Using electron beam radiation to simulate the dose distribution for whole body solar particle event proton exposure

As a part of the near solar system exploration program, astronauts may receive significant total body proton radiation exposures during a solar particle event (SPE). In the Center for Acute Radiation Research (CARR), symptoms of the acute radiation sickness syndrome induced by conventional radiation are being compared to those induced by SPE-like proton radiation, to determine the relative biological effectiveness (RBE) of SPE protons. In an SPE, the astronaut’s whole body will be exposed to radiation consisting mainly of protons with energies below 50 MeV. In addition to providing for a potentially higher RBE than conventional radiation, the energy distribution for an SPE will produce a relatively inhomogeneous total body dose distribution, with a significantly higher dose delivered to the skin and subcutaneous tissues than to the internal organs. These factors make it difficult to use a ⁶⁰Co standard for RBE comparisons in our experiments. Here, the novel concept of using megavoltage electron beam radiation to more accurately reproduce both the total dose and the dose distribution of SPE protons and make meaningful RBE comparisons between protons and conventional radiation is described. In these studies, Monte Carlo simulation was used to determine the dose distribution of electron beam radiation in small mammals such as mice and ferrets as well as large mammals such as pigs. These studies will help to better define the topography of the time-dose-fractionation versus biological response landscape for astronaut exposure to an SPE.     

Analysis of white blood cell counts in mice after gamma- or proton-radiation exposure

In the coming decades human space exploration is expected to move beyond low-Earth orbit. This transition involves increasing mission time and therefore an increased risk of radiation exposure from solar particle event (SPE) radiation. Acute radiation effects after exposure to SPE radiation are of prime importance due to potential mission-threatening consequences. The major objective of this study was to characterize the dose-response relationship for proton and γ radiation delivered at doses up to 2 Gy at high (0.5 Gy/min) and low (0.5 Gy/h) dose rates using white blood cell (WBC) counts as a biological end point. The results demonstrate a dose-dependent decrease in WBC counts in mice exposed to high- and low-dose-rate proton and γ radiation, suggesting that astronauts exposed to SPE-like radiation may experience a significant decrease in circulating leukocytes.     

Split-dose exposures versus dual ion exposure in human cell neoplastic transformation

Since radiation fields of space contain many-fold more protons than high atomic number, high energy (HZE) particles, cells in astronaut crews will experience on average several proton hits before an HZE hit. Thus radiation regimes of proton exposure before HZE particle exposure simulate space radiation exposure, and measurement of the frequency of neoplastic transformation of human primary cells to anchorage-independent growth simulates an initial step in cancer induction. Although previous investigations indicated a synergistic increase in transformation yields in the cells exposed to protons followed by HZE particles, these experiments did not differentiate between the effect of splitting of the dose into two fractions and that of changing the ion beams. To test this, we irradiated cells with split doses of either protons or HZE particles, then measured clonogenic survival and neoplastic transformation, as measured by colony formation in semi-solid soft agar medium. The data show that the split dose of 20 cGy plus 20 cGy of either H or HZE ions gave about the same effect as the 40 cGy uninterrupted dose, quite different from the effect of the mixed ion beam H + HZE irradiation. We also asked if lower proton doses than 20 cGy followed 15 min later by 20 cGy of HZE ions gave greater than additive transformation frequencies. Substantial increases in transformation levels were observed for all proton doses tested, including 1 cGy. These results point to the signal importance of protons in affecting the effect of space radiation on human cells.     

