Risk Of Exposure-induced Death Research Articles

Cancer and circulatory disease risks for the largest solar particle events in the space age

In this paper we use the NASA Space Cancer Risk (NSCR version 2022) model to predict cancer and circulatory disease risks using energy spectra representing the largest SPE's observed in the space age. Because tissue dose-rates behind shielding for large SPE's lead to low dose-rates (<0.2 Gy/h) we consider the integrated risk for several historical periods of high solar activity, including July–November, 1960 events and August–October 1989 events along with the February 1956 and August 1972 events. The galactic cosmic ray (GCR) contribution to risks is considered in predictions. Results for these largest historical events show risk of exposure induced death (REID) are mitigated to < 1.2 % with a 95 % confidence interval with passive radiation shielding of 20 g/cm2 aluminum, while larger amounts would support the application of the ALARA principle. Annual GCR risks are predicted to surpass the risks from large SPEs by ∼30 g/cm2 of aluminum shielding.

Secondary Cancer Risk of Radiotherapy and Imaging Examination for Two Different Malignancies in One Patient

Purpose: Radiotherapy (RT), which is considered one of the critical treatments for cancer patients is also known as adjuvant therapy and palliative care, and can be attempted alone or concurrent with chemotherapy. Although RT reduces the risk of recurrence, the scattered dose may enhance the risk of secondary cancer induction; this is raising some challenges in clinical practice. To the best of our knowledge, few studies to date have assessed such effects of brain cancer adjuvant radiotherapy.&#x0D; Materials and Methods: We estimated the RT-induced risk of secondary cancer for a 45-year-old patient who had undergone radiotherapy of the head and pelvis with a 6 MV photon beam in 15 and 10 sessions, respectively. The absorbed dose by the thyroid, breast, eye lenses, region overlying ovaries, and parotids was measured using Thermoluminescent Dosimeters (TLD). Since the patient was scanned before radiotherapy, it was decided to calculate their risk as well. To evaluate the cancer risk, radiobiological models for Excess Absolute Risk (EAR), as well as Excess Relative Risk (ERR) published by the Committee on the Biological Effects of Ionizing Radiation (BEIR) in report VII, were implemented. This study thus aimed to estimate the Risk of Exposure-Induced Death (REID) and assess the radiation dose delivered to patients from Computed Tomography (CT) scans and common diagnostic nuclear medicine examinations.&#x0D; Results: The mean risk of secondary cancer for sensitive organs was calculated 3 years after radiotherapy. The highest estimated ERR was related to the region overlying right and left ovaries for pelvic radiotherapy (47.82) and (51.17), and the next highest EAR followed by right and left eye lenses for brain radiotherapy (18.09) and (15.43), respectively. In addition, other cancers arising from CT scans had the highest REID values for solid cancer (0.0015) and bone scans revealed the highest REID values for other cancers (0.00121).&#x0D; Conclusion: Calculating the corresponding risks of RT is of great significance for the patients in procedural change. Choosing proper field sizes and adapted techniques to avoid excessive doses to healthy organs can thus be a great assistance in this regard.

Radiation dose and risk of exposure-induced death associated with chest CT examination during COVID-19 outbreak

As the use of chest CT examination during the COVID-19 outbreak is increased, patient radiation doses are become a concern. So, this survey was conducted to evaluate the patient radiation dose and estimated the cancer risk of chest CT examination during COVID-19 outbreak. From February 2020 to September 2021, 165 chest CT examination were acquired. For all patients, organ doses and the effective dose were calculated using ImpactDose software. The risk of exposure-induced death (REID) values was estimated by using models developed in the BEIR VII (Biological Effects of Ionizing Radiation VII)-Phase 2 report. Study population included 75 male and 90 female, with an average age of 51 years. The mean effective dose in females were greater than in the males (5.42 mSv versus 4.43 mSv, P &lt; 0.0001). The mean REID values in patients undergoing chest CT examination with a 16-MDCT scanner were 232 per million male and 351.3 per million female. The risk of lung cancer was high for both male and female. According to our study, it is proposed that to use low-dose CT chest protocol, as a reliable tool in detecting COVID-19 pneumonia in daily practice, can reduce radiation dose and estimated cancer risk.

