LET-Dependent Intertrack Yields in Proton Irradiation at Ultra-High Dose Rates Relevant for FLASH Therapy

FLASH radiotherapy delivers a high dose (≥10 Gy) at a high rate (≥40 Gy/s). In this way, particles are delivered in pulses as short as a few nanoseconds. At that rate, intertrack reactions between chemical species produced within the same pulse may affect the heterogeneous chemistry stage of water radiolysis. This stochastic process suits the capabilities of the Monte Carlo method, which can model intertrack effects to aid in radiobiology research, including the design and interpretation of experiments. In this work, the TOPAS-nBio Monte Carlo track-structure code was expanded to allow simulations of intertrack effects in the chemical stage of water radiolysis. Simulation of the behavior of radiolytic yields over a long period of time (up to 50 s) was verified by simulating radiolysis in a Fricke dosimeter irradiated by 60Co γ rays. In addition, LET-dependent G values of protons delivered in single squared pulses of widths, 1 ns, 1 µs and 10 µs, were obtained and compared to simulations using no intertrack considerations. The Fricke simulation for the calculated G value of Fe3+ ion at 50 s was within 0.4% of the accepted value from ICRU Report 34. For LET-dependent G values at the end of the chemical stage, intertrack effects were significant at LET values below 2 keV/µm. Above 2 keV/µm the reaction kinetics remained limited locally within each track and thus, effects of intertrack reactions remained low. Therefore, when track structure simulations are used to investigate the biological damage of FLASH irradiation, these intertrack reactions should be considered. The TOPAS-nBio framework with the expansion to intertrack chemistry simulation provides a useful tool to assist in this task.

One Stone Two Birds: Zr-Fc Metal-Organic Framework Nanosheet for Synergistic Photothermal and Chemodynamic Cancer Therapy

Metal-organic frameworks (MOFs) have been identified as promising materials for the delivery of therapeutics to cure cancer owing to their intrinsic porous structure. However, in a majority of cases, MOFs act as only a delivery cargo for anticancer drugs while little attention has been focused on the utilization of their intriguing physical and chemical properties for potential anticancer purposes. Herein for the first time, an ultrathin (16.4 nm thick) ferrocene-based MOF (Zr-Fc MOF) nanosheet has been synthesized for synergistic photothermal therapy (PTT) and Fenton reaction-based chemodynamic (CDT) therapy to cure cancer without additional drugs. The Zr-Fc MOF nanosheet acts not only as an excellent photothermal agent with a prominent photothermal conversion efficiency of 53% at 808 nm but also as an efficient Fenton catalyst to promote the conversion of H₂O₂ into hydroxyl radical (^•OH). As a consequence, an excellent therapeutic performance has been achieved in vitro as well as in vivo through this combinational effect. This work aims to construct an "all-in-one" MOF nanoplatform for PTT and CDT treatments without incorporating any additional therapeutics, which may launch a new era in the investigation of MOF-based synergistic therapy platforms for cancer therapy.

Ferrocene-catalyzed heterogeneous Fenton-like degradation mechanisms and pathways of antibiotics under simulated sunlight: A case study of sulfamethoxazole

Readily-available and efficient catalyst is essential for activating oxidants to produce reactive species for deeply remediating water bodies contaminated by antibiotics. In this study, Ferrocene (Fc) was introduced to establish a heterogeneous photo-Fenton system for the degradation of sulfonamide antibiotics, taking sulfamethoxazole as a representative. Results showed that the removal of sulfamethoxazole was effective in Fc-catalyzed photo-Fenton system. Electron spin resonance and radical scavenging experiments verified that there was a photoindued electron transfer process from Fc to H₂O₂ and dissolved oxygen resulting in the formation of OH that was primarily responsible for the degradation of sulfamethoxazole. The reactions of OH with substructure model compounds of sulfamethoxazole unveiled that aniline moiety was the preferable reaction site of sulfamethoxazole, which was verified by the formation of hydroxylated product and the dimer of sulfamethoxazole in Fc-catalyzed photo-Fenton system. This heterogeneous photo-Fenton system displayed an effective degradation efficiency even in a complex water matrices, and Fc represented a long-term stability by using the catalyst for multiple cycles. These results demonstrate that Fc-catalyzed photo-Fenton oxidation may be an efficient approach for remediation of wastewater containing antibiotics.

