Radiation Limits
for
Space Exploration
F.A. Cucinotta
NASA LBJ Space Center
Brown-bag Lunch Seminar
February 24, 2005
The Space Radiation Problem
Tracks Damage Large Numbers of Cells
in Integrated Functional Units
•
Space radiation is comprised of
high-energy protons and heavy
ions (HZE’s) and secondary
protons, neutrons, and heavy ions
produced in shielding
– Unique damage to biomolecules,
cells, and tissues occurs from HZE
ions
•
Protons similar, but not identical to x-rays
– No human data to estimate risk
– Animal models must be applied or
developed to estimate cancer,
CNS or other risks
– Solar particle events can not be
predicted with sufficient warning at
this time
– Shielding has excessive costs and
will not eliminate GCR
• SPE’s can be mitigated with
shielding
• GCR can not (energies too high)
Gamma-rays
Distribution
of DNA
breaks
HZE’s
Unique Damage
to DNA
Titanium
Radiation Risks
• Carcinogenesis
– Leukemia’s
– Solid Cancers
• Degenerative Tissue Effects
–
–
–
–
Potential Outcomes:
• Mortality: life-shortening
Heart disease
Cataracts
Respiratory disease
Digestive Diseases
• Mortality: In-flight-acute
from SPE
• Damage to the Central
Nervous System
• Reduced PerformanceMission success?
– Motor skills/performance
– Accelerated aging
• Acute Risks
– Death
– Vomiting/nausea
• Morbidity: post-flight
• Purpose of Radiation Limits?
– Legal Requirement
– Safety goals of the Agency
• Health of crew
• Mission success
– Mission, vehicle, surface habitats, spacesuit
design requirements
OSHA Regulations (Standards - 29 CFR)
Supplemental standards. - 1960.18
• In addition to complying with emergency temporary
standards issued under section 6 of the Act, an agency
head shall adopt such emergency temporary and
permanent supplementary standards as necessary and
appropriate for application to working conditions of
agency employees for which there exists no appropriate
OSHA standards. In order to avoid any possible
duplication of effort, the agency head should notify the
Secretary of the subject matter of such standard when the
development of the standard begins.
• Implications- NASA Administrator decides on standards
(limits) and is held accountable
Are acceptable levels of risk arbitrary?
We will review National Research Council (NRC)
and National Council on Radiation Protection
(NCRP) Guidance as well as others
Regis’s Guidance
Step 1. Calculate dose for Mars
mission
Step 2. Set Limits higher than
calculated dose
Dose and Biological Dose
• Physical Dose
– Amount of energy deposited in bulk matter
– Units of Gray (1 Gy = 100 rad)
– Dose = Fluence x LET (linear energy transfer)
• Biological Dose Equivalent
– Idea is a surrogate quantity to relate exposure to risk
– Terrestrial radiation protection uses Dose times quality factor (H=
D x Q)
– Units of Sievert (1 Sv = 100 rem)
– Quality factor approved by Committee using available
experimental data
• Depends on biological endpoint (cancer, acute risks, etc.)
– NASA requires more precise risk assessment tool
• Q for Exploration is Undefined (no Committee has approved)
Calculation of Risk
•
q(a) = probability to die for age a to a+1 based on US mortality rate,
M (all causes) and exposure dependent cancer rate, m
q( E , aE , a) =
•
M ( a ) + m( E , a E , a )
1
1 + [ M (a ) + m( E , aE , a )]
2
Probability to survive to age ‘a’
S(E, a E , a) =
a −1
∏[1 − q(E, a
E
, a)]
u=a E
•
Mortality rate for ion fluence F, of LET, L (ν=transfer model weight)
m( E , aE , a ) = [vERR( aE ) M c ( a ) + (1 − v ) EAR( aE , a )]
•
Excess lifetime risk (ELR)
ELR =
∞
∑ [ M (a) + m( E , a
E
, a )]S ( E , aE , a ) −
a = aE
•
Q ( L)
F ( L) L
DDREF
∞
∑ m( E , a
a = aE
∑ M (a)S (0, a
a = aE
Risk of exposure induced-death (REID)
REID =
∞
E
, a ) S ( E , aE , a )
E
, a)
Fatal Cancer Risk Life-loss
Projection of life-loss is important
for comparison of radiation risks to
other Mission risks
‰
US Males
(1 Sv Acute at age 35-yr)
35
Estimates can be made based on
gamma-ray data; however Heavy
ions and neutrons will have
shortened latency time
‰
Type
-Leukemia
-Solid Cancers:
Multiplicative
Transfer Model
Additive
Transfer Model
Years
20
12
20
30
Fatal Cancer Risk (%)
Estimate of Average Life-loss
from radiation cancer death for
ages 30-50 yr at exposure:
‰
Background no radiation
Background competing with IR
ELR (Solid Cancer)
REID (Solid Cancer)
25
20
15
10
5
0
40
50
60
70
Age, yr
80
90
100
High LET- time shift or increased incidence?