Response of primary human fibroblasts exposed to solar particle event protons

Solar particle events (SPEs) present a major radiation-related risk for manned exploratory missions in deep space. Within a short period the astronauts may absorb doses that engender acute effects, in addition to the risk of late effects, such as the induction of cancer. Using primary human cells, we studied clonogenic survival and the induction of neoplastic transformation after exposure to a worst case scenario SPE. We simulated such an SPE with monoenergetic protons (50, 100, 1000 MeV) delivered at a dose rate of 1.65 cGy min?1 in a dose range from 0 to 3 Gy. For comparison, we exposed the cells to a high dose rate of 33.3 cGy min?1. X rays (100 kVp, 8 mA, 1.7 mm Al filter) were used as a reference radiation. Overall, we observed a significant sparing effect of the SPE dose rate on cell survival. High-dose-rate protons were also more efficient in induction of transformation in the dose range below 30 cGy. However, as dose accumulated at high dose rate, the transformation levels declined, while at the SPE dose rate, the number of transformants continued to increase up to about 1 Gy. These findings suggest that considering dose-rate effects may be important in evaluating the biological effects of exposure to space radiation. Our analyses of the data based on particle fluence showed that lethality and transforming potential per particle clearly increased with increasing linear energy transfer (LET) and thus with the decreasing energy of protons. Further, we found that the biological response was determined not only by LET but also type of radiation, e.g. particles and photons. This suggests that using γ or X rays may not be ideal for assessing risk associated with SPE exposures.     

Cytoplasmic localization of programmed cell death 4 contributes to its anti-apoptotic function

Energetic protons are the most abundant particle type in space and can pose serious health risks to astronauts during long-duration missions. The health effects of proton exposure are also a concern for cancer patients undergoing radiation treatment with accelerated protons. To investigate the damage induced by energetic protons in vivo to radiosensitive organs, 6-week-old BALB/c male mice were subjected to 250 MeV proton radiation at whole-body doses of 0.1, 1, and 2 Gy. The gastrointestinal (GI) tract of each exposed animal was dissected 4 h post-irradiation, and the isolated small intestinal tissue was analyzed for histopathological and gene expression changes. Histopathologic observation of the tissue using standard hematoxylin and eosin (H&E) staining methods to screen for morphologic changes showed a marked increase in apoptotic lesions for even the lowest dose of 0.1 Gy, similar to X- or γ rays. The percentage of apoptotic cells increased dose-dependently, but the dose response appeared supralinear, indicating hypersensitivity at low doses. A significant decrease in surviving crypts and mucosal surface area, as well as in cell proliferation, was also observed in irradiated mice. Gene expression analysis of 84 genes involved in the apoptotic process showed that most of the genes affected by protons were common between the low (0.1 Gy) and high (1 and 2 Gy) doses. However, the genes that were distinctively responsive to the low or high doses suggest that high doses of protons may cause apoptosis in the small intestine by direct damage to the DNA, whereas low doses of protons may trigger apoptosis through a different stress response mechanism.     

Beta-radiation-induced resistance to MNNG initiation of papilloma but not carcinoma formation in mouse skin

We have shown previously that the risk of tumor initiation, promotion, and progression in animals initiated with alkylating agents can be drastically altered by hyperthermia treatments. We show here that ionizing radiation can also alter the risk of tumor initiation by alkylating agents. Using a two-step skin tumorigenesis protocol in female SENCAR mice (initiation by MNNG, promotion with TPA), we exposed the dorsal skin of the mice to various doses of 90Sr/90Y beta radiation near the time of initiation. The radiation produced a dose-dependent reduction in the number of papillomas which appeared after TPA promotion, with about a 20% reduction in animals receiving 0.5 Gy surface dose just before initiation, about 50% reduction after 2.5 Gy, and greater than 80% at doses above 5 Gy. A dose of 2.5 Gy in animals initiated with DMBA produced no significant reduction. One skin hyperthermia treatment (44 degrees C, 30 min) along with radiation in MNNG-initiated animals partially blocked the protective effect of radiation and increased the papilloma frequency. Radiation (2.5 Gy) given either 6 days before or after MNNG initiation was less effective but still reduced papilloma frequency about 20%. In sharp contrast to the marked reduction in papilloma formation, these same animals showed no change in carcinoma frequency with any of the doses or schedules of beta radiation. MNNG initiation alone produced three types of initiated cells. One type, produced in low yield, was promotion-independent with a high probability of progression to a carcinoma and appeared unaffected by the radiation. A second type, produced in intermediate yield, was promotion-dependent and also had a high progression probability, but was likewise unaffected by the radiation. The third and most abundant type was promotion-dependent with a very low progression probability. Radiation exposure resulted in a decrease in the risk of an MNNG initiation event which led only to the third type of cell. The data therefore indicate that the risk of some, but not all, tumor-initiating events caused by alkylating agents can be reduced by an exposure to ionizing radiation.     