Iron oxide-mediated semiconductor photocatalysis vs. heterogeneous photo-Fenton treatment of viruses in wastewater. Impact of the oxide particle size

The photo-Fenton process is recognized as a promising technique towards microorganism disinfection in wastewater, but its efficiency is hampered at near-neutral pH operating values. In this work, we overcome these obstacles by using the heterogeneous photo-Fenton process as the default disinfecting technique, targeting MS2 coliphage in wastewater. The use of low concentrations of iron oxides in wastewater without H₂O₂ (wüstite, maghemite, magnetite) has demonstrated limited semiconductor-mediated MS2 inactivation. Changing the operational pH and the size of the oxide particles indicated that the isoelectric point of the iron oxides and the active surface area are crucial in the success of the process, and the possible underlying mechanisms are investigated. Furthermore, the addition of low amounts of Fe-oxides (1mgL^-1) and H₂O₂ in the system (1, 5 and 10mgL^-1) greatly enhanced the inactivation process, leading to heterogeneous photo-Fenton processes on the surface of the magnetically separable oxides used. Additionally, photo-dissolution of iron in the bulk, lead to homogeneous photo-Fenton, further aided by the complexation by the dissolved organic matter in the solution. Finally, we assess the impact of the presence of the bacterial host and the difference caused by the different iron sources (salts, oxides) and the Fe-oxide size (normal, nano-sized).

A feasibility study of Fricke dosimetry as an absorbed dose to water standard for 192Ir HDR sources

High dose rate brachytherapy (HDR) using ¹⁹²Ir sources is well accepted as an important treatment option and thus requires an accurate dosimetry standard. However, a dosimetry standard for the direct measurement of the absolute dose to water for this particular source type is currently not available. An improved standard for the absorbed dose to water based on Fricke dosimetry of HDR ¹⁹²Ir brachytherapy sources is presented in this study. The main goal of this paper is to demonstrate the potential usefulness of the Fricke dosimetry technique for the standardization of the quantity absorbed dose to water for ¹⁹²Ir sources. A molded, double-walled, spherical vessel for water containing the Fricke solution was constructed based on the Fricke system. The authors measured the absorbed dose to water and compared it with the doses calculated using the AAPM TG-43 report. The overall combined uncertainty associated with the measurements using Fricke dosimetry was 1.4% for k = 1, which is better than the uncertainties reported in previous studies. These results are promising; hence, the use of Fricke dosimetry to measure the absorbed dose to water as a standard for HDR ¹⁹²Ir may be possible in the future.

Mineralization of wastewater from the pharmaceutical industry containing chloride ions by UV photolysis of H2O2/Fe(II) and ultrasonic irradiation

The mineralization of pharmaceutical wastewater containing chloride ions using a UV/H2O2/Fe(II) process was studied. The addition of Fe(II) to the UV/H2O2 system did not improve the degradation efficiency due to inhibition of the photo-Fenton reaction, at acid pH, in the presence of chloride ions in these wastewaters. The increase of pH from 2 to 7 increased the degree of mineralization under UV photolysis of H2O2 because more HO radicals are available by HOCl dissociation reaction. Under the selected operation conditions ([H2O2]o = 11,500 ppm, [Fe(II)] = 0 ppm, [TOC]o = 125 ppm and pH = 7), 100% of TOC removal was attained in 120 min. A significant synergistic effect of combining photolysis (UV/H2O2) and sonolysis was observed. Sonophotolysis (UV/H2O2/ultrasound) technique significantly increased the degree of mineralization (100% TOC removal in 90 min using 6500 ppm H2O2) when compared with each individual process. Sonochemical reaction was favored by the presence of chloride ions since the concentration of contaminants at the gas-liquid interface increased. Free radicals reaction was the controlling mechanism in the UV/H2O2/ultrasound system. HO radicals were the main oxidative intermediate species in the process, although hydroperoxyl radicals (HO2) also played a role. The contribution of thermal-pyrolytic reaction (in gas-phase) to sonophotolysis process was negligible.

Optimization of hydrous ferrous sulfate dehydration by microwave heating using response surface methodology

The work relates to assessing the ability of the microwave for dehydration of large amount of waste hydrous ferrous sulfate generated from the titanium pigment process industry. The popular process optimization tool of response surface methodology with central composite design was adopted to estimate the effect of dehydration. The process variables were chosen to be power input, duration of heating and the bed thickness, while the response variable being the weight loss. An increase in all the three process variables were found to significantly increase the weight loss, while the effect of interaction among the parameters were found to be insignificant. The optimized process conditions that contribute to the maximum weight loss were identified to be a power input of 960 W, duration of heating of 14 min and bed thickness of 5 cm, resulting in a weight loss of 31.44%. The validity of the optimization process was tested with the repeat runs at optimized conditions.

