoustic waves with frequency equal to marker's resonant frequency escape this confinement; the marker briefly acts as an acoustic transmitter.

A simplified Monte Carlo algorithm considering large-angle scattering for fast and accurate calculation of proton dose.

Purpose: The purpose of this study is to improve dose calculation accuracy of the simplified Monte Carlo (SMC) algorithm in the low-dose region. Because conventional SMC algorithms calculate particle scattering in consideration of multiple Coulomb scattering (MCS) only, they approximate lateral dose profiles by a single Gaussian function. However, it is well known that the low-dose region spreads away from the beam axis, and it has been pointed out that modeling of the low-dose region is important to calculated dose accurately.

Dual ring multilayer ionization chamber and theory-based correction technique for scanning proton therapy.

Purpose: To develop a multilayer ionization chamber (MLIC) and a correction technique that suppresses differences between the MLIC and water phantom measurements in order to achieve fast and accurate depth dose measurements in pencil beam scanning proton therapy.

Methods: The authors distinguish between a calibration procedure and an additional correction: 1-the calibration for variations in the air gap thickness and the electrometer gains is addressed without involving measurements in water; 2-the correction is addressed to suppress the difference between depth dose profiles in water and in the MLIC materials due to the nuclear interaction cross sections by a semiempirical model tuned by using measurements in water. In the correction technique, raw MLIC data are obtained for each energy layer and integrated after multiplying them by the correction factor because the correction factor depends on incident energy.

Evaluation of the influence of double and triple Gaussian proton kernel models on accuracy of dose calculations for spot scanning technique.

Purpose: The main purpose in this study was to present the results of beam modeling and how the authors systematically investigated the influence of double and triple Gaussian proton kernel models on the accuracy of dose calculations for spot scanning technique.

Methods: The accuracy of calculations was important for treatment planning software (TPS) because the energy, spot position, and absolute dose had to be determined by TPS for the spot scanning technique. The dose distribution was calculated by convolving in-air fluence with the dose kernel.

Development and evaluation of a short-range applicator for treating superficial moving tumors with respiratory-gated spot-scanning proton therapy using real-time image guidance.

Treatment of superficial tumors that move with respiration (e.g. lung tumors) using spot-scanning proton therapy (SSPT) is a high-priority research area.

A patient-specific aperture system with an energy absorber for spot scanning proton beams: Verification for clinical application.

Purpose: In the authors' proton therapy system, the patient-specific aperture can be attached to the nozzle of spot scanning beams to shape an irradiation field and reduce lateral fall-off. The authors herein verified this system for clinical application.

Methods: The authors prepared four types of patient-specific aperture systems equipped with an energy absorber to irradiate shallow regions less than 4 g/cm(2).

Biological effect of dose distortion by fiducial markers in spot-scanning proton therapy with a limited number of fields: a simulation study.

Purpose: In accurate proton spot-scanning therapy, continuous target tracking by fluoroscopic x ray during irradiation is beneficial not only for respiratory moving tumors of lung and liver but also for relatively stationary tumors of prostate. Implanted gold markers have been used with great effect for positioning the target volume by a fluoroscopy, especially for the cases of liver and prostate with the targets surrounded by water-equivalent tissues. However, recent studies have revealed that gold markers can cause a significant underdose in proton therapy.

Design of a ridge filter structure based on the analysis of dose distributions.

Dose distributions distorted by a periodic structure, such as a ridge filter, are analytically investigated. Based on the beam optics, the fluence distributions of scanned beams passing through the ridge filter are traced. It is shown that the periodic lateral dose distribution blurred by multiple Coulomb scattering can be expressed by a sum of cosine functions through Fourier transform.

Reduction of the number of stacking layers in proton uniform scanning.

Uniform scanning with a relatively large beam size can improve beam utilization efficiency more than conventional irradiation methods using scatterers and can achieve a large-field, long-range and large spread-out Bragg peak (SOBP). The SOBP is obtained by energy stacking in uniform scanning, but its disadvantage is that the number of stacking layers is large, especially in the low-energy region, because the Bragg peak of the pristine beam is very sharp. We applied a mini-ridge filter to broaden the pristine Bragg peak up to a stacked layer thickness of 1 or 2 cm in order to decrease the number of stacking layers.