Pulmonary Tumors with
fission neutrons in B6CF1 mice
Cataracts in Astronauts
(Fry et al., Env. Int. 1, (1972))
0.6
100
80 rad
(3.3x24 fract.)
0.5
240 rad
(10x24 fract.)
10
Controls
1
80 rad
(acute)
0.1
250
Probability of Cataract
Mortality Rate/ 10,000 Mice/ Day
All Cataracts
Low-dose Astroanauts
High-Dose astronauts
0.4
0.3
0.2
0.1
0.0
0
450
650
850
1050
Age, days
1250
5
10
15
20
Time after first-mission, yr
25
30
Einstein’s Twins Paradox & Space Travel
Degenerative Effects vs. Theory of Relativity ?
Radiation
induced aging
Time Dilation
Cancer and Non-cancer data
Human Carcinogenesis Data
Issues with data
-Statistics is significant to
low doses (~5 rad)
- Problems are in intercomparisons for several
reasons
-Radiation Type
-Whole vs. Partial
body
-Dose-rate
-Genetic background
4-6 crew to lunar surface for extended-duration stay
CEV:Earth-moon cruise – 4 days
Low lunar orbit (LLO) operations- 1 day
Untended lunar orbit operations – 4-14 days
Low lunar orbit operations – 1 day
Moon-Earth cruise – 4 days
Lunar Lander: Lunar
surface operations
60-90 days
4-6 crew to lunar surface for
long-duration stay
Lunar Habitat: Lunar surface
operations 60-90 days
2020
2015-2020
2030+
Crew TBD to Mars surface
Surface Habitat
2025+
2014
Crew TBD to Mars Vicinity
4-6 crew to Low Earth Orbit
Crew Exploration Vehicle: Launch Environment
LEO Environment
Earth entry, water (or land) recovery
Transit vehicle: Earth-Mars cruise – 6-9 months
Mars vicinity operations – 30-90 days
Mars-Earth cruise – 9-12 months
NASA ESMD
GCR and SPE Dose: Materials & Tissue
- GCR much higher energy producing secondary radiation
No Tissue Shielding
With Tissue Shielding
10000
GCR L. Hydrogen
GCR Polyethylene
GCR Graphite
GCR Aluminum
GCR Regolith
SPE Graphite
SPE Regolith
1000
Dose Equivalent, rem/yr
Dose Equivalent, rem/yr
10000
100
10
GCR L. Hydrogen
GCR Polyethylene
GCR Graphite
GCR Aluminum
GCR Regolith
SPE Graphite
SPE Regolith
1000
SPE L. Hydrogen
100
10
1
1
0
5
10
15
20
25
30
35
0
Shielding Depth, g/cm2
August 1972 SPE
5
10
15
20
25
Shielding Depth, g/cm2
30
35
Spiral Cancer Risks at Solar Minimum
(20 g/cm2 Aluminum Shielding)
Spiral
D, Gy
E, Sv
%Risk*
95% CI
Males (40-y)
3
0.03
0.071
0.28
[0.09, 0.96]
4
0.36
0.87
3.2
[1.0, 10.5]
5
0.41
0.96
3.4
[1.1, 11.0]
Females
3
0.03
0.071
0.34
[0.11, 1.2]
4
0.36
0.87
3.9
[1.2, 12.8]
5
0.41
0.96
4.1
[1.4, 14.4]
*Risk = Risk of Exposure Induced Death (REID) (lifetime)
Spiral Cancer Risks at Solar Maximum
(Males- 40 yr)
Spiral
D, Gy
E, Sv
%Risk
95% CI
5 g/cm2 Al
3
0.45
0.69
2.7
[0.92, 7.4]
4
0.63
1.21
4.4
[1.5, 13.3]
5
0.66
1.24
4.4
[1.5, 13.0]
20 g/cm2 Al
3
0.042
0.09
0.35
[0.11,1.2]
4
0.22
0.54
2.0
[0.65, 6.8]
5
0.25
0.60
2.1
[0.69, 7.2]
*Phi=1100 MV (solar modulation) with Aug. 1972 SPE in transit
Non-cancer Mortality Risks
Males (40-yr) H=1 Sv at low dose-rate
Females (40-yr) H=1 Sv at low dose-ate
12
15
Cancer
Heart Disease
Total
Probability of Death
Probability of Death
15
9
6
3
0
0.0
0.5
1.0
Dose, Sv
1.5
2.0
12
Cancer
Heart Disease
Total
9
6
3
0
0.0
0.5
1.0
Dose, Sv
1.5
2.0
National Research Council Guidance
• National Research Council report in 1970 provided basis
for Radiation Limits from 1970-1989
• Acceptable radiation protection guidelines- “acceptable to
who?”