Photoinduced Spin Transition for Iron(II) Compounds with Liquid-Crystal Properties

The iron(II) compounds [Fe(3Cn-L)2(NCS)2] (n = 6 (1), n = 8 (2), n = 10 (3), n = 12 (4), n = 14 (5), n = 16 (6), n = 18 (7), n = 20 (8), and n = 22 (9)) were synthesized and their physical properties characterized by polarizing optical microscopy, differential scanning calorimetry, and powder X-ray analysis, where 3Cn-L denotes bidentate Schiff-base ligands formed from the corresponding aniline derivatives and pyridine-2-carboxyaldehyde. The iron(II) compounds 4-8 exhibited crystal to liquid-crystal transitions at 318, 334, 345, 338, and 347 K, respectively. Variable-temperature magnetic susceptibility measurements revealed that the compounds 1-9 exhibit spin-crossover behavior between the high-spin and low-spin states and a photoinduced spin transition from a low-spin state to a metastable high-spin state. Therefore, the iron(II) compounds 4-8 can undergo spin-crossover and photoinduced spin transition as well as have liquid-crystal properties all in a single molecule. Compounds with multifunctions are important in the development of molecular switches and optical materials.

Growth morphology and spectroscopy of multiwall carbon nanotubes synthesized by pyrolysis of iron phthalocyanine

Carbon nanotubes (CNTs) were synthesized by Chemical Vapor Deposition (CVD) from the pyrolytic decomposition of Iron Phthalocyanine (FePc) molecules, on SiO2/Si(111) substrates in the presence of a hydrogen flow. FePc molecules contribute simultaneously both to the formation of the precursor Fe nanoparticles and also as a Carbon source. Different experimental conditions were examined. Samples were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and inverse photoemission. The resulting samples are highly oriented multiwall carbon nanotubes films, with heights in the range between: 4 and 20 microm. The tubes diameter is strongly dependent on growth temperature. Our experimental results show evidence of a transition in the growth mechanism, from a tip growth to a base growth mode, as the decomposition temperature is increased. Preliminary spectroscopic measurements performed on these MWCNTs, show the unoccupied density of states has several resonances close to Fermi level, related both to the graphene electronic structure and the formation of the tube.

Ion diffusion modelling of Fricke-agarose dosemeter gels

In Fricke-agarose gels, an accurate determination of the spatial dose distribution is hindered by the diffusion of ferric ions. In this work, a model was developed to describe the diffusion process within gel samples of finite length and, thus, permit the reconstruction of the initial spatial distribution of the ferric ions. The temporal evolution of the ion concentration as a function of the initial concentration is derived by solving Fick's second law of diffusion in two dimensions with boundary reflections. The model was applied to magnetic resonance imaging data acquired at high spatial resolution (0.3 mm) and was found to describe accurately the observed diffusion effects.

Dose imaging with gel-dosemeter layers: optical analysis and dedicated software

In radiotherapy involving thermal and epithermal neutrons, the knowledge of dose distributions, with separation of the contribution of each secondary radiation component, is of utmost importance. Layers of Fricke-Xylenol-Orange-infused gel dosemeters give the possibility of achieving such requirements because, owing to the layer-geometry, enriching or depleting the gel matrix of suitable isotopes does not sensibly alter neutron transport. The dosimetry method has been critically re-examined with the aim of improving its suitability to boron neutron capture therapy (BNCT) requirements, as it applies to the protocol of measurement and analysis, the sensitivity of the method and the range of the linearity of the dosemeters. Software has been developed and studied to obtain automatically the images of the various dose components with the established separation procedure.

Optical imaging of dose distributions in Fricke gels

Ferrous-sulphate infused gels, or 'Fricke gels', encounter great interest in the field of radiation dosimetry, due to their potential for 3D radiation dose mapping. Typically, magnetic resonance (MR) relaxation rates are determined in these systems in order to derive the absorbed dose. However, when large concentration gradients are present, diffusion effects before and during the MR imaging may not be negligible. In these cases, optical techniques may represent a viable alternative. This paper describes research aimed at measuring 3D dose distributions in a Fricke-xylenol orange gel by measuring optical density with a CCD camera. This method is inexpensive and fast. A series of early experiments is described, in which optical density profiles were measured with a commercial microdensitometer for film dosimetry. The light box of the device was modified to work at 567 nm, close to the maximum absorbance of the ferric ion-xylenol orange complex. Under these conditions, the gel shows linearity with dose and high sensitivity.