Estimating the entrance surface air kerma and diagnostic reference level in routine radiography examinations: A multi-center study

BackgroundAlthough the risk of radiographic imaging is negligible compared to its benefits, the overall risk to the population is significant due to a large number of these examinations. Therefore, this study was designed to estimate entrance surface air kerma (ESAK) and provide diagnostic reference level (DRL) in routine radiological examinations. MethodsIn this study 783 patients were involved who had a mean age of 41 ± 14 years. The study included five routine radiological examinations; lumbar vertebrae, pelvis, abdomen, chest, and skull from 11 radiology devices located in Yazd province. The ESAK was estimated for each examination, and then the DRL values as the third quartile of the distribution of the ESAK median values were calculated. The effective dose and risk of exposure-induced death (REID) were also estimated using PCXMC (a personal computer-based Monte Carlo) software. ResultsThe DRL values for the investigated examinations were obtained as: abdomen (anterior-posterior [AP]): 3.57, chest (AP): 1.23, chest (lateral [LAT]): 1.12, chest (posterior-anterior [PA]): 0.48, lumbar vertebrae (AP): 4.16, lumbar vertebrae (LAT): 8.81, lumbar vertebrae (oblique [OB]): 10.33, pelvis (AP): 2.77, pelvis (LAT): 2.82, skull (AP): 2.34, skull (LAT): 1.39, and skull (PA): 2.03 mGy. The lumbar vertebrae (OB) radiology examination had the highest effective dose (1.03 ± 0.76 mSv). In addition, the highest REID value in lumbar examination was associated to stomach cancer (7.03 ± 6.11 deaths per million). ConclusionBased on the results, although the DRL values were not higher than those of the international organizations' levels, they can be reduced by adequate training of radiographers.

Countermeasure development against space radiation-induced gastrointestinal carcinogenesis: Current and future perspectives

A significantly higher probability of space radiation-induced gastrointestinal (GI) cancer incidence and mortality after a Mars mission has been projected using biophysical and statistical modeling approaches, and may exceed the current NASA mandated limit of less than 3% REID (risk of exposure-induced death). Since spacecraft shielding is not fully effective against heavy-ion space radiation, there is an unmet need to develop an effective medical countermeasure (MCM) strategy against heavy-ion space radiation-induced GI carcinogenesis to safeguard astronauts. In the past, we have successfully applied a GI cancer mouse model approach to understand space radiation-induced GI cancer risk and associated molecular signaling events. We have also tested several potential MCMs to safeguard astronauts during and after a prolonged space mission. In this review, we provide an updated summary of MCM testing using the GI cancer mouse model approach, lessons learned, and a perspective on the senescence signaling targeting approach for desirable protection against space radiation-induced GI carcinogenesis. Furthermore, we also discuss some of the advanced senotherapeutic candidates/combinations as a potential MCM for space radiation-induced GI carcinogenesis.

Dose Descriptors and Assessment of Risk of Exposure-Induced Death in Patients Undergoing COVID-19 Related Chest Computed Tomography.

For more than two years, coronavirus disease 19 (COVID-19) has represented a threat to global health and lifestyles. Computed tomography (CT) imaging provides useful information in patients with COVID-19 pneumonia. However, this diagnostic modality is based on exposure to ionizing radiation, which is associated with an increased risk of radiation-induced cancer. In this study, we evaluated the common dose descriptors, CTDIvol and DLP, for 1180 adult patients. This data was used to estimate the effective dose, and risk of exposure-induced death (REID). Awareness of the extensive use of CT as a diagnostic tool in the management of COVID-19 during the pandemic is vital for the evaluation of radiation exposure parameters, dose reduction methods development and radiation protection.

Medical Countermeasure Requirements to Meet NASA's Space Radiation Permissible Exposure Limits for a Mars Mission Scenario.

