Health Effects of Radiation
Tissue organization
? Effects of radiation on tissues are related to the functional organization of
each tissue.
? Tissues are often organized into specialized cell types with limited ability to
divide.
? This tissue unit is supplied and regenerated by a population of “immortal”
stem cells.
Tissue effects depend on
? Inherent sensitivity of the cells
? Kinetics of the cell populations: “acute” vs “late” effects.
? Stem cells much more radiosentive than mature functioning cells.
? Cell death occurs as the cell tries to divide.
? Very large doses required to kill (stop the function) of a non-dividing cell.
M G
1
G
2
S
Differentiation
Stem cell
compartment
Mature
functioning
cells
Life
span
After Fig 8.1 in Hall, Eric J. Radiobiology for
the Radiologist, 5
th
ed. Philadephia PA:
Lippincott Williams & Wilkins, 2000
1
Timing of onset of symptoms correlates with the lifespan of the
functional cells.
Early Effects: stem cells are the “target”
? Effects occur in a few days to weeks
? Rapidly dividing cell populations
? Examples: skin epidermis, gastrointestinal tract, hematopoietic system
? Damage can be repaired. Stem cells repopulate rapidly.
Late Effects:
? Effects occur in months to years.
? Slowly proliferating tissues: lung, kidney, liver, CNS
? Damage never repaired completely
? Vascular damage or mature functional cells as the “target”?
Dose scales for manifestation of radiation effects
Life-shortening
Gen eff Cancer Fetus
Cataract
CNSGIBMB
Delayed effects Acute effects
0 10 100 1,000 10,000 rad
B White blood cell changes
BM Bone marrow syndrome
GI Gastrointestinal syndrome
CNS Central nervous system
GEN EFF Genetic Effects
Chronic effects of radiation exposure
Cells not killed, but damaged…..
? Cataract formation
? Genetic (hereditary) effects
? Effects on the fetus
? Carcinogenic effects (cancer)
2
Human Radiation Exposure Data
Japanese A-bomb survivors
? 93,000 survivors
? 27,000 non-exposed comparable individuals as controls
? Location at the time of the blast must be accounted for in the dosimetry.
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RERF: Radiation Effects Research Foundation
Joint US-Japan research foundation following all of the survivors for life.
www.rerf.or.jp
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Whole-body radiation exposure: Acute effects
Prodromial syndrome
? Nausea, vomiting
? Dose dependent
? Signs appear in minutes at very high doses
Central nervous syndrome (only at very high doses)
? Doses > 100 Gy
? Death in hours
? Cause not clear (cerebrovascular syndrome)
Gastrointestinal syndrome
? Doses above ~ 5 Gy
? Death in ~ 3-10 days
? Nausea, vomiting, diarrhea
Bone marrow syndrome
? Doses above ~ 2 Gy
? Death in several weeks
? Immune system failure
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Intestinal epithelium
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? Stem cells located in “crypts” at the base of the finger-like villi.
? Maturing cells migrate to the tip of the villus.
? Transit time: crypt to tip, ~ 5-10 days.
? Doses >10 Gy will sterilize most of the stem cells in the crypts.
? Death in 3-10 days, villi, denuded and flat.
? If some crypt cells survive, they will regenerate functional crypts and
repopulate the villi.
4
Hematopoietic System
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Stem cells located mostly in the bone marrow.
Stem cells very radiosensitive
Survival curve shows little or no shoulder
D
0
< 1 Gy
Little sparing from fractionation or low dose rate
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Lymphocytes most sensitive (~0.3 Gy will deplete)
Timing and extent of cell population decline is dose dependent.
Total body dose of 3-4 Gy will suppress the immune system.
5
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? Dogs irradiated uniformly with 250 kVp x rays
? Doses are in roentgens (~ cGy)
? Blood cell responses are very similar before recovery in survivors and non-
survivors.
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? There is only a narrow range of doses where bone marrow transplant might
be expected to help.
? Doses < ~ 8 Gy : most persons would survive with intensive hospital care.
? Doses > 10 Gy : most persons will die of the gastrointestinal syndrome.
? Death in 3-5 days from GI syndrome if D > 10 Gy.
? Dosimetry must be estimated.
? If dose ~ 7-10 Gy, bone marrow transplant may help.