Synthesis and Photochemical Properties of a Photochromic Iron(II) Complex of Hexaarylbiimidazole

The photochromic ligand bis(terpyridyl)hexaarylbiimidazole (bistpy-HABI) and the Fe(II) complex of bistpy-HABI with formula [{Fe(tpy)}2.bistpy-HABI](PF6)4.4H2O were synthesized and characterized. Bistpy-HABI is readily cleaved into a pair of terpyridyltriphenylimidazolyl radicals (tpy-TPI*) on irradiation with UV light. This photochemical reaction is completely reversible, and the light-induced radicals can thermally recombine to form bistpy-HABI in the dark. [{Fe(tpy)}2.bistpy-HABI]4+ is the first example of a transition-metal complex of an HABI derivative and was found to show photochromic reaction in solution. The spin state of the light-induced radical pair in a frozen matrix was investigated by ESR spectroscopy. The triplet state of the light-induced radical pair from [{Fe(tpy)}2.bistpy-HABI]4+, as well as that from bistpy-HABI, was confirmed to be a ground state or nearly degenerated with a singlet state. Kinetic studies on the radical recombination reaction in solution elucidated the decrease in the activation energy by forming the Fe(II) complex. This is the first observation of a decrease in the activation energy of the radical recombination reaction by the formation of a metal-coordinated radical complex. The syntheses, photochemical properties, and spin states of bistpy-HABI and [{Fe(tpy)}2.bistpy-HABI](PF6)4 are discussed.

Dependence of irradiation temperature in the response of iron salts

A potential dosimeter based on aqueous frozen solutions and solid-state salt are presented for the evaluation of the energy transferred during the interaction of high-energy radiation with matter at low temperature. The foundation of these dosimeters, both the solid state and the frozen solutions, is based on the measurement of the change of the iron oxidation state. The systems were irradiated with gamma radiation at different doses (up to 10 MGy), and at different temperatures (from 77 to 298 K). The irradiated samples were analysed by UV-spectroscopy and Mössbauer spectroscopy. A theoretical model was developed for the chemical reactions system. This model reproduces the experimental effects produced by the irradiation in aqueous solutions of ferrous salt. The results showed that the response of the dosimeters depends on the irradiation temperature. At low-radiation doses, the response was linear. In particular, this work can be applied to low-temperature dosimetry can be specially applied to simulation experiments of extraterrestrial bodies, as well as in general to space research.

Dosimetry of soft x-rays in thin liquid layers

Very thin material layers (<100 microm) partially absorb ionizing radiation of low energy. When irradiating monolayer cell cultures from above, attention must be paid to absorption by the medium. Frequently, the volume of the nutrient medium is variable, and this leads to differences in the radiation doses delivered to the cells. In the present work these conditions were investigated for x-rays of energies between 13 kV and 100 kV in comparison with 60Co gamma rays using chemical dosimetry to measure the absorption by liquid layers between 25 microm and 500 microm thick. When the dose as measured with the ionization chamber was held constant, the dose absorbed in the Fricke solution was shown to increase with decreasing thickness of the layer of liquid because of a dose gradient. The effect of the dose gradient disappeared, however, in thick liquid layers of the Fricke solution by mixing during spectrophotometry. Secondary (photoeffect and Compton) electrons produced in air or filters are responsible for this effect in plastic petri dishes where back scattering at the interface does not occur. This interpretation is suggested by the same results of an analogous experimental setup using gamma rays with a 5-mm-thick Perspex plate. This dose increase in very thin layers, however, could not be verified by irradiating monolayer cells in poured-out plastic petri dishes because the secondary electrons are already absorbed in the remaining liquid film above the cells.

A spin-spin relaxation rate investigation of the gelatin ferrous sulphate NMR dosimeter

Spin-spin NMR relaxation rate in the ferrous sulphate gelatin dosimeter has been studied in terms of pH, gelatin concentration, the addition of benzoic acid, and sample size. It is demonstrated that R2 is more sensitive to changes in Fe3+ ion concentration than R1 when measuring at frequencies of 64 and 100 MHz. pH has an important effect on dose response curves, and oxygen depletion occurs significantly more rapidly in FeSO4 gelatin than in the liquid FeSO4, resulting in a saturation dose of approximately 80 Gy at depths greater than approximately 3 mm in phantom. The concentration of gelatin can be increased to 12% by weight, and the dosimeter will continue to exhibit a linear dose response. Sensitivity is maintained at higher gel concentrations by pH compensation. Addition of low-concentration benzoic acid to the system does not alter the dose response of the gelatin FeSO4 system. Finally, spontaneous oxidation of Fe2+ ions does not significantly alter the shape of dose response curves but does result in increases in R2 by up to 4% per day.

