Topic 7  Dose Limits
Our Radiation Dose System 


 Meandering Through a Complicated Landscape….
 Cember, Ch 7, 8, 6
Categories of Exposure
 Occupational Exposure
 pregnant workers
 all other radiation workers
 Members of the General Public
 Medical Exposure
Dose Calculation Method(s)
 Dose, at a fundamental level:
 (Energy absorbed)/(mass of material)
 How do you compare doses delivered to different parts of the body?
 Uniform whole body irradiation
 (Most likely) external source
 Gamma emitter
 Non uniform irradiation
 (Most likely) internal source
 a, b, g, N, xray or other source
Dose Equivalent (H)
 Biological response varies by radiations
 Radiation weighting factors used to provide a common scale:
 H is the dose equivalent, expressed in Sv
 D_{T,R} is absorbed dose in tissue (T) from radiation (R)
 T is tissue (organ)
 R is radiation type R
 W_{R} is radiation weighting factor
Quality and Radiation Weighting Factors for Various Radiations
Radiation 
Q
(ICRP20) 
W
(ICRP 60) 
X, gamma, beta 
1 
1 
Neutrons 


Thermal 
2 
5 
0.01 MeV 
2.5 
10 
0.1 MeV 
7.5 
10 
0.5 MeV 
11 
20 
> 0.1  2 MeV 

