Spread-out Bragg Peak Proton Beam Research Articles

Calculation of biological effectiveness of SOBP proton beams: a TOPAS Monte Carlo study

Objective. This study aims to investigate the biological effectiveness of Spread-Out Bragg-Peak (SOBP) proton beams with initial kinetic energies 50–250 MeV at different depths in water using TOPAS Monte Carlo code. Approach. The study modelled SOBP proton beams using TOPAS time feature. Various LET-based models and Repair-Misrepair-Fixation model were employed to calculate Relative Biological Effectiveness (RBE) for V79 cell lines at different on-axis depths based on TOPAS. Microdosimetric Kinetic Model and biological weighting function-based models, which utilize microdosimetric distributions, were also used to estimate the RBE. A phase-space-based method was adopted for calculating microdosimetric distributions. Main results. The trend of variation of RBE with depth is similar in all the RBE models, but the absolute RBE values vary based on the calculation models. RBE sharply increases at the distal edge of SOBP proton beams. In the entrance region of all the proton beams, RBE values at 4 Gy i.e. RBE(4 Gy) resulting from different models are in the range of 1.04–1.07, comparable to clinically used generic RBE of 1.1. Moving from the proximal to distal end of the SOBP, RBE(4 Gy) is in the range of 1.15–1.33, 1.13–1.21, 1.11–1.17, 1.13–1.18 and 1.17–1.21, respectively for 50, 100, 150, 200 and 250 MeV SOBP beams, whereas at the distal dose fall-off region, these values are 1.68, 1.53, 1.44, 1.42 and 1.40, respectively. Significance. The study emphasises application of depth-, dose- and energy- dependent RBE values in clinical application of proton beams.

Determination of the proton LET using thin film solar cells coated with scintillating powder.

The amount of luminescent light detected in a scintillator is reduced with increased proton linear energy transfer (LET) despite receiving the same proton dose, through a phenomenon called quenching. This study evaluated the ability of a solar cell coated with scintillating powder (SC-SP) to measure therapeutic proton LET by measuring the quenching effect of the scintillating powder using a solar cell while simultaneously measuring the dose of the proton beam. SC-SP was composed of a flexible thin film solar cell and scintillating powder. The LET and dose of the pristine Bragg peak in the 14 cm range were calculated using a validated Monte Carlo model of a double scattering proton beam nozzle. The SC-SP was evaluated by measuring the proton beam under the same conditions at specific depths using SC-SP and Markus chamber. Finally, the 10 and 20 cm range pristine Bragg peaks and 5 cm spread-out Bragg peak (SOBP) in the 14 cm range were measured using the SC-SP and the Markus chamber. LETs measured using the SC-SP were compared with those calculated using Monte Carlo simulations. The quenching factors of the SC-SP and solar cell alone, which were slopes of linear fit obtained from quenching correction factors according to LET, were 0.027 and 0.070 µm/keV (R2 : 0.974 and 0.975). For pristine Bragg peaks in the 10 and 20 cm ranges, the maximum differences between LETs measured using the SC-SP and calculated using Monte Carlo simulations were 0.5 keV/µm (15.7%) and 1.2 keV/µm (12.0%), respectively. For a 5 cm SOBP proton beam, the LET measured using the SC-SP and calculated using Monte Carlo simulations differed by up to 1.9 keV/µm (18.7%). Comparisons of LETs for pristine Bragg peaks and SOBP between measured using the SC-SP and calculated using Monte Carlo simulations indicated that the solar cell-based system could simultaneously measure both LET and dose in real-time and is cost-effective.

Evaluations of a flat-panel based compact daily quality assurance device for proton pencil beam scanning (PBS) system.