Effect of saccharide additives on response of ferrous-agarose-xylenol orange radiotherapy gel dosimeters

Glucose, sucrose, starch, and locust bean gum have been used as additives to the ferrous-agarose-xylenol orange (FAX) gel dosimeter. The saccharide enhanced dosimeters were found to have a higher dose sensitivity over a standard FAX gel as measured by both optical density change and magnetic resonance imaging (MRI). With optical density measurement, OD-dose sensitivity increases were up to 55% for glucose, 122% for sucrose and 43% for starch, while locust bean gum did not give a consistent response. With MRI, R1-dose sensitivity increases were up to 178% with sucrose addition. The FAX gel with sucrose was studied in greatest detail. The OD-dose sensitivity dependence on cooling rate was reduced for the sucrose FAX gel over the standard FAX gel, which has significant implications for uniform dose sensitivity in large gel phantoms. The thermal oxidation rate in the sucrose FAX gel was up to 2.3 times higher than in the standard gel. The OD-dose sensitivity of oxygenated sucrose FAX gels was 4.3 times greater than standard FAX gels, while continued enhancement in OD-dose sensitivity with increased sucrose concentrations beyond 2.0 g/l was found only for the oxygenated sucrose FAX gels. Both the molar absorption coefficient of the ferric ion-xylenol orange complex at 543 nm and gel pH were not affected by the presence of sucrose, with the implication that the higher OD-dose sensitivity of gels with saccharides is due to increased chain reaction production of ferric ions.

Spatial resolution of magnetic resonance imaging Fricke-gel dosimetry is improved with a honeycomb phantom

The spatial accuracy of magnetic resonance imaging (MRI) Fricke-gel dosimetry is limited by diffusion of ferric ions. This paper describes a honeycomb structure to limit diffusion of Fe3+ ions in a three-dimensional phantom. Such a phantom containing the dosimeter gel was irradiated to a known dose distribution. Maps of dose distributions were produced from the MR images acquired at 2 and 24 hours after the dose was given. The dose distribution maps verified that the honeycomb structure precludes ion diffusion from one honeycomb cell to another, thus improving the usefulness of MRI Fricke-gel dosimetry.

A systematic review of the precision and accuracy of dose measurements in photon radiotherapy using polymer and Fricke MRI gel dosimetry

The purpose of this work is to undertake a critical appraisal of the evidence in the published literature concerning the basic parameters of accuracy and precision associated with the use of Fricke and polymer gels (in conjunction with MR imaging) as radiation dosimeters in photon radiotherapy, condensing and analysing the body of published information (to the end of April 2002). A systematic review was undertaken addressing specific issues of precision and accuracy asking defined questions of the published literature. Accuracy and precision in relation to gel dosimetry were defined. Information was obtained from published, peer-reviewed journals. A defined search strategy utilizing MeSH headings and keywords, with extensive use of cross-referencing, identified 115 references dealing with gel dosimetry. Exclusion criteria were used to select only data from publications which would give unequivocal evidence. For accuracy, results had to be compared with an ionization chamber as gold standard and all gel samples had to be manufactured in the same batch. For precision, in addition to gels being from the same batch, samples must all have been irradiated at the same time and scanned simultaneously (or within a short time frame). Many results were found demonstrating 'dose mapping' examples using gels. However, there were very few publications containing firm evidence of precision and accuracy. There was no evidence which fulfilled our criteria about accuracy or precision using Fricke gels. For polymer gels only one paper was found for accuracy (4% (Low et al 1999 Med. Phys. 26 1542-51)) and precision (1.7% (Baldock et al 1998 Phys. Med. Biol. 43695-702)); however, both were carried out at only one dose level. If the exclusion criteria were relaxed to include accuracy results comparing gel to a non gold standard dosimeter (e.g. TLD), results give a median accuracy of 10% (range 8-23.5%) for polymer gel (Cosgrove et al 2000 Phys. Med. Biol. 45 1195-210, De Deene et al 1998 Radiother: Oncol. 48 283-91, Farajollahi et al 2000 Br. J. Radiol. 72 1085-92, McJury et al 1999b Phys. Med. Biol. 44 2431-44, Murphy et al 2000b Phys. Med. Biol. 45 835-45, Oldham et al 2001 Med. Phys. 28 1436-45) and 5% for Fricke gel (Chan and Ayyangar 1995b Med. Phys. 22 1171-5). Evidence also points to accuracy worsening at lower dose levels for both gels. The precision data should be viewed with caution as repeated MR measurements were not performed with the same samples. The only precision data for Fricke gels was 1.5% (Johansson Back et al 1998 Phys. Med. Biol. 43 261-76), but for zero dose. In conclusion, despite the amount of published data, sparse research has been undertaken which provides clear evidence of the accuracy and precision for both gels. That which has been published has used higher doses than would be routine in radiotherapy. The basic radiation dosimeter qualities of accuracy and precision have yet to be fully quantified for polymer and Fricke gels at clinically relevant dose levels.

