Purpose. To provide Monte Carlo calculated beam quality correction factors (k Q ) for monoenergetic proton beams using the Monte Carlo code fluka. Materials and methods. The Monte Carlo code fluka was used to calculate the dose absorbed in a water-filled reference volume and the air-filled cavities of six plane-parallel and four cylindrical ionization chambers. The chambers were positioned at the entrance region of monoenergetic proton beams with energies between 60 and 250 MeV. Based on these dose values, f Q as well as k Q factors were calculated while factors were taken from Andreo et al (2020 Phys. Med. Biol. 65 095011). Results. k Q factors calculated in this work were found to agree with experimentally determined k Q factors on the 1%-level, with only two exceptions with deviations of 1.4% and 1.9%. The comparison of f Q factors calculated using fluka with f Q factors calculated using the Monte Carlo codes geant4 and penh showed a general good agreement for low energies, while differences for higher energies were pronounced. For high energies, in most cases the Monte Carlo codes fluka and geant4 lead to comparable results while the f Q factors calculated with penh are larger. Conclusion. fluka can be used to calculate k Q factors in clinical proton beams. The divergence of Monte Carlo calculated k Q factors for high energies suggests that the role of nuclear interaction models implemented in the different Monte Carlo codes needs to be investigated in more detail.
Objective: In FLASH radiotherapy (dose rates ≥ 40 Gy/s), a reduced normal tissue toxicity has been observed, while maintaining the same tumor control compared to conventional radiotherapy (dose rates ≤ 0.03 Gy/s). This protecting effect could not be fully explained yet. One assumption is that interactions between the chemicals of different primary ionizing particles, so-called inter-track interactions, trigger this outcome. In this work, we included inter-track interactions in Monte Carlo track structure simulations and investigated the yield of chemicals (G-value) produced by ionizing particles. 
Approach: For the simulations, we used the Monte Carlo toolkit TOPAS, in which inter-track interactions cannot be implemented without further effort. Thus, we developed a method enabling the simultaneous simulation of N original histories in one event allowing chemical species to interact with each other. To investigate the effect of inter-track interactions we analyzed the G-value of different chemicals using various radiation sources. We used electrons with an energy of 60 eV in different spatial arrangements as well as a 10 MeV and 100 MeV proton source. For electrons we set N between 1 and 60, for protons between 1 and 100.
Main results: In all simulations, the total G-value decreases with increasing N. In detail, the G-value for ●OH, H3O and eaq decreases with increasing N, whereas the G-value of OH- , H2O2 and H2 increases slightly. The reason is that with increasing N, the concentration of chemical radicals increases allowing for more chemical reactions between the radicals resulting in a change of the dynamics of the chemical stage.
Signifcance: Inter-track interactions resulting in a variation of the yield of chemical species, may be a factor explaining the FLASH effect. To verify this hypothesis, further simulations are necessary in order to evaluate the impact of varying G-values on the yield of DNA damages.
In particle therapy of lung tumors, modulating effects on the particle beam may occur due to the microscopic structure of the lung tissue. These effects are caused by the heterogeneous nature of the lung tissue and cannot be completely taken into account during treatment planning, because these micro structures are too small to be fully resolved in the planning CT. In several publications, a new material parameter called modulation power (P mod ) was introduced to characterize the effect. For various artificial lung surrogates, this parameter was measured and published by other groups and ranges up to approximately 1000 µm. Studies investigating the influence of the modulation power on the dose distribution during irradiation are using this parameter in the rang of 100 to 800 µm. More precise measurements for P mod on real lung tissue have not yet been published. In this work, the modulation power of real lung tissue was measured using porcine lungs in order to produce more reliable data of P mod for real lung tissue. For this purpose, ex-vivo porcine lungs were frozen in a ventilated state and measurements in a carbon ion beam were performed. Due to the way the lungs were prepared and transferred to a solid state, the lung structures that modulate the beam could also be examined in detail using micro CT imaging. An optimization of the established methods of measuring the modulation power, which takes better account of the typical structures within lung tissue, was developed as well.
Purpose. The purpose of this work is to investigate the feasibility of TOPAS-nBio for track structure simulations using tuple scoring and ROOT/Python-based post-processing. Materials and methods. There are several example applications implemented in GEANT4-DNA demonstrating track structure simulations. These examples are not implemented by default in TOPAS-nBio. In this study, the tuple scorer was used to re-simulate these examples. The simulations contained investigations of different physics lists, calculation of energy-dependent range, stopping power, mean free path and W-value. Additionally, further applications of the TOPAS-nBio tool were investigated, focusing on physical interactions and deposited energies of electrons with initial energies in the range of 10–60 eV, not covered in the recently published GEANT4-DNA simulations. Low-energetic electrons are currently of great interest in the radiobiology research community due to their high effectiveness towards the induction of biological damage. Results. The quantities calculated with TOPAS-nBio show a good agreement with the simulations of GEANT4-DNA with deviations of 5% at maximum. Thus, we have presented a feasible way to implement the example applications included in GEANT4-DNA in TOPAS-nBio. With the extended simulations, an insight could be given, which further tracking information can be gained with the track structure code and how cross sections and physics models influence a particle’s fate. Conclusion. With our results, we could show the potentials of applying the tuple scorer in TOPAS-nBio Monte Carlo track structure simulations. Using this scorer, a large amount of information about the track structure can be accessed, which can be analyzed as preferred after the simulation.
To treat lung tumours with particle therapy, different additional tasks and challenges in treatment planning and application have to be addressed thoroughly. One of these tasks is the quantification and consideration of the Bragg peak (BP) degradation due to lung tissue: as lung is an heterogeneous tissue, the BP is broadened when particles traverse the microscopic alveoli. These are not fully resolved in clinical CT images and thus, the effect is not considered in the dose calculation. In this work, a correlation between the CT histograms of heterogeneous material and the impact on the BP curve is presented. Different inorganic materials were scanned with a conventional CT scanner and additionally, the BP degradation was measured in a proton beam and was then quantified. A model is proposed that allows an estimation of the modulation power by performing a histogram analysis on the CT scan. To validate the model for organic samples, a second measurement series was performed with frozen porcine lunge samples. This allows to investigate the possible limits of the proposed model in a set-up closer to clinical conditions. For lung substitutes, the agreement between model and measurement is within ±0.05 mm and for the organic lung samples, within ±0.15 mm. This work presents a novel, simple and efficient method to estimate if and how much a material or a distinct region (within the lung) is degrading the BP on the basis of a common clinical CT image. Up until now, only a direct in-beam measurement of the region or material of interest could answer this question.
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