To evaluate the flat-panel detector quenching effect and clinical usability of a flat-panel based compact QA device for PBS daily constancy measurements. The QA device, named Sphinx Compact, is composed of a 20x20 cm2 flat-panel imager mounted on a portable frame with removable plastic modules for constancy checks of proton energy (100MeV, 150MeV, 200MeV), Spread-Out-Bragg-Peak (SOBP) profile, and machine output. The potential quenching effect of the flat-panel detector was evaluated. Daily PBS QA tests of X-ray/proton isocenter coincidence, the constancy of proton spot position and sigma as well as the energy of pristine proton beam, and the flatness of SOBP proton beam through the 'transformed' profile were performed and analyzed. Furthermore, the sensitivity of detecting energy changes of pristine proton beam was also evaluated. The quenching effect was observed at depths near the pristine peak regions. The flat-panel measured range of the distal 80% is within 0.9mm to the defined ranges of the delivered proton beams. X-ray/proton isocenter coincidence tests demonstrated maximum mismatch of 0.3mm between the two isocenters. The device can detect 0.1mm change of spot position and 0.1MeV energy changes of pristine proton beams. The measured transformed SOBP beam profile through the wedge module rendered as flat. Even though the flat-panel detector exhibited quenching effect at the Bragg peak region, the proton range can still be accurately measured. The device can fulfill the requirements of the daily QA tests recommended by the AAPM TG224 Report.

ML-EM algorithm for dose estimation using PET in proton therapy

Positron emission tomography (PET) has been extensively studied and clinically investigated for dose verification in proton therapy. However, the production distributions of positron emitters are not proportional to the dose distribution. Thus, direct dose evaluation is limited when using the conventional PET-based approach. We propose a method for estimating the dose distribution from the positron emitter distributions using the maximum likelihood (ML) expectation maximization (EM) algorithm combined with filtering. In experiments to verify the effectiveness of the proposed method, mono-energetic and spread-out Bragg-peak proton beams were delivered by a synchrotron, and a water target was irradiated at clinical dose levels. Planar PET measurements were performed during beam pauses and after irradiation over a total period of 200 s. In addition, we conducted a Monte Carlo simulation to obtain the required filter functions and analyze the influence of the number of algorithm iterations on estimation. We successfully estimated the 2D dose distributions even under statistical noise in the PET images. The accuracy of the 2D dose estimation was about 10% for both beams at the 1- values of relative error. This value is comparable to the deviations in the measured PET activity distributions. For the laterally integrated profile along the beam direction, a low error within 5% was obtained per irradiation value. Moreover, the difference of estimated proton ranges was within 1 mm, and 2D estimation from the PET images was completed in 21 ms. Hence, the proposed algorithm may be applied to real-time dose monitoring. Although this is the first attempt to use the ML-EM algorithm for dose estimation, the proposed method showed high accuracy and speed in the estimation of proton dose distribution from PET data. The proposed method is thus a step forward to exploit the full potential of PET for in vivo dose verification.

Application of activity pencil beam algorithm using measured distribution data of positron emitter nuclei for therapeutic SOBP proton beam

Recently, much research on imaging the clinical proton-irradiated volume using positron emitter nuclei based on target nuclear fragment reaction has been carried out. The purpose of this study is to develop an activity pencil beam (APB) algorithm for a simulation system for proton-activated positron-emitting imaging in clinical proton therapy using spread-out Bragg peak (SOBP) beams. The target nuclei of activity distribution calculations are (12)C nuclei, (16)O nuclei, and (40)Ca nuclei, which are the main elements in a human body. Depth activity distributions with SOBP beam irradiations were obtained from the material information of ridge filter (RF) and depth activity distributions of compounds of the three target nuclei measured by BOLPs-RGp (beam ON-LINE PET system mounted on a rotating gantry port) with mono-energetic Bragg peak (MONO) beam irradiations. The calculated data of depth activity distributions with SOBP beam irradiations were sorted in terms of kind of nucleus, energy of proton beam, SOBP width, and thickness of fine degrader (FD), which were verified. The calculated depth activity distributions with SOBP beam irradiations were compared with the measured ones. APB kernels were made from the calculated depth activity distributions with SOBP beam irradiations to construct a simulation system using the APB algorithm for SOBP beams. The depth activity distributions were prepared using the material information of RF and the measured depth activity distributions with MONO beam irradiations for clinical therapy using SOBP beams. With the SOBP width widening, the distal fall-offs of depth activity distributions and the difference from the depth dose distributions were large. The shapes of the calculated depth activity distributions nearly agreed with those of the measured ones upon comparison between the two. The APB kernels of SOBP beams were prepared by making use of the data on depth activity distributions with SOBP beam irradiations that were made from the depth activity distributions with MONO beam irradiations and sorted in terms of energy, SOBP width, and thickness of FD. The data on APB kernels of SOBP beams were determined as installment data for the simulation system using the APB algorithm for SOBP beam irradiations. A method of obtaining the depth activity distributions and the APB algorithm for clinical use of SOBP beams have been developed. It is suggested that the simulation system for imaging the clinical irradiated volume with the APB algorithm can be used in clinical proton therapy using SOBP beams by preparing and investigating the data on APB kernels of SOBP beams.