The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence

The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).

The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence

The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).

Stochastic simulation of gamma radiolysis of acidic ferrous sulfate solution at elevated temperatures

The effect of temperature on the oxidation of Fe2+ in the gamma radiolysis of acidic ferrous sulfate solutions has been modelled using the stochastic IRT method, incorporating simulated electron track structures. There is an 11% increase in the Fe3+ yield in aerated 0.4 M H2SO4 solution on going from 298 to 573 K (25 to 300 degrees C). The H2 yield in aerated solution increases about 24% from 0.41 at 298 K to 0.51 at 573 K. In the absence of oxygen, the increase in the yields of both Fe3+ and H2 is about 12%. Calculated yields are consistent with available experimental data at room, and elevated temperatures. Simulations for heavy water (D2O) show similar temperature effects to those found for ferrous sulfate solutions in light water (H2O).

Dose distribution measurements by MRI of a phantom containing lung tissue equivalent compartments made of ferrous sulphate gel

Gel dosimetry based on magnetic resonance imaging (MRI) has previously been shown to provide verification of calculated dose distributions in soft tissue equivalent homogeneous phantoms. This study demonstrates how measurements of dose distribution can also be achieved in a phantom containing porous, lung-equivalent, Fricke gel. A phantom was made of Fe2+ infused low-density gel and conventional ferrous sulphate gel, filled in separate compartments in a Perspex container. Absorbed dose measurements were accomplished by MR imaging and by calibrating the proton spin-lattice relaxation rate (R1) versus absorbed dose by means of TLD measurements. This study shows that the production of lung-equivalent low-density (LD) dosimeter gel (mean CT number of -610 HU) is feasible. The MR signal detected in the LD gel dosimeter was substantially more noisy (i.e. displayed larger random fluctuations) than the signal from conventional gel, as expected. A deviation between calculated (TPS) and measured dose of about 3% (6 MV) and 4-7% (15 MV) was found in the LD region of the phantom. These results correspond well with data from other studies of dose distribution in lung-equivalent phantoms. The Fe2+ infused LD gel therefore seems suitable for measurement of absorbed dose distribution in phantoms that contain lung tissue compartments.

Ferrous sulphate gel dosimetry and MRI for proton beam dose measurements

Ferrous sulphate gel dosimetry has the potential for measurement of absorbed dose distributions in proton therapy. The chemical properties of the gel are altered according to the radiation dose and these changes can be evaluated in three dimensions using MRI. The purpose of this work was to investigate the properties of a ferrous gel used with clinical proton beams. The gel was irradiated with both monoenergetic and range-modulated proton beams. It was then evaluated using MRI. The depth dose by means of the 1/T1 distribution was studied and compared with data from a plane-parallel plate ionization chamber. 1/T1 was shown to be proportional to the dose at a mean proton energy of approximately 90 MeV. The dose response was no different from that obtained using photon beams. However, on normalization at the entrance, the relative 1/T1 at the Bragg peak was 15-20% lower than the corresponding ionization chamber data for the monoenergetic proton beam. Better agreement was found for the modulated beam, but with significant differences close to the distal edge of the 1/T1 distribution. The change in sensitivity with depth was explained by means of a linear energy transfer dependence. This property was further studied using Monte Carlo methods.

Dose-response curve slope improvement and result reproducibility of ferrous-sulphate-doped gels analysed by NMR imaging

Ferrous sulphate gel analysed by relaxation time measurements with NMR imaging is considered a useful dosimeter for 3D determinations of absorbed dose. A protocol for the gel preparation with agarose SeaPlaque that leads to a dosimeter with very high dose sensitivity is described. The dose-response curve slope is about 0.2 s(-1) Gy(-1) and the G factor turns out to be approximately 185 ions per 100 eV of absorbed energy. A method for making the measurements and analysing the results that brings about good result reproducibility is suggested. A thorough experimental study of the dependence of the dosimeter response on the elapsed time from preparation to irradiation and from irradiation to NMR measurement has revealed good reproducibility. The above characteristic of the gel system is very interesting, because it shows the possibility of utilizing the dosimeter for absolute dose determinations with satisfactory reliability.

Determination of 3D dose distribution from intracavitary brachytherapy of cervical cancer by MRI of irradiated ferrous sulphate gel

BACKGROUND AND PURPOSE

MRI ferrous sulphate gel dosimetry has proven to be a valuable method for assessment of dose delivered in teletherapy. The intention of this study was to investigate ferrous sulphate gel as a possible dosimeter for intracavitary brachytherapy applications.

