Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5545_Библиотеки_им_академика_М_И_Перельмана.pdf
348
17 Radiation Biology
patients. Practitioners and regulatory authorities are increasingly concerned of such
high doses and are advocating for devising ways to minimize radiation exposure to
the patient and the public.
Of all diagnostic radiological procedures, CT scans and uoroscopic procedures
give the highest effective doses, whereas dental and chest x-rays contribute only
minimal effective doses. Gonadal doses are higher with uoroscopic procedures
than with head CT, chest x-ray, and dental procedures. This is primarily due to the
fact that the gonads are out of the eld of the latter procedures. It should be noted
that the mammographic procedure contributes only a little to the total body dose
compared to the breast. For obvious reasons, the highest gonadal dose comes from
the procedures involving hips and pelvis. The GSD is about 9.8 mrad (98 μGy) for
males compared to 20.9 mrad (209 μGy) for females (NCRP 100 1989).
The doses to different organs from different nuclear medicine procedures are
listed in Table 16.4 and the effective doses in Table 16.6 in Chap. 16. Radiation dose
is always higher with long-lived and β-emitting radionuclides. The GSD values for
females (1.9 mrad or 19 μGy) is almost twice those of males (1.1 mrad or 10.9 μGy)
(NCRP 100 1989).
Risks from diagnostic procedures include both somatic and genetic effects.
Normally, these effects are minimal from diagnostic procedures for humans because
doses from these procedures are considered low. Doses from nuclear medicine procedures are even lower than those from diagnostic x-ray procedures. However,
based on the LNT model, there is no reason to believe that there is no risk from
diagnostic exposures, no matter how small the doses are. There may not be acute
effects, but long-term effects such as carcinogenesis, teratogenic effects from fetal
exposure, and genetic effects in the future offspring can occur. The probabilities of
fatal cancers, nonfatal cancers, and hereditary effects have been estimated by the
ICRP to be 4.0, 0.8, and 0.8% per sievert, respectively, for adult radiation workers
and 5.0, 1.0, and 1.3% per sievert, respectively, for the whole population (ICRP
60 1990).
An important quantity in the assessment of risk from radiation exposures is the
collective effective dose, which is dened as the sum of the products of the effective
dose and the number of persons exposed for each diagnostic procedure. The total
annual collective dose to the US population from all sources of radiation in 2006 is
187,000,000 person-rem (1,870,000 person–Sv) (NCRP 160 2009). The annual collective dose for radiation workers in healthcare is 55,000 person-rem (550 person Sv) in 2006. Based on a collective dose of 55,000 person-rems (550 person-Sv), for
medical radiation workers, the risk from 1 year of working as a radiation worker is
22 fatal cancers, 4 non-fatal cancers, and 4 serious heritable defects. These risks are
quite low compared to the total radiation used annually.
The benet from diagnostic procedures (both x-ray and nuclear medicine) is the
immediate diagnosis of the disease that can lead to the appropriate treatment and its
ultimate cure. Argument should prevail in favor of the benet for the use of radiation
for diagnosis over the risks that may appear in later years in the individual himself
or the future offspring. However, a judicious use of these procedures is denitely
warranted, and a procedure that is not needed should not be done. This argument for
17.17 Questions
349
the prudent use of radiation also applies to different screening procedures using
x-ray, such as mammography, chest x-rays, and dental x-rays. Many individuals are
exposed to radiations from screening, but only a small number of people benet
from the early diagnosis, while most of the screened people turn out to be negative.
For this reason, the American College of Radiology has recommended annual mammography only for women above 40 years of age, excluding younger women who
are more radiosensitive, some of whom may likely develop breast cancer many
years after mammography.