TH‐C‐M100E‐10: A Correction Method for the MOSFET Energy Dependence Response to Therapeutic Proton Beams

The energy‐dependence response of the metal oxide semiconductor field‐effect transistor (MOSFET) dosimeter has been investigated with regard to therapeutic proton beams. The MOSFET configurations used include the commercial standard MOSFET (TN‐502RD), the microMOSFET (TN‐502RDM) dosimeters, and a prototype MOSFET with no encapsulation. Proton beams of non‐modulated pristine and modulated Spread‐Out Bragg Peak (SOBP) of 5 cm and 10 cm widths were used with beam ranges of 8.9cm, 15.9cm and 25.9cm in water. Each MOSFET dosimeter was calibrated at the center of the modulated 10 cm SOBP proton beam with conditions of 1 cGy per MU. The MOSFET energy‐dependence response was quantitatively evaluated by the ratio of measured doses between the MOSFET and an ionization chamber in a same condition. The three dosimeters showed a similar response for the pristine proton beams at various beam ranges. This indicates that the variation in dosimeter response is dominated by the change of the linear energy transfer (LET) for the used proton beam and not by the MOSFET encapsulation thickness. The observed MOSFET trends for various pristine proton beams have been modeled by an analytical function , where Rres is the residue range (distance to the distal 90% level dose), and A, B, C are constants. A similar modeling has been performed for modulated SOBP proton beams at different widths and for various beam ranges. The k (Rres) function has been used as a correction factor for patient dose measured by MOSFETs for corresponding residue ranges; this resulted in dose measurements with an uncertainty of less than 3.0% for various proton beams.

SU-FF-T-345: Measured Output Factors for Range-Modulated Spread-Out Bragg Peak Proton Beams

Purpose: The magnitude and the trend of variation of the output factor (OF) for range-modulated spread-out Bragg peak (SOBP) proton beams are investigated as a function of the range, modulated SOBP width and field size. Method and Materials: The proton delivery system of the fix horizontal beam line (FHBL) used at the Midwest Proton Radiotherapy Institute (MPRI) has been calibrated to give an output of 1.0 ± 1.0% cGy/MU at a reference condition. For field that deviates from reference is measured at center of the SOBP at the room “isocenter”. The OFs were measured with the fields that varied with range, SOBP width and size. Pristine curves for the various field sizes were also measured to study observed variation in the measured output factors. Results and Conclusion: For a field size of 10 cm in diameter, the OFs at the middle of a 10 cm SOBP vary 40% between 14 and 27 cm ranges in water and a similar trend with range is observed for a 6 cm SOBP. The OFs of a 17 cm range vary 60% between SOBP widths of 2.6 and 12.4 cm and a similar trend in SOBP width is observed for other ranges. For field sizes 2–10 cm diameters and a 10cm SOBP, 5% and 20% variations were observed for ranges of 12 and 27 cm, respectively. The trend in field size differs largely between different ranges. In theory, the trend in field size can be calculated from the ratios of Bragg-peak to entrance doses from measured pristine curves. However, the calculated output factors are significantly lower than the measured one for a small field size. Although the output factor can not be predicted theoretically, these measured output factors can be modeled analytically.