6
Skin
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Epidermis: site of early reactions
? ~ 100 μm thick (30-300 μm)
? Basal stem cell layer
? Non-dividing differentiated cell layers (10-20 layers) migrating to the
surface
? Keratinized cells at the surface
? Desquamation: loss of cells from the surface
? Transit time, stem cell to loss at the surface ~ 14 days (12-48).
Dermis: site of late reactions
? ~ 1,200 μm thick (1000-3000 μm )
? vascular network is in the dermis (no blood vessels in the epidermis)
? fibrous connective tissue
Two waves of skin reactions observed as a function of time after irradiation.
? Early: moist desquamation, ~ 10 days, stem cell damage, depletion of
differentiated cells. (~20 Gy single fraction)
? Late: dermal necrosis ~ months to occur, vascular damage and breakdown
of the skin.
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Erythema and hair loss
Cells in the hair follicle are sensitive.
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Hair Loss: Relation between the proportion of people with severe epilation
(loss of more than 2/3 of hair) and estimated radiation dose. Data from the
Japanese atomic bomb survivors (www.rerf.or.jp).
Threshold doses for skin reactions
3 Gy temporary hair loss
6 Gy erythema: reddening of the skin, occurs within hours to a few days.
15–20 Gy moist desquamation and dermal necrosis
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Hereditary Effects: effects on the offspring
? Radiation does not produce “new mutations”.
? Radiation increases the incidence of mutations that occur spontaneously
in the population.
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Fig. 11.1 in Hall, Eric J. Radiobiology for the Radiologist, 5
th
ed. Philadephia PA: Lippincott Williams & Wilkins, 2000.
Doubling dose: increases the natural background frequency by a factor of 2.
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Table 11.2 in [Hall].
Background mutation rate ~ 1-6%
Radiation-induced risk of hereditary disorder estimated at 0.6 x 10
-2
/Sv/per person
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Fig. 11.3 in [Hall].
Dose rate effect data from the “Mega mouse” Experiments at Oak Ridge National
Laboratory.
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Fetal Effects
? Lethal effects
? Malformations
? Growth disturbances
Principal factors
? Dose
? Gestation age at time of irradiation
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Data from rats given 200 rads at various time post-fertilization.
Japanese atomic bomb survivors irradiated in utero.
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Growth retardation: height, weight and head diameter
Mental retardation: observed in children irradiated at 8-15 weeks (only) of
pregnancy.
Effects observed at doses as low as 0.06 Gy.
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Cataracts: any detectable change in the normally transparent lens of the eye.
Deterministic response
? Cells are produced by mitosis in the germination zone (GZ) of the
epithelium.
? Differentiate into lens fibers in the meridional rows (MR).
? Cells in the central zone (CZ) do not normally divide.
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? No blood supply.
? No mechanism for removal of dead or damaged cells.
? Abnormal fibers migrate towards the posterior pole, the beginning of a
cataract.
? Single doses > 2 Gy will cause cataracts.
? Fractionation and low dose rate increase the threshold to 4-5 Gy.
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11
See Preston, Dale J., Yukiko Shimizu, Donald A. Pierce, Akihiko Suyama and Kiyohiko
Mabuchi. “Studies of Mortality of Atomic Bomb Survivors. Report 13: Solid Cancer and
Noncancer Disease Mortality: 1950-1997.” Radiation Research 160 (2003): 381-407.
Estimation of risk from radiation exposure
Excess risk: the excess cases of a particular health effect associated with
exposure to radiation. Excess risk can be described in various ways:
? Absolute Risk: the difference in the rate of occurrence of a particular
health effect in an exposed population and an equivalent population with no
radiation exposure. (units: excess number of cases per person-year-sievert).
? Relative Risk: the ratio of the rates in exposed and unexposed populations
(dimensionless)
? Excess Relative Risk (ERR): the ratio of the rate difference to the rate
in an unexposed population (N.B., ERR = the relative risk minus 1)
Excess risks depend on:
? radiation dose
? age at exposure
? time since exposure
? current age
? gender
Risk estimates are usually reported for a specific dose (often 1 gray or 1 sievert).
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Graph of Radiation dose of the uterus vs. Relative risk
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Figure 2.
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Figure 1.
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Plot of Excess Relative Risk (ERR) for different forms of cancer.
Estimates based on a model that accounts for age at exposure and gender.
The vertical dotted line represents no excess risk.
The solid vertical line represents the excess relative risk for all cancers.
