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Commentary, Cognition and Behavior

Comments on “New Concerns for Neurocognitive Function during Deep Space Exposures to Chronic, Low Dose Rate, Neutron Radiation”

Joseph J. Bevelacqua, James Welsh and S. M. Javad Mortazavi
eNeuro 19 December 2019, 7 (1) ENEURO.0329-19.2019; DOI: https://doi.org/10.1523/ENEURO.0329-19.2019
Joseph J. Bevelacqua
1Bevelacqua Resources, Richland, Washington 99352
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James Welsh
2Department of Radiation Oncology, Edward Hines Jr VA Hospital, Hines, Illinois 60141
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S. M. Javad Mortazavi
3Medical Physics Department, Shiraz University of Medical Sciences, Shiraz, IR
4Diagnostic Imaging Department, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111
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Abstract

Evaluations of the biological effects of space radiation must carefully consider the biological system response and the specific nature of the source term. Acharya et al. (2019) review neurocognitive function during deep space exposures to chronic, low dose rate, neutron radiation, but do not use a source term that reflects the actual space environment in terms of radiation types and their respective energies. In addition, important biological effects, including the adaptive response to the space radiation environment, are not addressed.

  • HZE
  • LET
  • neurocognitive function
  • neutrons
  • radiation
  • space

Significance Statement

Acharya et al. (2019) review neurocognitive function during deep space exposures to chronic, low dose rate, neutron radiation, but do not use a source term that reflects the actual space environment in terms of radiation types and their respective energies. In addition, important biological effects including adaptive response to the space radiation environment are not addressed.

This commentary addresses the article “New concerns for neurocognitive function during deep space exposures to chronic, low dose rate, neutron radiation” by Acharya et al. (2019). Considering the limitations of currently available technology for simulating the space radiation environment, this article (Acharya et al., 2019) outlines the use of a new neutron irradiation facility to simulate the low dose rates found in deep space. Their study showed neurobehavioral and electrophysiological defects in rodents subjected to continuous (6 month duration) exposure to low dose rate (1 mGy/d) neutron exposures. Despite the numerous strengths of this study, it has a few major shortcomings. The first shortcoming is due to ignoring the key point that in a realistic space environment, cells will be exposed to multiple low LET (linear energy transfer) protons before being traversed by intermediate and high-LET HZE (high charge and energy) particles. It is worth noting that a National Aeronautics and Space Administration report (Huff et al., 2016) clearly states that this sequential exposure can lead to the induction of adaptive responses in space that may significantly decrease the level of damages induced by high-LET HZE particles: “There have been several studies performed that indicate an adaptive response to low-dose ionizing radiation can provide a level of protection against future exposures (Bhattacharjee and Ito 2001; Mortazavi et al., 2003; Elmore et al., 2008, 2011; Rithidech et al., 2012). This may be particularly important for understanding risks in the space environment because the GCR [galactic cosmic radiation] environment is composed predominantly of protons, and it is realistic to expect that cells will be exposed to multiple hits of protons before being traversed by an HZE particle” (Huff et al., 2016). Moreover, an article authored by a large group of scientists from the United States, Canada, the United Kingdom, Russia, and Belgium have recently highlighted the cardinal role of adaptive response as an efficient method of biological protection in radiation risk reduction strategies for astronauts participating in space journeys (Cortese et al., 2018). It is worth noting that despite current controversies, some studies show cells pre-exposed to a low dose reveal decreased vulnerability to subsequent exposure to higher doses and produce a neuroprotective effect (Betlazar et al., 2016).

The second shortcoming is assuming that 252Cf neutron radiation can represent the biological effects of the HZE particles in a deep space mission. In ∼3% of decays, spontaneous fission occurs. This yields energetic fission products along with ∼3.75 neutrons per fission. The emitted neutrons are “fast” with a most probable energy of 0.7–1.0 MeV and an average energy of 2.1–2.3 MeV. Given this consideration, there are a number of issues with simulating solar particle events (SPEs) and GCR with a fission source, as follows:

1. The dominant dose component of 252Cf neutrons is not equivalent to more energetic protons and HZE particles. The energy differences are orders of magnitude apart (Bevelacqua, 2008, 2017).

