Combined effects of heavy ion exposure and simulated Lunar gravity on skeletal muscle.

BACKGROUND: The limitations to prolonged spaceflight include unloading-induced atrophy of the musculoskeletal system which may be enhanced by exposure to the space radiation environment. Previous results have concluded that partial gravity, comparable to the Lunar surface, may have detrimental effects on skeletal muscle. However, little is known if these outcomes are exacerbated by exposure to low-dose rate, high-energy radiation common to the space environment. Therefore, the present study sought to determine the impact of highly charge, high-energy (HZE) radiation on skeletal muscle when combined with partial weightbearing to simulate Lunar gravity. We hypothesized that partial unloading would compromise skeletal muscle and these effects would be exacerbated by radiation exposure. METHODS: For month old female BALB/cByJ mice were -assigned to one of 2 groups; either full weight bearing (Cage Controls, CC) or partial weight bearing equal to 1/6th bodyweight (G/6). Both groups were then divided to receive either a single whole body absorbed dose of 0.5 Gy of 300 MeV 28Si ions (RAD) or a sham treatment (SHAM). Radiation exposure experiments were performed at the NASA Space Radiation Laboratory (NSRL) located at Brookhaven National Laboratory on Day 0, followed by 21 d of CC or G/6 loading. Muscles of the hind limb were used to measure protein synthesis and other histological measures. RESULTS: Twenty-one days of Lunar gravity (G/6) resulted in lower soleus, plantaris, and gastrocnemius muscle mass. Radiation exposure did not further impact muscle mass. 28Si exposure in normal ambulatory animals (RAD+CC) did not impact gastrocnemius muscle mass when compared to SHAM+CC (p>0.05), but did affect the soleus, where mass was higher following radiation compared to SHAM (p<0.05). Mixed gastrocnemius muscle protein synthesis was lower in both unloading groups. Fiber type composition transitioned towards a faster isoform with partial unloading and was not further impacted by radiation. The combined effects of partial loading and radiation partially mitigated fiber cross-sectional area when compared to partial loading alone. Radiation and G/6 reduced the total number of myonuclei per fiber while leading to elevated BrdU content of skeletal muscle. Similarly, unloading and radiation resulted in higher collagen content of muscle when compared to controls, but the effects of combined exposure were not additive. CONCLUSIONS: The results of this study confirm that partial weightbearing causes muscle atrophy, in part due to reductions of muscle protein synthesis in the soleus and gastrocnemius as well as reduced peripheral nuclei per fiber. Additionally, we present novel data illustrating 28Si exposure reduced nuclei in muscle fibers despite higher satellite cell fusion, but did not exacerbate muscle atrophy, CSA changes, or collagen content. In conclusion, both partial loading and HZE radiation can negatively impact muscle morphology.

Positive impact of low-dose, high-energy radiation on bone in partial- and/or full-weightbearing mice.

Astronauts traveling beyond low Earth orbit will be exposed to galactic cosmic radiation (GCR); understanding how high energy ionizing radiation modifies the bone response to mechanical unloading is important to assuring crew health. To investigate this, we exposed 4-mo-old female Balb/cBYJ mice to an acute space-relevant dose of 0.5 Gy 56Fe or sham (n = ~8/group); 4 days later, half of the mice were also subjected to a ground-based analog for 1/6 g (partial weightbearing) (G/6) for 21 days. Microcomputed tomography (µ-CT) of the distal femur reveals that 56Fe exposure resulted in 65-78% greater volume and improved microarchitecture of cancellous bone after 21 d compared to sham controls. Radiation also leads to significant increases in three measures of energy absorption at the mid-shaft femur and an increase in stiffness of the L4 vertebra. No significant effects of radiation on bone formation indices are detected; however, G/6 leads to reduced % mineralizing surface on the inner mid-tibial bone surface. In separate groups allowed 21 days of weightbearing recovery from G/6 and/or 56Fe exposure, radiation-exposed mice still exhibit greater bone mass and improved microarchitecture vs. sham control. However, femoral bone energy absorption values are no longer higher in the 56Fe-exposed WB mice vs. sham controls. We provide evidence for persistent positive impacts of high-LET radiation exposure preceding a period of full or partial weightbearing on bone mass and microarchitecture in the distal femur and, for full weightbearing mice only and more transiently, cortical bone energy absorption values.

Exposure to Low-Dose X-Ray Radiation Alters Bone Progenitor Cells and Bone Microarchitecture.