MATERIALS AND METHODS

A plastic duplicate of a cervix ring applicator set was submerged in Fe2(+)-infused gelatin gel. The gel was subsequently irradiated by a stepwise moving 192Ir source, using automatic afterloading equipment (Microselectron, Nucletron-Oldelft International BV, Veenendaal, The Netherlands). A 3D dose distribution was reconstructed from MR images of the gel.

RESULTS

The gel dose measurements were found to be of the same accuracy as TLD measurements. Isodose curves based on gel dosimetry and isodose curves computed by a dose planning system were generally less than 2 mm apart. MR images showing the position of the applicator set in a patient during treatment were used to obtain images describing patient anatomy in the sagittal and ring planes of the applicator set. Isodose curves computed from the gel measurements were then superimposed on these images, illustrating one possible way of linking dosimetrical and anatomical data.

CONCLUSIONS

Our study shows that MRI ferrous sulphate gel dosimetry is a useful tool for studies of dose distributions in brachytherapy and their relation to critical organs. Possible improvements of the gel dosimeter lie in reducing the diffusion of ferric ions and increasing the radiation sensitivity of the gel.

Multiple-site fast exchange model for spin-lattice relaxation in the Fricke-gelatin dosimeter

The ferrous sulfate-doped gel dosimeters have been developed for three-dimensional magnetic resonance imaging of radiation dose distributions. When the gel dosimeter is irradiated, ferrous ions are converted to ferric ions and the nuclear magnetic spin relaxation of the dosimeter varies with dose. In this paper, a model is presented for the dose dependence of the spin-lattice relaxation rate R1 of the ferrous sulfate doped-gelatin dosimeter. The model is based on three basic physical quantities: the ferric ion yield and the ferrous and ferric ion relaxivities, r2+ and r3+, respectively. These relaxivities specify the ability of the ions to enhance the spin-lattice relaxation of water protons. The effects of gelatin and sulfuric acid concentration on the ferric ion yield and ion relaxivities are presented. The measured r2+ values agree with those predicted by a model in which the measured spin relaxation is considered the result of the fast exchange of water hydrating the ferrous ion with water in the bulk. The r3+ values are lower than predicted by the fast exchange model. The discrepancies in the measured and predicted r3+ values are shown to result from the complexing of ferric ions arising from pH variation caused by changes in gelatin or sulfuric acid concentrations. A modified version of the R1-dose response model accounting for ferric ion complexing is presented and tested spectrophotometrically.

Oxidation yield of the ferrous ion in a Fricke solution irradiated with monochromatic synchrotron soft X-rays in the 1.8-10 keV region

The oxidation yield of ferrous ion in a Fricke solution was measured in the soft X-ray region from 1.8 to 10 keV. The standard Fricke solution was irradiated with monochromatic X-rays from synchrotron radiation. The yield decreased with decreasing X-ray energy, as several theoretical calculations have predicted. No significant changes were observed in the K-shell photo-absorption of the iron and sulphur contained in the standard Fricke solution. The high-LET nature of soft X-rays is discussed.

Radiation therapy verification in brachytherapy combining iron sulphate gel irradiation and magnetic resonance imaging (MRI)

We developed a verification method that allows the three-dimensional qualitative delineation of the dose inside an applicator during intracavitary brachytherapy using MRI of dose-dependent changes of iron sulphate. In addition to computer calculations, the verification of the dose distribution is depicted now within the applicator. Therefore one can get a qualitative information about the relative position of the applicator according to the tumor volume and the desired brachytherapy radiation volume.

Magnetic circular dichroism study of cytochrome ba3 from Thermus thermophilus: spectral contributions from cytochromes b and a3 and nanosecond spectroscopy of CO photodissociation intermediates

Near-UV-vis magnetic and natural circular dichroism (MCD and CD) spectra of oxidized, reduced, and carbonmonoxy-complexed cytochrome ba3, a terminal oxidase from the bacterium Thermus thermophilus, and nanosecond time-resolved MCD (TRMCD) and CD (TRCD) spectra of the unligated species formed after photodissociation of the CO complex are presented. The spectral contributions of individual cytochromes b and a3 to the Soret region MCD are identified. TRMCD spectroscopy is used to follow the spin state change (S = 0 to S = 2) in cytochrome a3(2+) following photodissociation of the CO complex. There is prompt appearance of the high-spin state after photolysis, as found previously in mammalian cytochrome oxidase [Goldbeck, R. A., Dawes, T. D., Einarsdóttir, O., Woodruff, W. H., & Kliger, D. S. (1991) Biophys. J. 60, 125-134]. Peak shifts of 1-10 nm appear in the TRMCD, TRCD, and time-resolved UV-vis absorption spectra of the photolyzed enzyme throughout its observable lifetime, indicating that the photolyzed enzyme does not relax to its equilibrium deliganded form before recombination with CO occurs hundreds of milliseconds later. Direct heme-heme interaction is not found in cytochrome ba3, but red-shifts in the MCD and absorption spectra of both cytochromes b and (photolyzed) a3 are correlated with a CO-liganded form of the protein. The long time (tau approximately greater than 1 s) needed for relaxation of the cytochrome b and a3 peaks to their static positions suggests that CO binding to a3 induces a global conformational change in the protein that weakly perturbs the MCD and absorption spectra of b and photolyzed a3. Fea3 binds CO more weakly in cytochrome ba3 than in cytochrome aa3. The MCD spectrum of reduced enzyme solution placed under 1 atm of CO contains a peak at 446 nm that shows approximately 30% of total cytochrome a3 remains pentacoordinate, high-spin.