2. The interactions from the neutrons include elastic and inelastic scattering and a limited number of reaction channels such as (n,gamma), (n,p), and (n,d; Bevelacqua, 1999, 2008, 2009, 2016). GCR and SPE open numerous higher-energy channels with the productions of pions, muons, and a host of spallation products and their associated hadronic cascades (Bevelacqua, 1999, 2008, 2009, 2016). The biological effects of these various species are not readily equated to a low-energy fission source.

3. In addition to direct reactions in tissue, the reactions with the spacecraft shell and components will vary significantly (Bevelacqua, 2008).

4. Characterizing a fission source in terms of delivered biological dose is significantly easier than determining the dose from a spectrum of protons and HZE particles of much greater energy (Bevelacqua, 2008).

5. The absorbed dose does not correspond to a biological detriment. This dose must be weighted with an appropriate factor [e.g., RBE (relative biological effectiveness), quality factor, or radiation weighting factor] to obtain a biological dose. Determining these factors is a nontrivial exercise (Bevelacqua, 2010). Using a stochastic International Commission on Radiologic Protection methodology (Bevelacqua, 1999, 2009), it is clear that energy dependence and radiation type are significant factors. Neutron factors are not the same as those for protons and HZEs.

Footnotes

  • The authors declare no competing financial interests.

  • Response to the Commentary from Bevelacqua et al. https://doi.org/10.1523/ENEURO.0439-19.2019

This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

References

  1. ↵
    Acharya MM, Baulch JE, Klein PM, Baddour AAD, Apodaca LA, Kramar EA, Alikhani L, Garcia C Jr., Angulo MC, Batra RS, Fallgren CM, Borak TB, Stark CEL, Wood MA, Britten RA, Soltesz I, Limoli CL (2019) New concerns for neurocognitive function during deep space exposures to chronic, low dose rate, neutron radiation. eNeuro 6:ENEURO.0094-19.2019.
    OpenUrl
  2. ↵
    Betlazar C, Middleton RJ, Banati RB, Liu G-J (2016) The impact of high and low dose ionising radiation on the central nervous system. Redox Biol 9:144–156. doi:10.1016/j.redox.2016.08.002 pmid:27544883
    OpenUrlCrossRefPubMed
  3. ↵
    Bevelacqua JJ (1999) Basic health physics: problems and solutions. New York: Wiley.
  4. ↵
    Bevelacqua JJ (2008) Health physics in the 21st century. New York: Wiley.
  5. ↵
    Bevelacqua JJ (2009) Contemporary health physics: problems and solutions. New York: Wiley.
  6. ↵
    Bevelacqua JJ (2010) Feasibility of using internal radiation-generating devices in radiotherapy. Health Physics 98:614–620. doi:10.1097/HP.0b013e3181c8f6ac pmid:20220369
    OpenUrlCrossRefPubMed
  7. ↵
    Bevelacqua JJ (2016) Health physics: radiation-generating devices, characteristics, and hazards. New York: Wiley.
  8. ↵
    Bevelacqua JJ (2017) Radiation protection consequences of the emerging space tourism industry. J Earth Science 1:1–11.
    OpenUrl
  9. ↵
    Bhattacharjee D, Ito A (2001) Deceleration of carcinogenic potential by adaptation with low dose gamma irradiation. In Vivo 15:87–92.
    OpenUrlPubMed
  10. ↵
    Cortese F, Klokov D, Osipov A, Stefaniak J, Moskalev A, Schastnaya J, Cantor C, Aliper A, Mamoshina P, Ushakov I, Sapetsky A, Vanhaelen Q, Alchinova I, Karganov M, Kovalchuk O, Wilkins R, Shtemberg A, Moreels M, Baatout S, Izumchenko E, et al. (2018) Vive la radiorésistance! Converging research in radiobiology and biogerontology to enhance human radioresistance for deep space exploration and colonization. Oncotarget 9:14692–14722. doi:10.18632/oncotarget.24461 pmid:29581875
    OpenUrlCrossRefPubMed
  11. ↵
    Elmore E, Lao X-Y, Kapadia R, Giedzinski E, Limoli C, Redpath JL (2008) Low doses of very low-dose-rate low-LET radiation suppress radiation-induced neoplastic transformation in vitro and induce an adaptive response. Radiat Res 169:311–318.
    OpenUrlCrossRefPubMed
  12. ↵
    Elmore E, Lao X, Kapadia R, Swete M, Redpath J (2011) Neoplastic transformation in vitro by mixed beams of high-energy iron ions and protons. Radiat Res 176:291–302. doi:10.1667/rr2646.1 pmid:21732791
    OpenUrlCrossRefPubMed
  13. ↵
    Huff J, Carnell L, Blattnig S, Chappell L, Kerry G, Lumpkins S, Simonsen L, Slaba T, Werneth C (2016) Evidence report: risk of radiation carcinogenesis. Houston, TX: National Aeronautics and Space Administration, Lyndon B. Johnson Space Center.
  14. ↵
    Mortazavi SMJ, Cameron JR, Niroomand-rad A (2003) Is the Adaptive Response an Efficient Protection Against the Detrimental Effects of Space Radiation. Proc 28th Intl Cosmic Ray Conf. Tsukuba, Japan, pp. 4299–4302.
  15. ↵
    Rithidech KN, Lai X, Honikel L, Reungpatthanaphong P, Witzmann FA (2012) Identification of proteins secreted into the medium by human lymphocytes irradiated in vitro with or without adaptive environments. Health Phys 102:39. doi:10.1097/HP.0b013e31822833af pmid:22134077
    OpenUrlCrossRefPubMed