Exposure to high-dose ionizing radiation during medical treatment exerts well-documented deleterious effects on bone health, reducing bone density and contributing to bone growth retardation in young patients and spontaneous fracture in postmenopausal women. However, the majority of human radiation exposures occur in a much lower dose range than that used in the radiation oncology clinic. Furthermore, very few studies have examined the effects of low-dose ionizing radiation on bone integrity and results have been inconsistent. In this study, mice were irradiated with a total-body dose of 0.17, 0.5 or 1 Gy to quantify the early (day 3 postirradiation) and delayed (day 21 postirradiation) effects of radiation on bone microarchitecture and bone marrow stromal cells (BMSCs). Female BALBc mice (4 months old) were divided into four groups: irradiated (0.17, 0.5 and 1 Gy) and sham-irradiated controls (0 Gy). Micro-computed tomography analysis of distal femur trabecular bone from animals at day 21 after exposure to 1 Gy of X-ray radiation revealed a 21% smaller bone volume (BV/TV), 22% decrease in trabecular numbers (Tb.N) and 9% greater trabecular separation (Tb.Sp) compared to sham-irradiated controls (P < 0.05). We evaluated the differentiation capacity of bone marrow stromal cells harvested at days 3 and 21 postirradiation into osteoblast and adipocyte cells. Osteoblast and adipocyte differentiation was decreased when cells were harvested at day 3 postirradiation but enhanced in cells isolated at day 21 postirradiation, suggesting a compensatory recovery process. Osteoclast differentiation was increased in 1 Gy irradiated BMSCs harvested at day 3 postirradiation, but not in those harvested at day 21 postirradiation, compared to controls. This study provides evidence of an early, radiation-induced decrease in osteoblast activity and numbers, as well as a later recovery effect after exposure to 1 Gy of X-rays, whereas osteoclastogenesis was enhanced. A better understanding of the effects of radiation on osteoprogenitor cell populations could lead to more effective therapeutic interventions that protect bone integrity for individuals exposed to low-dose ionizing radiation.

Physical Considerations.

It seems likely that the biological consequences of radiation exposure are the result of sequences of biochemical processes initiated by the chemical changes caused by energy deposited as ionization or excitation. Based on this assumption, it would appear that a detailed knowledge of the energy deposited in relevant biological targets would provide a basis for predicting the biological risk. However, there is generally insufficient knowledge of the biological processes for such predictions to be successful. However, knowledge of energy deposition characteristics can help to determine if specific models of biological processes are tenable. Furthermore, an alternative to absorbed dose may provide a way to characterize the energy deposition, which is more easily related to the health risk and less likely to be misconstrued when used to describe low levels of radiation exposure. Some examples of limits on models and alternatives to absorbed dose are described.

Simulated Response of a Tissue-equivalent Proportional Counter on the Surface of Mars.

Uncertainties persist regarding the assessment of the carcinogenic risk associated with galactic cosmic ray (GCR) exposure during a mission to Mars. The GCR spectrum peaks in the range of 300(-1) MeV n to 700 MeV n(-1) and is comprised of elemental ions from H to Ni. While Fe ions represent only 0.03% of the GCR spectrum in terms of particle abundance, they are responsible for nearly 30% of the dose equivalent in free space. Because of this, radiation biology studies focusing on understanding the biological effects of GCR exposure generally use Fe ions. Acting as a thin shield, the Martian atmosphere alters the GCR spectrum in a manner that significantly reduces the importance of Fe ions. Additionally, albedo particles emanating from the regolith complicate the radiation environment. The present study uses the Monte Carlo code FLUKA to simulate the response of a tissue-equivalent proportional counter on the surface of Mars to produce dosimetry quantities and microdosimetry distributions. The dose equivalent rate on the surface of Mars was found to be 0.18 Sv y(-1) with an average quality factor of 2.9 and a dose mean lineal energy of 18.4 keV µm(-1). Additionally, albedo neutrons were found to account for 25% of the dose equivalent. It is anticipated that these data will provide relevant starting points for use in future risk assessment and mission planning studies.

A novel algorithm for solving the true coincident counting issues in Monte Carlo simulations for radiation spectroscopy.

Coincident counts can be observed in experimental radiation spectroscopy. Accurate quantification of the radiation source requires the detection efficiency of the spectrometer, which is often experimentally determined. However, Monte Carlo analysis can be used to supplement experimental approaches to determine the detection efficiency a priori. The traditional Monte Carlo method overestimates the detection efficiency as a result of omitting coincident counts caused mainly by multiple cascade source particles. In this study, a novel "multi-primary coincident counting" algorithm was developed using the Geant4 Monte Carlo simulation toolkit. A high-purity Germanium detector for 6°Co gamma-ray spectroscopy problems was accurately modeled to validate the developed algorithm. The simulated pulse height spectrum agreed well qualitatively with the measured spectrum obtained using the high-purity Germanium detector. The developed algorithm can be extended to other applications, with a particular emphasis on challenging radiation fields, such as counting multiple types of coincident radiations released from nuclear fission or used nuclear fuel.

Effect of wall thickness on measurement of dose for high energy neutrons.