Nuclear magnetic relaxation characterization of irradiated Fricke solution

The spin-lattice relaxation rate R1(= T1(-1) of irradiated Fricke solution was studied as a function of the absorbed dose D. The R1 increases linearly with dose up to D approximately 400 Gy after which the response saturates. A model describing the R1 of a solution of either ferrous (Fe2+) or ferric (Fe3+) ions is presented; it is based on the spin relaxation of protons on water molecules in the bulk and protons on water molecules in the coordination shells of the ions with fast exchange occurring between the two water environments. All inherent relaxation parameters of the different proton groups are determined empirically at NMR frequencies of 9 and 25 MHz. An extension of the model is made to describe the spin-lattice relaxation behavior of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin-lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus potentially creating an absolute NMR dosimetry technique. Various practical aspects of the NMR-Fricke system, such as the optimal initial ferrous concentration and the NMR frequency dependence of the sensitivity, are described.

Dose-response curves for Fricke-infused agarose gels as obtained by nuclear magnetic resonance

The radiation-response characteristics of agarose gels prepared with Fricke dosemeter solution have been studied. The response mechanism is an increase in the NMR longitudinal relaxation rate of protons caused by ferric ions. It has been observed that: (i) oxygen saturation assures consistent and maximum sensitivity; (ii) agarose concentrations in the range 1.0-2.0% have no effect upon sensitivity; (iii) the initial G value is 150 Fe3+/100 eV for gels containing 0.5 mM Fe2+ ions; (iv) increasing NMR frequencies only causes a moderate increase in sensitivity; (v) the gel dosemeters are dose rate independent in the range 4.7-24.2 Gy min-1; (vi) sensitivity is pH dependent, being zero at pH 7; (vii) freshly prepared gels are slightly more sensitive than those more than 24 h old; and (ix) the diffusion coefficient for ferric ions in a 1.0% agarose gel containing 0.0125 M H2SO4 is 1.83 x 10(-2) cm2 h-1, and this will require consideration for the NMR imaging of dose distributions.

MR imaging of absorbed dose distributions for radiotherapy using ferrous sulphate gels

The measurement of absorbed dose distributions using dosemeter gel and magnetic resonance imaging (MRI) in a standard geometry has been investigated. Absorbed depth-dose curves and profiles measured with this new technique show good agreement with corresponding measurements using diodes. This was proven in a 60Co beam as well as an electron beam. The dosemeter gel is made of agarose and ferrous sulphate solution. The dose response is linear (r = 0.9996) in the investigated dose interval, 0-40 Gy. The sensitivity is a factor of about six higher compared to ordinary ferrous sulphate solution, known as 'Fricke'. This is a true 3D dose measurement technique which will have a number of applications in radiation therapy, since it is possible to mould the gel to arbitrary geometries, mix different radiation qualities and integrate the absorbed dose from different kinds of fields.

[Comparison of the measurement of absorbed doses by ferrous sulfate dosimetry and by ionization chambers]

A comparison has been carried out among six radiotherapy centres in Canada to determine the absorbed dose by means of ferrous sulphate dosemeters and ionization chambers. Previous to these measurements, epsilon was found to be 2196+/-7 dm3 mol-1 cm-1. The results show that: for 60Co at four centres, two are better than 1% and the other two differ by 2 to 5%; for high energy electrons at four centres, one is better than 1%, another differs by 1 to 3% and the last two differ by more than 3% (the maximum discrepancies are +7% and -6%); for high energy photons at five centres, two are equal to 1%, two others differ by 2 to 4% and the last one differs by + 12%. The chemical dosemeter with irradiation cells made of quartz appeared to be an excellent dosemeter for this type of comparison. The standard error of the means was about 0.3%.