17.16 Risks toPregnant Women
Since radiation can cause a devastating effect on the embryo and fetus in pregnant
women, diagnostic radiological and nuclear medicine procedures are contraindicated
in pregnant women, despite only a small risk involved with the individual exposed
from these procedures. This is particularly important in nuclear medicine procedures,
because radiopharmaceuticals reside in the body following a biological half-life and
are likely to cross the placenta to cause the fetal damage. β-emitting radionuclides
are more damaging than γ-emitting radionuclides. Radioiodine administered orally
to pregnant women during the gestation period of 15–22 weeks can cross the placenta and localize in the fetal thyroid to the extent of 50–75%. The fetal thyroid dose
at 6 weeks of gestation is of the order of 2.1 Gy/MBq (7.8 rad/mCi) (Watson 1991).
In most cases, radiologic procedures are avoided in pregnant women by proper
screening such as asking them prior to the procedure if they are pregnant or when
they had their last menstrual period. However, at times, it is discovered after the
procedure that the women is pregnant. In such situations, steps should be taken to
estimate the dose received by the embryo or fetus based on the dosimetry parameters of the radiopharmaceutical. Depending on the period of pregnancy, the question
of therapeutic abortion may be considered if the dose is excessive. Some experts
believe that a dose of 10 cGy (10 rad) is a reasonable value above which therapeutic
abortion should be considered. However, the decision to abort depends on a number
of socio-personal factors.
In radionuclide therapy, pregnant women are absolutely excluded because of the
anticipated excessive fetal dose.
ited unless benet outweighs the risk of the fetus from therapy. Besides the in utero
effects, there is a small probability of thyroid cancer induced by the
hyperthyroidism.
131
I treatment of pregnant women is almost prohib-
131
I therapy of
17.17 Questions
1. (a) What are the mechanisms of radiation damage?
(b) Does the direct action or indirect action contribute more to radiation dam-
age? Why?
(c) Which are the free radicals that are most damaging to cells?
(d) Does the presence of oxygen increase or decrease radiation damage?
350
17 Radiation Biology
2. (a) Why are erythroblasts more radiosensitive than red blood cells?
(b) Which phase of the cell cycle is most radiosensitive?
(c) Which molecule of the cell structure is most radiosensitive?
(d) What are the different factors affecting radiation damage?
3. (a) Dene D0, Dq, and n as illustrated in the cell survival curve.
(b) Dq is smaller for high-LET radiations than for low-LET radiations. True
or false?
(c) D0 is smaller for high-LET radiations than for low-LET radiations. True
or false?
(d) What is the value of n for mammalian cells?
4. (a) The cell survival curve is steeper at high radiation doses than at low radia-
tion doses. Explain why and its implication.
(b) Does the shape of the cell survival curve vary with high-LET radiations and
at very high-dose rates?
5. (a) Choose the dose in rad that has been suggested as a practical threshold for
radiation-induced abortion: (i) 2; (ii) 5; (iii) 10; (iv) 20; (v) 50.
(b) How many days after conception can prenatal death occur as a result of in
utero irradiation?
(c) Which one of the following organs is most affected to be malformed by
prenatal radiation exposure? (i) heart; (ii) stomach; (iii) head; (iv) gonads;
(v) upper extremities.
(d) What is the period of pregnancy during which the incidence of abnormali-
ties and malformations in human neonates is expected to be the highest?
(e) What are the effects of radiation on the fetus?
(f) The incidence of childhood leukemia after in utero irradiation with a few
rad of diagnostic x-rays increases by a factor of (i) 1.5–2.0; (ii) 2.5–3.0;
(iii) 3.5–6.0.
6. (a) What are the dose ranges and approximate time limits of death for hemato-
poietic, gastrointestinal, and cerebrovascular syndromes?
(b) What are the prodromal syndromes and when do they appear?
(c) What is the dose at which almost total immunosuppression occurs
in humans?
7. (a) Dene the oxygen enhancement ratio (OER).
(b) Why is the oxygen effect absent for high-LET radiations?
(c) What are radiosensitizers? Name some of them.
(d) What is the maximum OER for γ- and x-rays?
(e) Is misonidazole a radiosensitizer or radioprotector for hypoxic cells?
(f) What is the specic composition of radioprotectors and how do they
function?
8. (a) What is the doubling dose and what is its value for humans?