Calibration of MTT assay in proton beams using radiochromic films

PurposeThis study provides methodology of calibrating as well as controlling the output for an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay irradiated in a low energy proton beam using EBT3-model GAFCHROMIC^TM film, without correcting for quenching effect.MethodsA calibrated Markus ionization chamber was used to measure the depth dose and beam output for 26.5 MeV protons produced by a CS30 cyclotron. A time-controlled aluminum cylinder was added in front of the horizontal beam-exit serving as a radiation shutter. Following the TRS-398 reference dosimetry protocol for proton beams, the output was calibrated in water at a reference depth of 3 mm. EBT3 film was calibrated for doses up to 8 Gy at the same depth. To verify the dose distribution for each 96-well MTT assay plate, EBT3 film was placed at the reference depth during irradiation and cell doses were scaled by measured percent depth dose (PDD) data.ResultsThe radiochromic film dosimetry system in this study provides dose measurements with an uncertainty better than 3.3% for doses higher than 1 Gy. From a single exposure and utilizing the Gaussian shape of the beam, multiple dose points can be obtained within different wells of the same plate ranging from 6.9 Gy (sigma ∼4%) in the central well, and 2 Gy (sigma ∼8%) for wells positioned closer to the periphery.ConclusionsWe described a methodology for radiochromic film-based dose monitoring system, using low-energy protons, which can be used for the MTT assay in any proton beam, except within Bragg peak region.     

Effect of radiation dose on the recovery of aerobic and anaerobic bacteria from mice

The presence of aerobic and anaerobic bacteria in the blood, spleen, and liver was investigated in mice that were exposed to 7, 8, 9, or 10 Gy 60Co radiation. Microorganisms were detected more often in animals exposed to higher doses of radiation. The number of mice that were culture positive and the number of isolates in one site increased with increasing dose. Bacteria were recovered in mice killed at various times after radiation, in 3 of 100 mice exposed to 7 Gy, in 13 of 100 irradiated with 8 Gy, in 23 of 90 exposed to 9 Gy, and in 34 of 87 irradiated with 10 Gy. The predominant organisms recovered were Escherichia coli, anaerobic Gram-positive cocci, Proteus mirabilis, Staphylococcus aureus, and Bacteroides spp. Escherichia coli and anaerobes were more often isolated in animals exposed to 10 Gy, while S. aureus was more often recovered in those irradiated with 9 Gy. These data demonstrate a relationship between the dose of radiation and the rate of infection due to enteric aerobic and anaerobic bacteria.     

Evaluation of the cataractogenic effect of 9 GeV protons

A study was made of the regularities of formation of lenticular opacity in mice exposed to 9 GeV protons and 60Co-gamma-rays. The RBE coefficients, calculated by the nonparametric method, were found to depend upon dose and time after irradiation. It was shown that after small radiation doses (0.25 to 0.50 Gy) the RBE coefficients increased from 1 to 8 with increasing period of observation. With higher doses (up to 5.0 Gy) the RBE coefficient increase in time was less pronounced.     

Changes in cellular response to the damage induced in PC-3 prostate cancer cells by proton microbeam irradiation

The aim of this research was to find out whether the passage number effect may influence on the PC-3 cells (the human prostate cancer line derived from bone metastases) response to proton radiation. 2 MeV horizontally focused proton microbeam was used as a radiation source. The cells were treated with a counted number of H(+) ions (50-8000) corresponding to doses of 1.3-209 Gy/cell. For comparison, cell death was also induced by UVC radiation. All cells were stained with Hoechst 33342 and propidium iodide and visualized under a fluorescence microscope. Necrosis was observed at: a) 8000 protons per cell (corresponding to ～209 Gy/cell) after 2-4 passages, b) 3200 protons per cell (corresponding to ～84 Gy/cell) for cells after 11-14 passages and c) only 800 protons per cell (corresponding to ～2 Gy/cell ) after 47-50 passages. Apoptosis was efficiently induced, by protons, only in cells after 50 passages. The results showed that the laboratory conditions affected cellular response of PC-3 cell line to the proton irradiation. The cellular response to the radiation treatment strongly depends on number of passages.     