– Must conform with Federal Radiation Regulations on basis of riskversus-gain-philosophy
– In-line with consensus of the informed Radiobiological and
Medical communities
– Acceptable in terms of technical and fiscal responsibility to the
design and flight engineers and those responsible for Federal
appropriations
– The judgment of risk-versus-gain requires specific numbers
relative to the nations space goals
• Outside the scope of NRC panel
– ALARA Should be followed
NCRP Guidance
• 1989 Recommendation (NCRP 98)
– It would be inappropriate to allow unrestricted radiation exposures
– No exact comparison with other risks can be made
• Differential life loss
• Mission success criteria
– Types of comparisons
• Other risks astronauts face
• Risks in “less-safe” industries (2-3%)
• Radiation limits of ground workers (~3%)
– ALARA should be followed
• 2000 Recommendation (NCRP 132)
– Comparison to “less-safe” industries maybe too restrictive
Comparison to Occupational Fatality Rates
Table-3. Occupational death rates (National Safety Council) and life-time risks for 40-yr careers
for the less-safe and safe industries.
Occupation
Deaths per 10,000 Workers
Life-time Risk (%) of
per year
Occupational Death
1977
1987
2002
1977
1987
2002
Agriculture
5.4
4.9
2.1
2.2
2.0
Mining
6.3
3.8
2.9
2.5
1.5
0.8 Less-safe
1.2 industries
Construction
5.7
3.5
1.3
2.3
1.4
0.6
Transportation
3.1
2.8
1.0
1.2
1.1
0.5
Manufacturing
0.9
0.6
0.28
0.4
0.2
0.1
Government
1.1
0.8
0.26
0.4
0.3
0.1
All
1.4
1.0
0.36
0.6
0.4
0.2
Astronauts Occupation Hazards
1.0
Occupational
~7.5%
Survival Probability
0.9
Astronauts
0.8
0.7
US Males
0.6
0.5
30
35
40
45
50
55
Age, yr
60
65
70
75
Comparison to Other NASA Standards
• Human Missions Requirements– Cumulative probability safe crew return > 0.99 (1 x 10-2)
• Chemical Carcinogen Limits (morbidity)
– NASA SMAC’s limit exposure to cancer rate of 1 in
1000 (1x10-3)
– EPA limit to cancer rate of 1 in 10,000 (1x10-4)
• Radiation Limits for LEO (mortality)
– 3% Cancer Mortality (3.3 x 10-2)
Equating Risks?
• No precise way to equate risks
• Considerations
– Mission Success
– Costs to limit risks
– Mortality is equivalent endpoint, however life-loss
needs to be considered
• Example
Risk Limit
Ave. Life Loss (yr)
Weighted Risk
N x Weighted Risk
Program Risk
Chemical
Risk
Radiation
Risk
0.01
35
0.35
1.4
0.001
15
0.015
0.015
0.0303
15
0.455
0.468
N=no. of crew; Radiation life loss based on gamma ray data
Our Recommended Approach
•
•
•
•
NASA should follows similar approach used in LEO for exploration
Chronic limits set at REID of 3 to 5% with uncertainty assessment
Acute limits to prevent occurrence of clinically significant risk during
mission
Revisions to Acute risk limits
– Separate limit for CNS effects
– Probabilistic interpretation for SPE shielding design
•
Revisions to Chronic risk limits
– Total mortality risks (cancer+ heart + other) weighted by life-loss
– Program goals to reduce uncertainties in order to ensure cost-effective
counter-measures
– Risks be calculated using cross sections rather than dose
•
Revision to ALARA approach to make Mission focused
– Material requirements and realistic cost analysis program
– Role of Individual sensitivity
Acute Mortality Risks
Estimates based on
Human and Animal Data
(NRC, 1967)
Estimates based on Human and Animal Data
50
100
Model
Japanese survivors
Normal
80
Fisherman
Mexican Family
30
Mortality(%)
LD50, Gy
40
20
Lower bound (all survived)
60
Patients
40
10
20
0
0.001
0.01
0.1
1
10
100
0
Dose Rate, Gy/hr
1
2
3
4
5
6
Total Dose, Gy
Acute response: Large sparing at low dose-rates
Tail of distribution becomes important if response is depressed due to spaceflight
Factors (stress, micro-gravity, etc.)