Cancer deaths between 1950 and 1990 among Life Span Study -
Survivors with significant exposures (> 0.005 Sv)
Dose Range
Number of
Cancer
Deaths
Estimated
Excess Deaths
Attributable
Fraction (excess
lifetime risk)
0.005-0.2 Sv
3391 63 2%
(=100 x 63/3391)
0.2 – 0.5 Sv
646 76 12%
0.5 – 1 Sv
342 79 23%
> 1 Sv
308 121 39%
All
4687 339 7%
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Excess Lifetime Risk
? Based on observed cancer incidence to date
? Depends on dose, age at exposure, sex.
Lifetime cancer risks for atomic bomb survivors who received an
acute dose of 0.2 Sv
Age at exposure
(years)
Excess lifetime
risk
Background lifetime
risk
Excess relative risk
(ERR)
MEN
10 0.03 0.26 12%
(= 100 x 0.03/0.26)
30 0.02 0.28 7%
50 0.01 0.18 6%
WOMEN
10 0.05 0.19 26%
30 0.03 0.20 15%
50 0.01 0.15 7%
(Data from http://www.rerf.or.jp)
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Numbers of cancer deaths by cancer type and strength of evidence
for a radiation effect
SITE
TOTAL
DEATHS
ESTIMATED
EXCESS
EVIDENCE
FOR EFFECT
Stomach 2529 65 strong
Lung 939 67 strong
Liver 753 30 strong
Uterus 476 9 moderate
Colon 347 23 strong
Rectum 298 7 weak
Pancreas 297 3 weak
Esophagus 234 14 strong
Gallbladder 228 12 moderate
F. Breast 211 37 strong
Ovary 120 10 strong
Bladder 118 10 strong
Prostate 80 2 weak
Bone 32 3 moderate
Other solid 948 47 strong
Lymphoma 162 1 weak
Myeloma 51 6 strong
? Statistical significance may vary, but excess risks are seen for all types of
cancer.
? Supports the notion that radiation increases the risk for ALL types of cancer.
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Radiation in Medicine: therapy and diagnosis
a) thyroid cancer: 1930s and 1940s use of x-rays to shrink enlarged thymus
in children.
b) Ringworm of the scalp: 1940s and 1950s x rays used to cause temporary
hair loss (several Gy), treat hair follicles more effectively. Increases in
thyroid cancer, leukemia, brain tumors (10,000 patients in Israel, 2215 in
New York).
c) In Britain, ~14,000 patients with a congenital spinal cord problem known
as ankylosing spondylitis were irradiated to relieve pain. Increased
incidence of leukemia.
d) Female tuberculosis patients undergoing repeated fluoroscopy procedures
showed an increase in breast cancer.
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Fig. 10.5 in [Hall].
Radium dial painters
? Practice continued up to 1925. Ingestion of
226
Ra (bone seeker) caused an
increased incidence of bone cancer.
? At autopsy, bone was analyzed for radium content.
? Note, that there appears to be a threshold below which no effects are seen.
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Fig. 10.6 in [Hall].
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What are the risks from low-doses of radiation?
All human data are relatively high dose and delivered at high dose
rates…..
…..and extrapolated down to the low dose region at low dose rates.
This is a source of significant and continuing controversy.
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Risk Estimation Models
? linear no-threshold
? threshold
? linear-quadratic
The choice of model, and the estimated risk, has serious implications for
radiation protection.
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Data from high dose rate exposures is extrapolated to low doses and low dose
rates.
Low dose rate exposure is significantly less damaging.
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Is there a 4
th
curve??
Hormesis
a term coined to describe the behavior of an agent that is lethal at high doses but
beneficial at low doses.
(e.g., nickel, chromium, hormones, ultraviolet light)
The radiation effects paradigm:
? Radiation exposure is harmful.
? Radiation exposure is harmful at all doses.
? There are no effects at low doses that cannot be predicted from the effects at
high-dose levels.
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Graph: “Mortality risk from leukemia in Hiroshima and Nagasaki and
dose-response curve. After UNSCEAR (1994).
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Attitude of the General Public Towards “Risk”
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Refer to Sandman, Peter. Explaining Environmental Risk: “Important If True.” Originally
published by EPA’s Office of Toxic Substances, Nov. 1986. [updated 10 Nov 2001, cited
29 March 2004]. http://www.psandman.com/articles/explain1.htm
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