Effect of Linear Energy Transfer on Cystamine's Radioprotective Activity: A Study Using the Fricke Dosimeter with 6-500 MeV per Nucleon Carbon Ions-Implication for Carbon Ion Hadrontherapy

(1) Background: Radioprotective agents have garnered considerable interest due to their prospective applications in radiotherapy, public health medicine, and situations of large-scale accidental radiation exposure or impending radiological emergencies. Cystamine, an organic diamino-disulfide compound, is recognized for its radiation-protective and antioxidant properties. This study aims to utilize the aqueous ferrous sulfate (Fricke) dosimeter to measure the free-radical scavenging capabilities of cystamine during irradiation by fast carbon ions. This analysis spans an energy range from 6 to 500 MeV per nucleon, which correlates with "linear energy transfer" (LET) values ranging from approximately 248 keV/μm down to 9.3 keV/μm. (2) Methods: Monte Carlo track chemistry calculations were used to simulate the radiation-induced chemistry of aerated Fricke-cystamine solutions across a broad spectrum of cystamine concentrations, ranging from 10-6 to 1 M. (3) Results: In irradiated Fricke solutions containing cystamine, cystamine is observed to hinder the oxidation of Fe2+ ions, an effect triggered by oxidizing agents from the radiolysis of acidic water, resulting in reduced Fe3+ ion production. Our simulations, conducted both with and without accounting for the multiple ionization of water, confirm cystamine's ability to capture free radicals, highlighting its strong antioxidant properties. Aligning with prior research, our simulations also indicate that the protective and antioxidant efficiency of cystamine diminishes with increasing LET of the radiation. This result can be attributed to the changes in the geometry of the track structures when transitioning from lower to higher LETs. (4) Conclusions: If we can apply these fundamental research findings to biological systems at a physiological pH, the use of cystamine alongside carbon-ion hadrontherapy could present a promising approach to further improve the therapeutic ratio in cancer treatments.

Assessment of Cystamine's Radioprotective/Antioxidant Ability under High-Dose-Rate Irradiation: A Monte Carlo Multi-Track Chemistry Simulation Study

(1) Background: cystamine and its reduced form cysteamine have radioprotective/antioxidant effects in vivo. In this study, we use an in vitro model system to examine the behavior of cystamine towards the reactive primary species produced during the radiolysis of the Fricke dosimeter under high dose-rate irradiation conditions. (2) Methods: our approach was to use the familiar radiolytic oxidation of ferrous to ferric ions as an indicator of the radioprotective/antioxidant capacity of cystamine. A Monte Carlo computer code was used to simulate the multi-track radiation-induced chemistry of aerated and deaerated Fricke-cystamine solutions as a function of dose rate while covering a large range of cystamine concentrations. (3) Results: our simulations revealed that cystamine provides better protection at pulsed dose rates compared to conventional, low-dose-rate irradiations. Furthermore, our simulations confirmed the radical-capturing ability of cystamine, clearly indicating the strong antioxidant profile of this compound. (4) Conclusion: assuming that these findings can be transposable to cells and tissues at physiological pH, it is suggested that combining cystamine with FLASH-RT could be a promising approach to further enhance the therapeutic ratio of cancer cure.

A Computer Modeling Study of Water Radiolysis at High Dose Rates. Relevance to FLASH Radiotherapy

"FLASH radiotherapy" is a new method of radiation treatment by which large doses of radiation are delivered at high dose rates to tumors almost instantaneously (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. To date, no definitive mechanism has been proposed to explain the different responses of the tumor and normal tissue to radiation. As a first step, and given that living cells and tissues consist mainly of water, we studied the effects of high dose rates on the transient yields (G values) of the radical and molecular species formed in the radiolysis of deaerated/aerated water by irradiating protons, using Monte Carlo simulations. Our simulation model consisted of two steps: 1. The random irradiation of a right circular cylindrical volume of water, embedded in nonirradiated bulk water, with single and instantaneous pulses of N 300-MeV incident protons ("linear energy transfer" or LET ∼ 0.3 keV/µm) traveling along the axis of the cylinder; and 2. The development of these N proton tracks, which were initially contained in the irradiated cylinder, throughout the solution over time. The effect of dose rate was studied by varying N, which was calibrated in terms of dose rate. For this, experimental data on the yield G(Fe3+) of the super-Fricke dosimeter as a function of dose rate up to ∼1010 Gy/s were used. Confirming previous experimental and theoretical studies, significant changes in product yields were found to occur with increasing dose rate, with lower radical and higher molecular yields, which result from an increase in the radical density in the bulk of the solution. Using the kinetics of the decay of hydrated electrons, a critical time (τc), which corresponds to the "onset" of dose-rate effects, was determined for each value of N. For the cylindrical irradiation model, τc was inversely proportional to the dose rate. Moreover, the comparison with experiments with pulsed electrons underlined the importance of the geometry of the irradiation volume for the estimation of τc. Finally, in the case of aerated water radiolysis, we calculated the yield of oxygen consumption and estimated the corresponding concentration of consumed (depleted) oxygen as a function of time and dose rate. It was shown that this concentration increases substantially with increasing dose rate in the time window ∼1 ns-10 µs, with a very pronounced maximum around 0.2 µs. For high-dose-rate irradiations (>109 Gy/s), a large part of the available oxygen (∼0.25 mM for an air-saturated solution) was found to be consumed. This result, which was obtained on a purely water radiation chemistry basis, strongly supports the hypothesis that the normal tissue-sparing effect of FLASH stems from temporary hypoxia due to oxygen depletion induced by high-dose-rate irradiation.