Long-term dose response of trabecular bone in mice to proton radiation

Astronauts on exploratory missions will experience a complex environment, including microgravity and radiation. While the deleterious effects of unloading on bone are well established, fewer studies have focused on the effects of radiation. We previously demonstrated that 2 Gy of ionizing radiation has deleterious effects on trabecular bone in mice 4 months after exposure. The present study investigated the skeletal response after total doses of proton radiation that astronauts may be exposed to during a solar particle event. We exposed mice to 0.5, 1 or 2 Gy of whole-body proton radiation and killed them humanely 117 days later. Tibiae and femora were analyzed using microcomputed tomography, mechanical testing, mineral composition and quantitative histomorphometry. Relative to control mice, mice exposed to 2 Gy had significant differences in trabecular bone volume fraction (-20%), trabecular separation (+11%), and trabecular volumetric bone mineral density (-19%). Exposure to 1 Gy radiation induced a nonsignificant trend in trabecular bone volume fraction (-13%), while exposure to 0.5 Gy resulted in no differences. No response was detected in cortical bone. Further analysis of the 1-Gy mice using synchrotron microCT revealed a significantly lower trabecular bone volume fraction (-13%) than in control mice. Trabecular bone loss 4 months after exposure to 1 Gy highlights the importance of further examination of how space radiation affects bone.     

Effects of 28Si Ions, 56Fe Ions,and Protons on the Induction of Murine Acute Myeloid Leukemia and Hepatocellular Carcinoma

Estimates of cancer risks posed to space-flight crews by exposure to high atomic number, high-energy (HZE) ions are subject to considerable uncertainty because epidemiological data do not exist for human populations exposed to similar radiation qualities. We assessed the carcinogenic effects of 300 MeV/n ²⁸Si or 600 MeV/n ⁵⁶Fe ions in a mouse model for radiation-induced acute myeloid leukemia and hepatocellular carcinoma. C3H/HeNCrl mice were irradiated with 0.1, 0.2, 0.4, or 1 Gy of 300 MeV/n ²⁸Si ions, 600 MeV/n ⁵⁶Fe ions or 1 or 2 Gy of protons simulating the 1972 solar particle event (1972SPE) at the NASA Space Radiation Laboratory. Additional mice were irradiated with ¹³⁷Cs gamma rays at doses of 1, 2, or 3 Gy. All groups were followed until they were moribund or reached 800 days of age. We found that ²⁸Si or ⁵⁶Fe ions do not appear to be substantially more effective than gamma rays for the induction of acute myeloid leukemia. However, ²⁸Si or ⁵⁶Fe ion irradiated mice had a much higher incidence of hepatocellular carcinoma than gamma ray irradiated or proton irradiated mice. These data demonstrate a clear difference in the effects of these HZE ions on the induction of leukemia compared to solid tumors, suggesting potentially different mechanisms of tumorigenesis. Also seen in this study was an increase in metastatic hepatocellular carcinoma in the ²⁸Si and ⁵⁶Fe ion irradiated mice compared with those exposed to gamma rays or 1972SPE protons, a finding with important implications for setting radiation exposure limits for space-flight crew members.     

Hindlimb Suspension and SPE-Like Radiation Impairs Clearance of Bacterial Infections

A major risk of extended space travel is the combined effects of weightlessness and radiation exposure on the immune system. In this study, we used the hindlimb suspension model of microgravity that includes the other space stressors, situational and confinement stress and alterations in food intake, and solar particle event (SPE)-like radiation to measure the combined effects on the ability to control bacterial infections. A massive increase in morbidity and decrease in the ability to control bacterial growth was observed using 2 different types of bacteria delivered by systemic and pulmonary routes in 3 different strains of mice. These data suggest that an astronaut exposed to a strong SPE during extended space travel is at increased risk for the development of infections that could potentially be severe and interfere with mission success and astronaut health.     