Neutrons produced from the interaction between galactic cosmic rays and spacecraft materials are responsible for a very important portion of the dose received by astronauts. The neutron energy spectrum depends on the incident charged particle spectrum and the scattering environment but generally extends to beyond 100 MeV. Tissue-equivalent proportional counters (TEPC) are used to measure the dose during the space mission, but their weight and size are very important factors for their design and construction. To achieve ideal neutron dosimetry, the wall thickness should be at least the range of a proton having the maximum energy of the neutrons to be monitored. This proton range is 0.1 cm for 10 MeV neutrons and 7.6 cm for 100 MeV neutrons. A 7.6 cm wall thickness TEPC would provide charged particle equilibrium (CPE) for neutrons up to 100 MeV, but for space applications it would not be reasonable in terms of weight and size. In order to estimate the errors in measured dose due to absence of CPE, MCNPX simulations of energy deposited by 10 MeV and 100 MeV neutrons in sites with wall thickness between 0.1 cm and 8.5 cm were performed. The results for 100 MeV neutrons show that energy deposition per incident neutron approaches a plateau as the wall thickness approaches 7.6 cm. For the 10 MeV neutrons, energy deposition per incident neutron decreases as the wall thickness increases above 0.1 cm due to attenuation.

Dosimetry of the 198Au source used in interstitial brachytherapy.

The American Association of Physicists in Medicine Task Group 43 reports, AAPM TG-43 and its update TG-43U1, provide an analytical model and a dosimetry protocol for brachytherapy dose calculations, as well as documentation and results for some sealed sources. The radionuclide 198Au (T(1/2)=2.70 days, Egamma=412 keV) has been used in the form of seeds for brachytherapy treatments including brain, eye, and prostate tumors. However, TG-43 reports have no data for 198Au seeds, and none have previously been obtained. For that reason, and because of the conversion of most treatment planning systems to TG-43 based methods, both Monte Carlo calculations (MCNP 4C2) and thermoluminescent dosimeters (TLDs) are used in this work to determine these data. The geometric variation in dose is measured using an array of TLDs in a solid water phantom, and the seed activity is determined using a high purity germanium detector (HPGe) and a well ionization chamber. The results for air kerma strength, Sk, per unit apparent activity, are 2.063 (MCNP) and 2.089 (measured) U mCi(-1), values close to those published in 1991 in the AAPM Task Group 32 report. The dose rate constant, lambda, is found equal to 1.115 (MCNP) and 1.095 (measured) cGy h(-1) U(-1). The radial dose function, g(r), anisotropy function, F(r, theta), and anisotropy factor, phi(an)(r), are also given.

Testing for spatial correlation in nonstationary binary data, with application to aberrant crypt foci in colon carcinogenesis.

In an experiment to understand colon carcinogenesis, all animals were exposed to a carcinogen, with half the animals also being exposed to radiation. Spatially, we measured the existence of what are referred to as aberrant crypt foci (ACF), namely, morphologically changed colonic crypts that are known to be precursors of colon cancer development. The biological question of interest is whether the locations of these ACFs are spatially correlated: if so, this indicates that damage to the colon due to carcinogens and radiation is localized. Statistically, the data take the form of binary outcomes (corresponding to the existence of an ACF) on a regular grid. We develop score-type methods based upon the Matern and conditionally autoregressive (CAR) correlation models to test for the spatial correlation in such data, while allowing for nonstationarity. Because of a technical peculiarity of the score-type test, we also develop robust versions of the method. The methods are compared to a generalization of Moran's test for continuous outcomes, and are shown via simulation to have the potential for increased power. When applied to our data, the methods indicate the existence of spatial correlation, and hence indicate localization of damage.

Opportunities for nutritional amelioration of radiation-induced cellular damage.

The closed environment and limited evasive capabilities inherent in space flight cause astronauts to be exposed to many potential harmful agents (chemical contaminants in the environment and cosmic radiation exposure). Current power systems used to achieve space flight are prohibitively expensive for supporting the weight requirements to fully shield astronauts from cosmic radiation. Therefore, radiation poses a major, currently unresolvable risk for astronauts, especially for long-duration space flights. The major detrimental radiation effects that are of primary concern for long-duration space flights are damage to the lens of the eye, damage to the immune system, damage to the central nervous system, and cancer. In addition to the direct damage to biological molecules in cells, radiation exposure induces oxidative damage. Many natural antioxidants, whether consumed before or after radiation exposure, are able to confer some level of radioprotection. In addition to achieving beneficial effects from long-known antioxidants such as vitamins E and C and folic acid, some protection is conferred by several recently discovered antioxidant molecules, such as flavonoids, epigallocatechin, and other polyphenols. Somewhat counterintuitive is the protection provided by diets containing elevated levels of omega-3 polyunsaturated fatty acids, considering they are thought to be prone to peroxidation. Even with the information we have at our disposal, it will be difficult to predict the types of dietary modifications that can best reduce the risk of radiation exposure to astronauts, those living on Earth, or those enduring diagnostic or therapeutic radiation exposure. Much more work must be done in humans, whether on Earth or, preferably, in space, before we are able to make concrete recommendations.