The space radiation environment consists of a complex mixture of ionizing particles that pose significant health risks to crew members. NASA currently requires that an astronaut's career Risk of Exposure Induced Death (REID) for cancer mortality should not exceed 3% at the upper 95% confidence level. This career radiation limit is likely to be exceeded for even the shortest round-trip mission scenario to Mars. As such, NASA has begun to pursue more vigorously approaches to directly reduce radiation risks, despite the large uncertainties associated with such projections. A recent study considered cohort studies of aspirin and warfarin as possible medical countermeasures (MCMs) acting to reduce background cancer mortality rates used in astronaut risk projections. It was shown that such MCMs can reduce the REID for specific tissues in restricted time intervals over which the drugs were administered; however, the cumulative effect on total lifetime REID was minimal. As an extension, the present work addresses more general MCM requirements that would be needed to meet current NASA radiation limits for a Mars mission scenario. A sensitivity analysis is performed within the major components of the NASA cancer risk model that would likely be modified by MCM interventions. This includes the background cancer incidence and mortality rates, epidemiologically based hazard rates derived from acute terrestrial exposures, and radiation quality factors used to translate terrestrial exposures to space radiation. Relationships between possible MCMs and each of these components are discussed. Results from this study provide important information regarding MCM requirements needed to meet NASA limits for planned Mars missions. Insight into the types of countermeasures expected to yield greatest reductions in crew risk is also gained.

Patterns and trends of population radiation exposure and projected the risk of exposure-induced death from gamma camera examinations in Yazd Province

IntroductionThe use of nuclear medicine examinations as imaging modalities has recently increased. This study aimed to assess the radiation dose delivered to patients from common diagnostic nuclear medicine examinations and estimate the risk of exposure-induced death (REID). In addition, the frequency of nuclear medicine procedures were evaluated from 2015-2018. Materials and MethodsThe data were collected from adult patients who had undergone cardiac, skeletal, renal, lacrimal, and thyroid imaging. For each patient, the effective dose was calculated using dose conversion factors, and REID was estimated using PCXMC. The frequency of examinations between 2015 and 2018 was obtained from the hospital information system (HIS). ResultsThe highest estimated effective dose was attributed to the scans of myocardial stress (8.09 ± 1.28 mSv), myocardial rest (5.59 ± 1.27 mSv), and thyroid imaging (3.93 ± 0.55 mSv). In addition, cardiac stress examination had the highest REID values for solid cancers (212.5 ± 67.5) and bone scans had the highest REID values for leukemia (11.5 ± 2.5). A large increase in the number of myocardial perfusion scans was a significant contributor to an increase in collective effective dose from 23.37 man-Sv in 2015 to 49.47 man-Sv in 2018, a compound annual growth rate (CAGR) of 26%. ConclusionAlthough the annual frequency and per capita effective dose of nuclear medicine procedures in Yazd Province increased continuously, they were comparably lower than those reported for other countries. Despite this, the cancer risks of nuclear medicine scans at the individual level are negligible (around 0.01% excess fatal cancer risk), yet the increasing tendency for these examinations could be of concern.

The risk of induced cancer and ischemic heart disease following low dose lung irradiation for COVID-19: estimation based on a virtual case

Background Recently, low dose radiotherapy delivered to the whole lung has been proposed as treatment for the pneumonia due to COVID-19. Although there is biological plausibility for its use, the evidence supporting its effectiveness is scarce, and the risks associated with it may be significant. Thus, based on a virtual case simulation, we estimated the risks of radiation-induced cancer (RIC) and cardiac disease. Methods Lifetime attributable risks (LAR) of RIC were calculated for the lung, liver, esophagus, and breast of female patients. The cardiovascular risk of exposure-induced death (REID) due to ischemic heart disease was also calculated. The doses received by the organs involved in the treatment were obtained from a simulation of conformal radiotherapy (RT) treatment, delivering a dose of 0.5 Gy–1.5 Gy to the lungs. We considered a LAR and REID <1% as acceptable, 1–2% cautionary, and >2% unacceptable. Results The lung was at the highest risk for RIC (absolute LAR below 5200 cases/100,000 and 2250 cases/100,000 for women and men, respectively). For women, the breast had the second-highest LAR, especially for young women. The liver and esophagus had LARs below 700/100,000 for both sexes, with a higher incidence of esophageal cancer in women and liver cancer in men. Regarding the LAR cutoff, we observed an unacceptable or cautionary LAR for lung cancer in all women and men <60 years with an RT dose >1 Gy. LAR for lung cancer with an RT dose of 1 Gy was cautionary for women >60 years of age and men <40 years of age. No LAR estimation was unacceptable for the RT dose ≤0.7 Gy in all groups irrespective of sex or age at exposure. Only 0.5 Gy had an acceptable REID. Conclusions A RT dose ≤0.5 Gy provides an acceptable LAR estimate (≤1%) for RIC and REID, irrespective of sex and age. The current ongoing trials should initially use doses ≤0.5 Gy to maintain the risks at an acceptable level and include only patients who fail or do not have any other treatment option.