Out-of-field doses from secondary radiation produced in proton therapy and the associated risk of radiation-induced cancer from a brain tumor treatment

PurposeTo determine out-of-field doses produced in proton pencil beam scanning (PBS) therapy using Monte Carlo simulations and to estimate the associated risk of radiation-induced second cancer from a brain tumor treatment.MethodsSimulations of out-of-field absorbed doses were performed with MCNP6 and benchmarked against measurements with tissue-equivalent proportional counters (TEPC) for three irradiation setups: two irradiations of a water phantom using proton energies of 78–147 MeV and 177–223 MeV, and one brain tumor irradiation of a whole-body phantom. Out-of-field absorbed and equivalent doses to organs in a whole-body phantom following a brain tumor treatment were subsequently simulated and used to estimate the risk of radiation-induced cancer. Additionally, the contribution of absorbed dose originating from radiation produced in the nozzle was calculated from simulations.ResultsOut-of-field absorbed doses to the TEPC ranged from 0.4 to 135 µGy/Gy. The average deviation between simulations and measurements of the water phantom irradiations was about 17%. The absorbed dose contribution from radiation produced in the nozzle ranged between 0 and 70% of the total dose; the contribution was however small in absolute terms. The absorbed and equivalent doses to the organs ranged between 0.2 and 60 µGy/Gy and 0.5–151 µSv/Gy. The estimated lifetime risk of radiation-induced second cancer was approximately 0.01%.ConclusionsThe agreement of out-of-field absorbed doses between measurements and simulations was good given the sources of uncertainties. Calculations of out-of-field organ doses following a brain tumor treatment indicated that proton PBS therapy of brain tumors is associated with a low risk of radiation-induced cancer.     

Calculation and measurement of doses in the surface layers of a phantom when using Tomotherapy

BackgroundThe calculation and measurement on the surface of the skin presents a significant dosimetric problem because of numerous factors which have an influence on the dose distribution in this region.AimThe overall aim of this study was to check the agreement between doses measured with thermoluminescent detectors (TLD) during tomotherapy photon beam irradiation of the skin area of a solid water cylindrical phantom with doses calculated with Hi-Art treatment planning system (TPS).Material and MethodThe measurements of the dose were made with the use of a solid water cylindrical phantom - Cheese Phantom. Two bolus phantoms were used: 5 mm and 10 mm Six different planning treatments were generated. The doses were measured using TL detectors.ResultsIn the case of a tumor located near the surface of the skin, the mean dose for 0.5 cm bolus was - 1.94 Gy, and for 1 cm bolus - 2.03 Gy. For the tumor located inside the phantom and organ at risk on the same side that TL detectors, for a 0.5 cm bolus, mean dose was 0.658 Gy, and for a 1 cm bolus, 0.62 Gy.ConclusionThe analysis of results showed that the relative percentage difference between measured and planned dose in the field of irradiation was less than 10%, while the largest differences were on the board of the field of radiation and outside of the field of irradiation, where the dose was 0.08 Gy to 1 Gy.     

Improved normal tissue protection by proton and X-ray microchannels compared to homogeneous field irradiation

The risk of developing normal tissue injuries often limits the radiation dose that can be applied to the tumour in radiation therapy. Microbeam Radiation Therapy (MRT), a spatially fractionated photon radiotherapy is currently tested at the European Synchrotron Radiation Facility (ESRF) to improve normal tissue protection. MRT utilizes an array of microscopically thin and nearly parallel X-ray beams that are generated by a synchrotron. At the ion microprobe SNAKE in Munich focused proton microbeams (“proton microchannels”) are studied to improve normal tissue protection. Here, we comparatively investigate microbeam/microchannel irradiations with sub-millimetre X-ray versus proton beams to minimize the risk of normal tissue damage in a human skin model, in vitro. Skin tissues were irradiated with a mean dose of 2 Gy over the irradiated area either with parallel synchrotron-generated X-ray beams at the ESRF or with 20 MeV protons at SNAKE using four different irradiation modes: homogeneous field, parallel lines and microchannel applications using two different channel sizes. Normal tissue viability as determined in an MTT test was significantly higher after proton or X-ray microchannel irradiation compared to a homogeneous field irradiation. In line with these findings genetic damage, as determined by the measurement of micronuclei in keratinocytes, was significantly reduced after proton or X-ray microchannel compared to a homogeneous field irradiation. Our data show that skin irradiation using either X-ray or proton microchannels maintain a higher cell viability and DNA integrity compared to a homogeneous irradiation, and thus might improve normal tissue protection after radiation therapy.     