Historically Largest SPE’s
Range in materialsfew 10’s of cm’s (g/cm2)
High E data??
Oct./Nov. 2003 Solar Particle Event
Cumulative Dose Equivalent, cSv
1.E+02
Al thickness,
g/cm2:
Current 30-day limit
1.E+01
0
1
3
5
10
15
20
30
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
0
50
100
150
Time, h
200
250
300
August 1972 Solar Particle Event
1.E+02
Dose Rate, cSv/h
1.E+01
1 rem/h
1.E+00
Al thickness,
g/cm2:
1.E-01
0
1
3
5
10
15
20
30
1.E-02
1.E-03
1.E-04
BFO Dose, rem (cSv)
Depth Alum
Poly
5
61.1
39.1
10
20.9
10.7
20
4.57
1.83
1.E-05
0
40
80
120
Time, h
160
200
240
Summary- Acute Mortality Risk
CH2
ƒ Risk is small for more than 98% of historical SPE’s
¾ Doses to BFO are at low dose-rates (<5 cGy/hr) reducing acute
risks
¾ Shielding solution is known (2-3 in thick wall of polyethylene)
™Requires accurate monitoring/warning especially EVA
¾ Use of average BFO dose is conservative due to protective
effects of heterogeneous dose distribution
ƒ Remaining questions
¾ Characterization of high energy components of large (<2%)
of SPE’s (future measurements or data mining)
¾ Is sensitivity increased due to immuno-suppression, stress, etc.?
SPE Design Requirement
Design to
here
Backup Material
Non-Cancer Risks in A-Bomb Survivors
(Shimizu et al., 1999)
Stroke
ERR/Sv 0.09
P 0.02
0.8
Heart Disease
ERR/Sv 0.13
P 0.003
0.4
ERR
0.0
Respiratory
ERR/Sv 0.18
P 0.005
Digestive
ERR/Sv 0.11
P 0.05
0.8
0.4
0.0
Infectious
ERR/Sv -0.01
P > 0.5
0.8
Other
ERR/Sv 0.01
P > 0.5
0.4
0.0
0
1
2
0
3
Dose (Sv)
ERR: Excess Relative Risk per Sv
1
2
3
NASA Revised Standard for Dose Limits
(LEO)
ISS Mission Nominal Fatal Cancer Risk
‰ Revised standard applies a
‰
95% confidence is
conservative
¾ Specific risk probabilities of
individuals
¾ Narrows range of increased
risk
¾ Uncertainties¾
¾
¾
¾
Epidemiology data
Dose-rate effects
Radiation Quality (QF)
Dosimetry/transport
codes
0.0150
Risk Distribution
D = 100 mGy
E = 252 mSv
Q = 2.52
R0 = 1.0 %
95% C.I. = [0.41, 3.02%]
0.0125
Probability
95% confidence level to the
career limit of 3% additional risk
of lifetime fatal cancer
¾ Approved by NASA Medical
Policy Board
0.0100
0.0075
0.0050
0.0025
0.0000
0
1
2
3
4
5
6
% Fatal Risk per
Monte-Carlo simulation of risk estimates
Including range of quality factors, dose-rate
Factors, epidemiology data, and errors in
Dosimetry or transport codes.
Other uncertainties in Risk Projections?
•
Are responses linear at low doses
and dose-rates?
– Difficult to achieve good statistics
in experiments
0.4
Sub-linear dose responses at low
dose or dose-rates?
0.3
– Bystander mechanisms
– Sensitive sub-populations
– Bending due to cell sterilization or
competing risks at higher doses
masks linearity
Incidence
•
Sensitive sub-population
"Normal" population
Total
0.2
0.1
0.0
•
Importance to NASA
– Shielding is less effective if “true”
biological response is sub-linear
– Countermeasure design impacted
by mechanisms of low dose
responses
0.0
0.2
0.4
0.6
Dose, Gy
0.8
1.0