Galactic Cosmic Radiation in the Interplanetary Space Through a Modern Secular Minimum

AbstractRecent solar conditions indicate a persistent decline in solar activity—possibly similar to the past solar grand minima. During such periods of low solar activity, the fluxes of galactic cosmic rays (GCRs) increase remarkably, presenting a hazard for long‐term crewed space missions. We used data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) to examine the correlation between the heliospheric magnetic field, solar wind speed, and solar modulation potential of the GCRs through Cycle 24. We used this correlation to project observations from past secular solar minima, including the Dalton minimum (1790–1830) and the Gleissberg minimum (1890–1920), into the next cycle. For the case of conditions similar to the Dalton (or Gleissberg) minimum, the heliospheric magnetic field could drop to 3.61 (or 4.06) nT, leading to a dose rate increase of ∼75% (or 34%). We showed that accounting for a floor in the modulation potential, invoked by the Badhwar‐O'Neill 2014 model, moderates the projected radiation levels in Cycle 25. We used these results to determine the most conservative permissible mission duration (PMD, and days for 45‐year‐old male and female astronauts, respectively) based on a 3% risk of exposure‐induced death (REID) at the upper 95% confidence interval in interplanetary space.

Benchmarking risk predictions and uncertainties in the NSCR model of GCR cancer risks with revised low let risk coefficients

We report on the contributions of model factors that appear in projection models to the overall uncertainty in cancer risks predictions for exposures to galactic cosmic ray (GCR) in deep space, including comparisons with revised low LET risks coefficients. Annual GCR exposures to astronauts at solar minimum are considered. Uncertainties in low LET risk coefficients, dose and dose-rate modifiers, quality factors (QFs), space radiation organ doses, non-targeted effects (NTE) and increased tumor lethality at high LET compared to low LET radiation are considered. For the low LET reference radiation parameters we use a revised assessment of excess relative risk (ERR) and excess additive risk (EAR) for radiation induced cancers in the Life-Span Study (LSS) of the Atomic bomb survivors that was recently reported, and also consider ERR estimates for males from the International Study of Nuclear Workers (INWORKS). For 45-y old females at mission age the risk of exposure induced death (REID) per year and 95% confidence intervals is predicted as 1.6% [0.71, 1.63] without QF uncertainties and 1.64% [0.69, 4.06] with QF uncertainties. However, fatal risk predictions increase to 5.83% [2.56, 9.7] based on a sensitivity study of the inclusion of non-targeted effects on risk predictions. For males a comparison using LSS or INWORKS lead to predictions of 1.24% [0.58, 3.14] and 2.45% [1.23, 5.9], respectively without NTEs. The major conclusion of our report is that high LET risk prediction uncertainties due to QFs parameters, NTEs, and possible increase lethality at high LET are dominant contributions to GCR uncertainties and should be the focus of space radiation research.

A methodology for investigating the impact of medical countermeasures on the risk of exposure induced death