20 
> 2  20 MeV 

5 
Unknown Energy 
10 

Highenergy protons 
10 
5 
Alpha particles, fission fragments, heavy nuclei 
20 
20 
Example use of w_{R}
Radiation 
Measured dose rate µGy h^{1} 
W_{R} 
H = D*W_{R} 
Gammas 
5 
1 
5 
Thermal neutrons 
2 
5 
10 
Fast neutrons > 2 MeV 
1 
5 
5 
Does equivalent, H 
20 µSv h^{1} 
Traditional Units
 H is the dose equivalent, expressed in rem
 D_{T,R} is absorbed dose in tissue (T) from radiation (R)
in rads
 T is tissue (organ)
 R is radiation type R
 Q_{R} is radiation weighting factor
Internal Dosimetry
 Dose to a person must consider
 Radiation parameters
 Energy
 Halflife
 Radiation type(s)
 Biological parameters
 Organ/body weight
 Biological behavior of contaminant
 Elimination rates
 Calculational techniques
 Metabolic models
 Competing methodologies
 Biological modifiers for radiation types
Accounting for Biological Effects
 Important Note
 The techniques used to calculate biological dose have changed
substantially in the last 30 years
 The terminology used to describe biological dose has not
 The resultant dose predictions have changed, sometimes a lot,
sometimes not at all
 Sorry
 Dose equivalent
 Modifies absorbed dose by a modifying factor
 Accounts for varying biological impacts of different radiations
 Conventional system:
 Rem (“Roentgen equivalent man”)
 Systeme Internationale:
So…Biological Dose Today
 Measure
 Exposure (X or roentgen) or
 Absorbed dose (rad, Gy)
 Calculate
 Dose equivalent for each effected organ (Gy*WR)
 Result expressed in rem, Sv
History of Internal Dose Calc’s
 1959
 ICRP 2  Recommendations for Internal and External Exposure
 The recommendations of this organization provided dose limits
from external irradiation (total body) and limited doses to specific
(critical) orgrans. Occupational limits were established:
 total body and gonads, 5 rem/y
 thyroid, skin  30 rem/y
 skeleton (bone, not marrow)  0.56 rem/wk (0.1 μCi of
Ra226)
 all other organs, 15 rem/y
 1959
 1960
 1961
 FRC  Regulations  occupational limits and public exposures
 1977
 ICRP 26 – Recommendations
 New dose limits and the concept of effective dose equivalent
were first introduced.
 1979
 ICRP 30  Limits of intakes for workers
 Represents the first complete set of recommendations since the
1950s. Committed dose equivalent, ALI and DACs were introduced for
the first time. Various parts of ICRP 30 were published in the
years following 1979, with updates to dosimetric and metabolic models
being made periodically.
 ~1985
 DOE adopts ICRP 26/30 recommendations (somewhat)
 1989
 ICRP 56  Age dependent doses to the public from the intake
of radionuclides
 1990
 10 CFR 20 Revision
 NRC publishes the final rule effective 1/1/94.
 Basically adopts ICRP 26 and ICRP 30 with some changes and developments.
 Coins use of TEDE, CEDE, CDE, EDE.
 1990
 NESHAPS Limits
 EPA limits radioactive air emissions to 10 mrem dose to public
using ICRP 26/30.
 1991
 ICRP 60 and 61  Latest ICRP recommendations, and ALIs/DACs
 New dose limit recommendations based on reevaluation of risk,
including detriments in addition to fatality.
 New and additional w_{T} values.
 New w_{R}(Q) values.
 Slightly different definitions of terms.
 1993
 ICRP 67  Latest ICRP Model
 1994
 ICRP 66  Latest ICRP Human Respiratory Tract Model
 1995
 10 CFR 834  (Draft) DOE Dose Limits
ICRP 2 Methodology
 Simple concept
 Still seen in a few regulations
 Assumptions for dose calculations:
 Material defined as soluble
 Can cross body barriers (lung, GI) to move into tissues
 Materials defined as Insoluble (doesn’t cross barriers)
 When transmitted to organ(s)
 Uniformly distributed
 Focus on organ with highest concentration or dose
 Focus on occupational exposure
 40 hrs/wk, 50 wks/yr, 50 yr working life
 Also consider continuous exposure, 168 hrs/wk
 Assume pure parent on intake of nuclide
 Progeny occur after intake
 Compartment model
 Every organ in the body is assumed to be a single compartment
with exponential elimination:
 Employed concept of whole body and critical organ
 Whole body (photon) irradiation can be used to calculate external
dose
 Critical organ
 Calculate dose to a mass of tissue
 Compare doses to all organs
 One with the highest dose is the “critical” organ
 Dose Rate (D) in organ
 k = proportionality constant (dimensional equalizer)
 q = activity in total body
 f_{2} = fraction in organ of concern
 e = effective absorbed energy (MeV/dis)
 m = mass of organ
 Buildup to equilibrium from continuous intake also considered:
 Person with continuous intake and constant loss will experience
increase in organ burden over time
Key parameters in ICRP2
 e
 effective absorbed energy
 MeV/dis
 Function of size of tissue
 Values tabulated for various organs
 f_{2}
 fraction in organ of concern
 tabulated for various organs
 tabulated for elements
ICRP 26/30 Philosophy
 Stochastic effects
 were defined as those effects for which the probability
of the effect occuring, rather than its severity, is a function
of dose without a threshold.
 cancer (fatal)
 hereditary (next two generations)
 Non Stochastic Effects
 were those effects for which the severity is a
function of dose and a threshold may exist.
 cataract of lens of the eye (0.15 Sv/y)
 almost any organ in the body can have a nonstochastic effect,
but typically the doses are quite large
 Commited Dose Equivalent
 That dose averaged throughout tissue T over the 50 years after
intake of the radioactive material
 T = tissue T
 t = time
 H = Commited Dose Equivalent
Effective Dose Equivalent (H_{E})
 Different tissues respond differently to same radiation dose
 Tissue weighting factors used to provide a common scale:
 H_{E} is the effective dose equivalent
 W_{T} is the tissue weighting factor
H_{E} restated
 Effective Dose Equivalent
 H_{E}
 Applies only to stochastic effects
 where w_{T} is the tissue weighting factor
 the fraction of the total stochastic risk associated with
the irradiation of tissue T.
Procedure for Deriving w_{T}
 Identify tissues susceptible to radiation induced cancer
 Establish risk
 Probablity of induction per unit dose equivalent
 Include
 Radiosensitivity
 Essentiallness
 Treatablility
 w_{T}= (risk for tissue T)/(Total Risk)
Deriving Weighting Factors
Tissue 
Effect 
Risk Coefficient 
ICRP26 W_{t} 
Gonads 
Hereditary 
4 * 10^{3} Sv^{1} 
0.25 
Breast 
Cancer 
2.5 * 10^{3} Sv^{1} 
0.15 
Lung 
Cancer 
2 * 10^{3} Sv^{1} 
0.12 
RBM 
Leukemia 
2 * 10^{3} Sv^{1} 
0.12 
Thyroid 
Cancer 
5 * 10^{4} Sv^{1} 
0.03 
Bone Surfaces 
Cancer 
5 * 10^{4} Sv^{1} 
0.03 
Remainder* 
Cancer 
5 * 10^{3} Sv^{1} 
0.3 (0.06/tissue) 
Sum Total Risk 
1.65 * 10^{2} Sv^{1} 