(b) What are the doses at which permanent sterility can be induced in (a) males
and (b) females?
(c) Dene the genetically signicant dose (GSD).
(d) What is the GSD for humans?
(e) Which one of all medical radiations contributes most to the GSD?
References and Suggested Readings
351
(f) What are the factors that inuence the GSD?
9. (a) The mean latent period for radiation-induced leukemia is about (i)
5–10years; (ii) 12–20years; (iii) 21–30years.
(b) The mean latent period for radiation-induced solid tumors is about (i)
5–10years; (ii) 12–20years; (iii) 21–30years.
(c) Cataract can be induced in humans with (i) 10–30rad (10–30cGy); (ii)
100–110rad (100–110cGy); (iii) 200rad (200cGy).
(d) What is the risk of cancer in the general population from small doses of
low-LET radiation exposure?
10. What are the two most common chromosome aberrations that are responsible
for carcinogenesis?
11. In the linear quadratic model of the cell survival, what is the value of parameter
β for high-LET doses?
12. Elucidate the mechanisms of sublethal damage repair and potentially lethal
damage repair. Give an example of the latter.
13. What is the recent value of cancer deaths attributable to radiation exposure in
Japanese survivors of the atomic bomb? What are the different dose-response
models preferred for solid tumors and leukemia?
14. What are the risk estimates of fatal cancer by the ICRP and the BEIR VII for the
general population?
15. Describe apoptosis.
References and Suggested Readings
American Cancer Society. Radiation Exposure and Cancer. Atlanta, GA; 2003.
BEIR V Committee. The Effects on Populations of Exposure to Low Levels of Ionizing Radiations.
Washington DC: National Academy of Sciences/National Research Council; 1990.
BEIR VII, Phase 2. Health Risks from Exposure to Low Levels of Ionizing Radiations. Washington,
DC: National Academy of Sciences/National Research Council; 2005.
Hall EJ and Giaccia AJ. Radiobiology for the Radiologist. 7th ed. Philadelphia: JB Lippincott
Williams & Wilkins; 2011.
ICRP report no. 26. Recommendations of the International Commission on Radiological
Protection. NewYork: Pergamon; 1977.
ICRP report no. 60. 1990 Recommendations of the International Commission on Radiological
Protection. NewYork: Pergamon, 1991.
Jaworowski Z.Radiation risk and ethics. Physics Today. 1999; 52:24.
Mettler FA, Upton AC. Medical Effects of Ionizing Radiations. 3rd ed. Philadelphia:
W.B.Saunders; 2008.
Murphy PH.Acceptable risk as a basis for regulation. Radiographics. 1991; 11:889–897.
NCRP report no. 93. Ionizing Radiation Exposure of the Population of the United States. Bethesda,
MD: NCRP; 1987.
NCRP report no. 100. Exposure of the U.S. Population from Diagnostic Medical Radiation.
Bethesda, MD: NCRP; 1989.
NCRP report no. 160. Ionizing Radiation Exposure of the Population of the United States.
Bethesda, MD: NCRP; 2009.
Nias AHW. An Introduction to Radiobiology. 2nd ed. Hoboken, NJ: Wiley; 1998.
Pizzarello DJ, Witcofski RL. Medical Radiation Biology. 2nd ed. Philadelphia: Lea & Febiger; 1982.
Prasad KN. Handbook of Radiobiology. 2nd ed. Boca Raton, FL: CRC Press; 1995.
352
Ring JP.Radiation risks and dirty bombs. Health Phys. 2004; 86:S42–S47.
Rotblat J, Lindop P.Long-term effects of a single whole body exposure of mice to ionizing radia-
tion, II.Causes of death. Proc R Soc Lond (Biol). 1961; 154:350–368.
Travis EL. Primer of Medical Radiobiology. 2nd ed. Chicago: Year Book Medical Publishers; 1989.
United Nations Scientic Committee on the Effects of Atomic Radiation (UNSCEAR). Ionizing
Radiations: Sources and Biological Effects. NewYork: United Nations, 1982.