Yield of the Fricke dosimeter irradiated with the recoil α and Li ions of the 10B(n,α)7Li nuclear reaction: effects of multiple ionization and temperature

Monte Carlo track chemistry simulations were used to investigate the effects of multiple ionization (MI) of water on the yields (G values) of the ferrous sulfate (Fricke) dosimeter, which was irradiated with low-energy α and lithium ion recoils from the 10B(n,α)7Li nuclear reaction as a function of temperature from 25 to 350 °C. Calculations were performed individually for 1.47 MeV α-particles and 0.84 MeV lithium nuclei with dose-average linear energy transfer (LET) values of ∼196 and 225 keV/µm at 25 °C, respectively. The total yields were obtained by summing the G values for each recoil α and Li ion weighted with its fraction of the total energy absorbed. At room temperature, our G(Fe3+) values calculated under aerated and deaerated conditions only agreed well with the experimental results, provided the MI of water was incorporated in the simulations. This strongly supports the importance of the role of MI of water in the high-LET radiolysis of water. We also simulated the effects of MI of water on G-values for the primary species of the radiolysis of deaerated 0.4 M H2SO4 aqueous solutions by 10B(n,α)7Li recoils. As with the Fricke dosimeter, the best agreement between experiment and simulation was found at 25 °C when the MI of water was included in the simulations. It was also shown that G(Fe3+) decreases slightly as a function of temperature over the range of 25–350 °C. However, at elevated temperatures, no experimental data were available with which to compare our results.

A review of simulation codes and approaches for radiation chemistry

Historically, the field of radiation chemistry began shortly after the discovery of radioactivity, and its development has been closely related to discoveries in other related fields such as radiation and nuclear physics. Radiolysis of water and radiation chemistry have been very important in elucidating how radiation affects living matter and how it induces DNA damage. Nowadays, we recognize the importance of chemistry to understanding the effects of radiation on cells; however, it took several decades to obtain this insight, and much is still unknown. The radiolysis of water and aqueous solutions have been the subject of much experimental and theoretical research for many decades. One important concept closely related to radiation chemistry is radiation track structure. Track structure results from early physical and physicochemical events that lead to a highly non-homogenous distribution of radiolytic species. Because ionizing radiation creates unstable species that are distributed non-homogenously, the use of conventional reaction kinetics methods does not describe this chemistry well. In recent years, several methods have been developed for simulating radiation chemistry. In this review, we give a brief history of the field and the development of the simulation codes. We review the current methods used to simulate radiolysis of water and radiation chemistry, and we describe several radiation chemistry codes and their applications.

Linear energy transfer dependence of transient yields in water irradiated by 150 keV – 500 MeV protons in the limit of low dose rates

FLASH radiotherapy is a new irradiation method in which large doses of ionizing radiation are delivered to tumors almost instantly (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. Although this technique is primarily studied in the context of electron and photon therapies, proton delivery at high dose rates can also reduce the adverse side effects on normal cells. So far, no definitive mechanism has been proposed to explain the differences in the responses to radiation between tumor and normal tissues. Given that living cells and tissues consist mainly of water, we set out to study the effects of high dose rates on the radiolysis of water by protons in the energy range of 150 keV – 500 MeV (i.e., for linear energy transfer (LET) values between ∼72.2 and 0.23 keV/μm, respectively) using Monte Carlo simulations. To validate our methodology, however, we, first, report here the results of our calculations of the yields (G values) of the radiolytically produced species, namely the hydrated electron ([Formula: see text]), •OH, H•, H2, and H2O2, for low dose rates. Overall, our simulations agree very well with the experiment. In the presence of oxygen, [Formula: see text] and H• atoms are rapidly converted into superoxide anion or hydroperoxyl radicals, with a well-defined maximum of [Formula: see text] at ∼1 μs. This maximum decreases substantially when going from low-LET 500 MeV to high-LET 150 keV irradiating protons. Differences in the geometry of the proton track structure with increasing LET readily explain this diminution in [Formula: see text] radicals.

The physical stage of radiolysis of solvated DNA by high-energy-transfer particles: insights from new first principles simulations

Electron dynamics simulations based on density functional theory are carried out on nanometric molecular systems to decipher the primary processes following irradiation of bio-macromolecules by high energy transfer charged particles.