The space radiation environment is composed of ionizing particles that may pose health risks to crew members during Low Earth Orbit (LEO) and deep space missions. NASA has established astronaut career radiation limits for cancer of 3% Risk of Exposure Induced Death (REID) at the 95% confidence level. The REID is the increased lifetime risk of death from cancer due to radiation exposure in comparison to an unexposed background population and has been traditionally mitigated by passive shielding design concepts and limiting safe days in space. Additional reduction in radiation exposure risk may be achieved with Medical Countermeasures (MCM). Recent meta-analyses have demonstrated the efficacy of aspirin in the reduction of the background colorectal cancer incidence and mortality rates for specific cohorts. Additional studies of warfarin in patients greater than 50 years of age have indicated statistically significant decreases in stomach, bladder, brain, prostate, and lung cancer incidence as compared to control groups. While ultimate selection of suitable countermeasures will be the responsibility of flight surgeons, this paper presents a general methodology for incorporating MCM into the NASA Space Radiation Cancer Risk model and includes modifications of the background mortality rates (hazard rates) and the radiation risk coefficients to numerically quantify the benefits of MCM. As examples of the method, aspirin and warfarin will be employed as MCM in a sensitivity analysis to compute the REID for astronauts embarking on a one-year deep space mission scenario.

Radiation dose and risk of exposure-induced death associated with common computed tomography procedures in Yazd Province

PurposeThis survey was conducted to evaluate the radiation dose delivered to the patients through different CT procedures along with its corresponding risks in Yazd province. MethodData for the five most common procedures were collected from six institutions from September 2017. The effective dose for each patient was estimated by ImpactDose software. The risk of exposure-induced death (REID) was calculated by coupling the BEIR VII model and ICRP 103 data for the Asian population. ResultsThe median effective doses for the five most common procedures, in descending order, were as follows: 5.19 mSv for abdomen-pelvis, 3.30 mSv for routine chest, 3.00 mSv for chest HRCT, 0.76 mSv for Brain and 0.47 mSv for sinus. Averaged over all the procedures, the ratio of maximum to minimum of effective dose was 90-fold. The highest associated risk was tied to high-resolution CT scans for women which was estimated to be 1 exposure-induced death related to 2096 scans performed on a 20-year old patient. ConclusionThe estimated effective doses for all the procedures except sinus were comparably low, however, the variations among patient doses, even for a given procedure, were substantially high, indicating that further optimization processes need to be undertaken. Moreover, it was estimated that approximately 2000 scans of abdomen-pelvis performed on 20 year-old women could result in 1 radiation-induced cancer death, which in large scales may cause drastic health issues. In this regard, several approaches, one of which is the implementation of DRLs, can be suggested to reduce the collective dose from CT scans.

A novel, population-based approach to astronaut radiation risk assessment

Risk due to space radiation exposure is one of the prime concerns for astronaut health and safety on deep space missions. NASA's current radiation risk limit is 3% risk of exposure-induced death (REID) at the upper 95% tolerance limit. Sex and age dependence in existing radiation risk models and large uncertainties, mostly due to lack of knowledge of biological effects of highly energetic and heavy charged particles, presently limit demographics to older, male astronauts for participation in long-duration missions to Mars. To address this issue, a novel method is introduced for evaluating radiation risk for proposed missions, where the combined risk for the population is used in place of age- and sex-specific risks. The age and sex distribution of active and trainee astronauts as of 2018 and operational NASA tools for evaluating astronaut radiation risk are used to demonstrate the proposed method for a variety of missions. Discrete and continuous methods representing the population probability density function are tested. Mission-specific astronaut population risk distributions are constructed using random sampling from the astronaut age and sex distributions, which are then used to calculate the corresponding upper 95% tolerance limit. It is found that the population-combined REID for a conjunction class Mars mission in solar maximum conditions is around 4% at upper 95% tolerance limit, which is less than the REID for female astronauts of age less than 55 years. It is also found that the probability density function and organ-specific risks for the population-combined REID are very close to that of 30-year-old male and 60-year-old female astronauts. The present work demonstrates a methodology for performing space radiation risk assessments independent of astronaut age and sex.