Shielding of relativistic protons

Protons are the most abundant element in the galactic cosmic radiation, and the energy spectrum peaks around 1 GeV. Shielding of relativistic protons is therefore a key problem in the radiation protection strategy of crewmembers involved in long-term missions in deep space. Hydrogen ions were accelerated up to 1 GeV at the NASA Space Radiation Laboratory, Brookhaven National Laboratory, New York. The proton beam was also shielded with thick (about 20 g/cm²) blocks of lucite (PMMA) or aluminium (Al). We found that the dose rate was increased 40–60% by the shielding and decreased as a function of the distance along the axis. Simulations using the General–Purpose Particle and Heavy-Ion Transport code System (PHITS) show that the dose increase is mostly caused by secondary protons emitted by the target. The modified radiation field after the shield has been characterized for its biological effectiveness by measuring chromosomal aberrations in human peripheral blood lymphocytes exposed just behind the shield block, or to the direct beam, in the dose range 0.5–3 Gy. Notwithstanding the increased dose per incident proton, the fraction of aberrant cells at the same dose in the sample position was not significantly modified by the shield. The PHITS code simulations show that, albeit secondary protons are slower than incident nuclei, the LET spectrum is still contained in the low-LET range (<10 keV/μm), which explains the approximately unitary value measured for the relative biological effectiveness.     

Response of rat skin to high-dose unidirectional x-ray microbeams: a histological study

There is growing interest in evaluating microbeam radiation therapy as a potential clinical modality. Microbeam radiation therapy uses arrays of parallel, microscopically thin (<100 microm) planes of synchrotron-generated X rays (microplanar beams, or microbeams). Due to the relatively low beam energies involved in microbeam radiation therapy (a median beam energy of 120 keV was used in the present study), the dose penetration of microbeams in tissue is lower than that used in conventional radiotherapy. This lower energy necessitates using a significantly elevated dose to the skin's surface during clinical microbeam therapy to ensure an adequate dose distribution in the target tumor. The findings of the present study, using a rat skin model, indicated that the skin had an extremely high tolerance to microbeam radiation at doses considerably in excess of those that were therapeutically effective in preclinical studies. A histological study was undertaken to evaluate the biological mechanisms underlying this high tolerance. The irradiation configuration employed single-exposure, unidirectional microbeams 90 microm wide, with 300 microm beam spacing on-center. The in-beam skin-surface absorbed doses were in the range 835-1335 Gy. Monte Carlo simulations of the dose distribution indicated that the "valley" dose, i.e. the radiation leakage between adjacent microbeams, was about 2.5% of the in-beam dose. The high tolerance of the rats' skin to microbeams and the rapid regeneration of the damaged segments of skin were attributed to the surviving clonogenic cells situated between the adjacent microplanar beams. In the epidermis, clonogenic cells in the hair follicular epithelium appeared to play a key role in the regeneration process.     

Verification of electron beam parameters in an intraoperative linear accelerator using dosimetric and radiobiological response methods

BackgroundThe availability of linear accelerators (linac) for research purposes is often limited and therefore alternative radiation sources are needed to conduct radiobiological research. The National Centre for Radiation Research in Poland recently developed an intraoperative mobile linac that enables electron irradiation at energies ranging from 4 to 12 MeV and dose rates of 5 or 10 Gy/min. The present study was conducted to evaluate the electron beam parameters of this intraoperative linac and to verify the set-up to evaluate out-of-field doses in a water phantom, which were determined through dosimetric and biological response measurements.Materials and methodsThe distribution of radiation doses along and across the radiation beam were measured in a water phantom using a semiconductor detector and absolute doses using an ionisation chamber. Two luminal breast cancer cell lines (T-47D and HER2 positive SK-BR-3) were placed in the phantom to study radiation response at doses ranging from 2 to 10 Gy. Cell response was measured by clonogenic assays.Results and ConclusionThe electron beam properties, including depth doses and profiles, were within expected range for the stated energies. These results confirm the viability of this device and set-up as a source of megavoltage electrons to evaluate the radiobiological response of tumour cells.