United Nations Scientic Committee on the Effects of Atomic Radiation (UNSCEAR). Sources,
Effects and Risks of Ionizing Radiation. NewYork, United Nations, 1988.
Watson EE. Radiation absorbed dose to the human fetal thyroid. In: 5th International
Radiopharmaceutical Dosimetry Symposium. Oak Ridge, TN, May 7–10, 1991.
17 Radiation Biology
Radiation Protection
andRegulations
18.1 Introduction
Radiation hazards to humans are well documented. To minimize their risks, international and national organizations have been established to set guidelines for the safe
handling of radiation. As mentioned before, the ICRP and NCRP are two such organizations. They make recommendations and guidelines for radiation workers to follow in handling radiation. The Nuclear Regulatory Commission (NRC) and state
agencies adopt many of these recommendations into regulations to implement radiation protection programs in the United States. The NRC regulations are published
in the Federal Register in the form of the Code of Federal Regulations (CFR). The
regulations pertinent to the practices of nuclear medicine are briey described here.
Until 2005, the NRC regulated all reactor-produced by-product materials, while
the individual State controlled the naturally occurring and accelerator-produced
radioactive materials (NARM). The Congress passed the Energy Policy Act in 2005,
broadening the denition of by-product material to include NARM products and
authorized the NRC to take control of all by-product materials. Over the years, the
NRC developed regulatory policies to include NARM products and implemented
them as of August 2009.
For convenience of operation, at present, 39 states have entered into agreement
with the NRC to implement regulations concerning the use of radioactive materials
(RAM) and thus to take control of regulatory management of RAM. These states are
called the Agreement States. The rules and regulations implemented by the agreement states must be at least as strict as, if not stricter than, those of the NRC.They
issue the license to use RAM and monitor their use and disposal.
18
© The Author(s), under exclusive license to Springer Science+Business Media, LLC,
part of Springer Nature 2025
G. B. Saha, Physics and Radiobiology of Nuclear Medicine,
https://doi.org/10.1007/978-1-0716-4816-2_18
353
354
18 Radiation Protection andRegulations
18.2 Radiation Protection
Rules and regulations pertaining to radiation protection set by the NRC are contained in 10CFR20. Because it is beyond the scope of this book to include the entire
10CFR20, only the relevant highlights are included.
18.2.1 Definition ofTerms
Several terms related to absorbed dose, as dened in the 10CFR20 are given here.
Committed dose equivalent (H
erence (T) that will be received from an intake of radioactive material by an
individual during the 50-year period following the intake.
Committed effective dose equivalent (H
ing factors applicable to each of the body organs or tissues that are irradiated and
the committed dose equivalent to these organs or tissues.
Controlled area means an area, beyond a restricted area but inside the site boundary,
access to which is limited by the license for any reason.
Deep-dose equivalent (Hd), which applies to the external whole-body exposure, is
the dose equivalent at a tissue depth of 1cm (1000mg/cm2).
Shallow-dose equivalent (Hs), which applies to the external exposure of the skin or
an extremity, is the dose equivalent at a tissue depth of 0.007cm (7 mg/cm2)
averaged over an area of 1cm2.
Tissue weighting factor (WT) for an organ or tissue is the proportion of the risk of
stochastic effects resulting from irradiation of that organ or tissue to the total risk
of stochastic effects when the total body is irradiated uniformly. The values of WT
from the 10CFR20 are given in Table 16.5.
Effective dose equivalent (HE) is the sum of the products of the committed dose
equivalent to each of the body organs and tissues and the weighting factor of the
corresponding organ or tissue (H
effective dose as of 1990.
Derived air concentration (DAC) is the concentration of a given radionuclide in air
that, if breathed by the reference man for a working year of 2000h under condi-
tions of light work, results in an intake of ALI.DAC values are given in Table1,
column 3 of Appendix B in 10CFR20.
Annual limit on intake (ALI) is the derived limit on the amount of radioactive mate-
rial allowed to be taken into the body of an adult worker by inhalation or inges-
tion in a year. These values are given in 10CFR20 (Table1, Appendix B).