Synthesis

Reviewing Editor: Mark Baxter, Mount Sinai School of Medicine

Decisions are customarily a result of the Reviewing Editor and the peer reviewers coming together and discussing their recommendations until a consensus is reached. When revisions are invited, a fact-based synthesis statement explaining their decision and outlining what is needed to prepare a revision will be listed below. The following reviewer(s) agreed to reveal their identity: NONE.

Equating the exposure of neutrons with low-LET and high-LET heavy charged particles is not as simplistic as equating cumulated dose exposures. There are complex biological and nuclear processes that differ based on radiation species, energy, dose-rate, etc., that can not be ignored when (presuming) a biological impact. The author elucidates these by providing clarification to the non-expert (e.g. nuclear physicist, etc.) on how and why the radiation species and energy is important to ascertaining the biological impact at the cellular scale.

The letter is well written and makes excellent points that may not be readily apparent to the non expert. The author justifies the shortcomings of the manuscript under question with sound scientific arguments. One change that should be considered is the language in lines 41-42 which details the decay mechanisms of 252Cf, explicitly stating that the majority by product are 6.1 MeV alphas. This information is not necessary to the argument presented, without further clarification, and could confuse readers when reading lines 48-49 (reason 1). Here the reader may not understand that these range out in less than 0.5 cm of air and may convolute the “dominant dose component of 252Cf neutrons...” with neutrons and alphas.

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Comments on “New Concerns for Neurocognitive Function during Deep Space Exposures to Chronic, Low Dose Rate, Neutron Radiation”
Joseph J. Bevelacqua, James Welsh, S. M. Javad Mortazavi
eNeuro 19 December 2019, 7 (1) ENEURO.0329-19.2019; DOI: 10.1523/ENEURO.0329-19.2019

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Comments on “New Concerns for Neurocognitive Function during Deep Space Exposures to Chronic, Low Dose Rate, Neutron Radiation”
Joseph J. Bevelacqua, James Welsh, S. M. Javad Mortazavi
eNeuro 19 December 2019, 7 (1) ENEURO.0329-19.2019; DOI: 10.1523/ENEURO.0329-19.2019
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