*To use in claculating the effective dose equlivalent,
calculate the dose to the remaining organs, and apply the value of
0.06 to the 5 mostdosed of the remainder, then throw out the rest.
You are then calculating the H_{E} to include up to 11 tissues 
Example
 Laboratory accident results in 185 kBq of ^{131}I uptake
by technician
 37 kBq to thyroid
 148 kBq uniformly throughout body
 Doses estimated via scanning/bioassay as:
 61.5 mSv to thyroid
 0.13 mSv to whole body
 What was H_{E}?
 H_{E} = 0.05·(61.5) + 0.95 ·(0.13) = 3.2 mSv
ICRP 30 continued
 ICRP 30, the revised 10 CFR 20, and draft DOE 10 CFR 834 are based
on the principle
 that the risk should be equal whether the whole body is irradiated
(external or internal) or whether there is nonuniform, part body
irradiation (external or internal).
 This condition is met if:
 wb = whole body
 w_{T }= tissue weighting factor
 H_{T }= dose equivalent received by tissue T
 H_{wb}= stochastic dose equivalent limit for uniform
wb irradiation
Limits for Intake by Workers
 Risk estimates of cancer & hereditary effects
 made using the linear dose response relationship.
 Total dose equivalent over the organ/tissue determines the effect,
regardless of the time over which it is delivered.
 For intake limits, 50 years is used as the occupational lifetime.
 Committed Dose Equivalent (CDE) is the total dose equivalent
in any tissue over the 50 years after intake (ref man).
 Committed Effective Dose (Equivalent) (CEDE) is the riskweighted
sum of committed dose equivalents to tissues (H_{T,50}).
Basic Limits  Occupational Exposure
 The ICRP 26 basic annual limits recommended for exposure to workers
 To limit nonstochastic effects
 0.5 Sv to all tissues except the lens of the eye
 0.15 Sv to the lens of the eye
 To limit stochastic effects
 To meet the ICRP 26 basic limits for exposure to workers the intakes
of radioactive material in any year must be limited to satisfy the following
conditions (assuming only internal exposure):
 and
 H_{T,50} is the committed dose equivalent in tissue, T,
resulting from intakes of radioactive material from all sources during
the year in question.
 The first relationship limits the stochastic effects and the
second limits the nonstochastic effects arising from the intake
of radioactive material.
 Only the annual intake is limited, not the rate of intake.
 Note: 10 CFR 20 defines the Total Effective Dose Equivalent
(TEDE) as the sum of the deep dose equivalent (external dose) and the
committed effective dose equivalent (internal dose). The dose limit
is then based on the TEDE.
Annual Limit on Intake
 Activity of a radionuclide, which if taken in alone, would irradiate
an individual to the limit set by the ICRP for each year of occupational
exposure
 Intake rate: quantity per year
 Pure parent assumption
 Can have an ALI each year
 No time constraint set on exposure period
 (rate can be instantaneous or up to a year)
 Two limits – stochastic and nonstochastic considered.
 Secondary limit
 designed to meet the basic limits for occupational exposure
 derived from the previous two relationships.
 The ALI is the greatest value of the annual intake, I, which
satisfies both of the following inequalities:
 Where
 I (Bq) is the annual intake of the specified radionuclide (by
ingestion or inhalation).
 S = stochastic limit
 N = nonstochastic limit
 H_{T,50} per unit intake (Sv Bq^{1}) is the
committed dose equivalent in tissue (T) from the intake of unit
activity of the nuclide by the specified route.
 If I_{s} not exceeded, then stochastic limits met
 If I_{n} not exceeded, then nonstochastic limits met
 Select value of I which satisfies both inequalities to determine
limiting value.
Calculating an ALI inhalation example ^{239 }Pu intake by inhalation
Tissue 
H_{50,T} Sv Bq^{1} 
W_{T} 
H_{50,T} Sv * W_{T} 
Lungs 
3.2 * 10^{4} 
0.12 
309 * 10^{5} 
Red Marrow 
7.6 * 10^{5} 
0.12 
9.1 * 10^{6} 
Bone surfaces 
9.5 * 10^{4} 
0.03 
2.9 * 10^{5} 
Liver 
2.1 * 10^{4} 
0.06 
1.2 * 10^{4} 
Sum 
8.9 * 10^{5} Sv Bq^{1} 
ICRP 30
Calculating ALI (stochastic)
Calculating ALI (nonstochastic)
Calculating ALI: ^{239}Pu
 Stochastic limit
 Nonstochastic limit
 Uncertainites in metabolic models result in listed ALI at one significant
figure:
Derived Air Concentrations
 Revised version of ICRP 2 MPC_{air}
 Maximum permissible concentration (air)
 Old MPCs were misused.
 They were maximum permissible concentrations intended
to control exposure over prolonged periods (> 3 mos).
 They have been used to infer overexposure for even short exposure
times.
 The limit for inhalation of a radionuclide is the appropriate
ALI.
 The concentration of a radionuclide in air during any year is
limited as follows:
 That concentration of a radionuclide in air, which if breathed for
one working year, would result in one ALI by inhalation:
 2000 hrs per working year
 1.2 m^{3} h^{1} working breathing rate
 Note: ALI is the main limit and the DAC is a derived limit.