On prognostic estimates of radiation risk in medicine and radiation protection

The problem of expressing cumulative detrimental effect of radiation exposure is revisited. All conventionally used and computationally complex lifetime or time-integrated risks are based on current population and health statistical data, with unknown future secular trends, that are projected far into the future. It is shown that application of conventionally used lifetime or time-integrated attributable risks (LAR, AR) should be limited to exposures under 1 Gy. More general quantities, such as excess lifetime risk (ELR) and, to a lesser extent, risk of exposure-induced death (REID), are free of dose constraints, but are even more computationally complex than LAR and AR and rely on the unknown total radiation effect on demographic and health statistical data. Appropriate assessment of time-integrated risk of a specific outcome following high-dose (more than 1 Gy) exposure requires consideration of competing risks for other radiation-attributed outcomes and the resulting ELR estimate has an essentially non-linear dose response. Limitations caused by basing conventionally applied time-integrated risks on current population and health statistical data are that they are: (a) not well suited for risk estimates for atypical groups of exposed persons not readily represented by the general population; and (b) not optimal for risk projections decades into the future due to large uncertainties in developments of the future secular trends in the population-specific disease rates. Alternative disease-specific quantities, baseline and attributable survival fractions, based on reduction of survival chances are considered here and are shown to be very useful in circumventing most aspects of these limitations. Another main quantity, named as radiation-attributed decrease of survival (RADS), is recommended here to represent cumulative radiation risk conditional on survival until a certain age. RADS, historically known in statistical literature as “cumulative risk”, is only based on the radiation-attributed hazard and is insensitive to competing risks. Therefore, RADS is eminently suitable for risk projections in emergency situations and for estimating radiation risks for persons exposed after therapeutic or interventional medical applications of radiation or in other highly atypical groups of exposed persons, such as astronauts.

280. VATS (Video Assisted Thoracoscopic Surgery) with radiological guidance in hybrid operating room: Technique standardization, organizational aspects and dose optimization

Purpose The aim of this work was to optimize the Video Assisted Thoracoscopic Surgery (VATS) procedure through a new rotational geometry avoiding also the interference of the flat panel with infusion lines, vital monitoring equipment and intubation instrumentation. Methods The VATS technique was performed in the Hybrid Room environment that can be used at the same time as a diagnostic room and a surgical room. The C-Arm robotic angiography acquires cone beam CT with reduced angle rotation around the patient permitting the lateral decubitus access to thoracic surgeons with two different protocols: 12sDCTLargeVolume (12s) and 5sDRBODYCARE (5s). The first one allows to scan large volumes by twisting twice around the patient. The second protocol allows to shrink the field to the region of interest with a single pass. Organ dose and REID (Risk of Exposure-Induced Death) were estimated with Monte Carlo methods in PCXMC. Results The initial and final positions of the C-Arm rotation were changed from −170, +30 to −100, +100 in the 5s protocol reducing the risk of collision in case of critical cases (external lesions), wherever the bed is located. After optimization we reduced the dose of 80% in the 12s protocol and of 8.5 times in the 5s. the organ dose and REID are shown in the table. Conclusions VATS is a rapidly emerging surgical technique for the excision of lung nodules that permits to perform radical surgery of lung cancers to a larger cohort of patients and with a longer expectancy of life respect to conventional surgery. We optimized the protocol by changing rotational departure and arrival position avoiding collisions and reducing the dose.

Estimated radiation risk of cancer from dental cone-beam computed tomography imaging in orthodontics patients

BackgroundRadiation dose evaluation is important to cone-beam computed tomography (CBCT) for routine orthodontic treatment planning, especially for a significant proportion of children in orthodontic patients. This study evaluated the patient radiation dose and estimated the radiation cancer risk on dental CBCT according to the calculations by the Monte Carlo simulation method.MethodsThe dental CBCT scanner evaluated in this project was the i- CAT® (Imaging Sciences International Inc., PA, U.S.A.) device. Organ doses and effective doses were calculated by using personal computer-based Monte Carlo simulation (PCXMC 2.0 Rotation) software. The cancer risk resulting from the exposure to ionizing radiation was estimated by using the BEIR VII (Biologic Effects of Ionizing Radiation VII) report model, and the risk of exposure-induced death (REID) was assessed by PCXMC 2.0 Rotation software.ResultsThe largest contribution to the organ dose and effective dose at Zref 83 cm positioned in the dental CBCT x-ray beam centerline was from the salivary glands (738.29μGy, 7.38 μSv). The different organ doses showed the maximum values at the different Zref locations, and the largest contribution to the organ dose and effective dose of all simulated positions was from the thyroid (928.77μGy, 37.5 μSv). The REID values in the 10-year olds (22.6 × 10− 7, female; 19 × 10− 7, male) were approximately double than those in 30-year olds (10.4 × 10− 7, female; 8.88 × 10− 7, male) for all cancers. The highest change during age range from 10 to 30 was shown in breast cancer of females.ConclusionsAlthough individual cancer risk estimates as a function of gender and age are small, the concern about the risks from dental CBCT is related to the rapid increase in its use for orthodontic practice, especially in children patients.

Space: The Final Frontier-Research Relevant to Mars.

A critically important gap in knowledge surrounds the health consequences of exposure to radiation received gradually over time. Much is known about the health effects of brief high-dose exposures, such as from the atomic bombings in Japan, but the concerns today focus on the frequent low-dose exposures received by members of the public, workers, and, as addressed in this paper, astronauts. Additional guidance is needed by the National Aeronautics and Space Administration (NASA) for planning long-term missions where the rate of radiation exposure is gradual over years and the cumulative amounts high. The direct study of low doses and low-dose rates is of immeasurable value in understanding the possible range of health effects from gradual exposures and in providing guidance for radiation protection, not only of workers and the public but also astronauts. The ongoing Million Person Study (MPS) is 10 times larger than the study of the Japanese atomic bomb survivors of 86,000 survivors with estimated doses. The number of workers with >100 mSv career dose is substantially greater. The large study size, broad range of doses, and long follow-up indicate substantial statistical ability to quantify the risk of exposures that are received gradually over time. The study consists of 360,000 U.S. Department of Energy workers from the Manhattan Project; 150,000 nuclear utility workers from the inception of the nuclear age; 115,000 atomic veterans who participated in above-ground atmospheric tests at the Nevada Test Site and the Bikini and Enewetak Atolls and Johnston Island in the Pacific Proving Grounds (PPG); 250,000 radiologists and medical workers; and 130,000 industrial radiographers. NASA uses an individual risk-based system for radiation protection in contrast to the system of dose limits for occupational exposures used by terrestrial-based organizations. The permissible career exposure limit set by NASA for each astronaut is a 3% risk of exposure-induced death (REID) from cancer at a 95% confidence level to account for uncertainties in risk projections. The large size of the MPS will reduce the uncertainty in the risk estimates, narrowing the 95% confidence interval, and thus allow more time in space for astronauts. Further differences between men and women in their response to radiation can be more fully examined, and non-cancer outcomes, such as neurological disorders and cardiovascular disease, can be evaluated in a way not hitherto possible.

Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration?

AbstractThe Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show stopper for long‐duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% risk of exposure‐induced death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23–24 minimum. The time to 3% REID is estimated to be ∼20% lower in the coming cycle 24–25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations.

Space radiation risks for astronauts on multiple International Space Station missions.

Mortality and morbidity risks from space radiation exposure are an important concern for astronauts participating in International Space Station (ISS) missions. NASA’s radiation limits set a 3% cancer fatality probability as the upper bound of acceptable risk and considers uncertainties in risk predictions using the upper 95% confidence level (CL) of the assessment. In addition to risk limitation, an important question arises as to the likelihood of a causal association between a crew-members’ radiation exposure in the past and a diagnosis of cancer. For the first time, we report on predictions of age and sex specific cancer risks, expected years of life-loss for specific diseases, and probability of causation (PC) at different post-mission times for participants in 1-year or multiple ISS missions. Risk projections with uncertainty estimates are within NASA acceptable radiation standards for mission lengths of 1-year or less for likely crew demographics. However, for solar minimum conditions upper 95% CL exceed 3% risk of exposure induced death (REID) by 18 months or 24 months for females and males, respectively. Median PC and upper 95%-confidence intervals are found to exceed 50% for several cancers for participation in two or more ISS missions of 18 months or longer total duration near solar minimum, or for longer ISS missions at other phases of the solar cycle. However, current risk models only consider estimates of quantitative differences between high and low linear energy transfer (LET) radiation. We also make predictions of risk and uncertainties that would result from an increase in tumor lethality for highly ionizing radiation reported in animal studies, and the additional risks from circulatory diseases. These additional concerns could further reduce the maximum duration of ISS missions within acceptable risk levels, and will require new knowledge to properly evaluate.