Total effective dose equivalent (TEDE) is the sum of the deep-dose equivalent (for
external exposure) and the committed effective dose equivalent (for internal
exposure).
Restricted area is an area where an individual could receive in excess of 5 mrem
(0.05mSv) per hour at 30cm from a radiation source.
) is the dose equivalent to organs or tissues of ref-
T,50
) is the sum of the products of the weight-
E,50
=ΣWT·H
E
). It is the currently used term
T,50
18.2 Radiation Protection
355
High-radiation area is an area where an individual could receive from a radiation
source a dose equivalent in excess of 100 mrem (1mSv) in 1h at 30cm from
the source.
Very high-radiation area is an area where an individual could receive from radiation
sources an absorbed dose in excess of 500rad (5Gy) in 1h at 1m from the source.
Unrestricted area is an area in which an individual could receive from an external
source a dose of 2 mrem (20μSv)/h and 50 mrem (0.5mSv)/yr.
18.2.2 Caution Signs andLabels
The NRC requires that specic signs, symbols, and labels be used to warn people of
possible danger from the presence of radiation. These signs use magenta, purple, or
black color on a yellow background; some typical signs are shown in Fig.18.1.
Fig. 18.1 Various radiation caution signs and labels
356
18 Radiation Protection andRegulations
Caution: Radiation Area. This sign must be posted in radiation areas.
Caution: High Radiation Area or Danger: High-Radiation Area. This sign must be
posted in high-radiation areas.
Caution: Radioactive Material or Danger: Radioactive material. This sign is posted
in areas or rooms in which 10 times the quantity of any licensed material speci-
ed in Appendix C of 10CFR20 is used or stored. All containers with quantities
of licensed materials exceeding those specied in Appendix C of 10CFR20
should be labeled with this sign. These labels must be removed or defaced before
disposal of the container in the unrestricted areas.
Caution signs are not required in rooms storing the sealed sources, provided the
radiation exposure at 1 foot (30cm) from the surface of the source reads less than 5
mrem (50μSv)/h. Caution signs are not needed in rooms where radioactive materials are handled for less than 8h, during which time the materials are constantly
attended.
18.2.3 Occupational Dose Limits
The annual limit of the occupational dose to an individual adult is the more limiting
of (a) TEDE of 5rem (0.05Sv) or (b) the sum of the deep-dose equivalent and the
committed dose equivalent to any individual organ or tissue other than the lens of
the eye being equal to 50rem (0.5Sv). It should be noted that there is no lifetime
cumulative dose limit in 10CFR20, although the NCRP recommends a lifetime
cumulative dose of 1rem (10mSv)×age in years.
The annual limit on the occupational dose to the lens of the eye is 15 rem
(0.15Sv).
The annual limit of the occupational dose to the skin and other extremities is the
shallow-dose equivalent of 50rem (0.5Sv).
Depending on the license conditions, both internal and external doses have to be
summed to comply with the limits. A licensee may authorize under planned special
procedures an adult worker to receive additional dose in excess of the prescribed
annual limits, provided no alternative procedure is available. The total dose from all
planned procedures plus all doses in excess of the limits must not exceed the dose
limit (5rem or 50mSv) in a given year, nor must it exceed ve times the annual
dose limits in the individual’s lifetime.
The annual occupational dose limits for minors is 10% of the annual dose limits
for adults. The dose limit to the fetus/embryo during the entire pregnancy (gestation
period) due to occupational exposure of a declared pregnant woman is 0.5rem
(5mSv), which amounts to approximately 50mrem (0.5mSv) per month, assuming
a 10-month pregnancy.
The total effective dose equivalent to individual members of the public is 0.1rem
(1mSv) per year. However, this limit can be increased to 0.5rem (5mSv) provided
the need for such a higher limit is demonstrated.
18.2 Radiation Protection
357
18.2.4 ALARA Program
The established dose limits are the upper limits for radiation exposure to individuals. The NRC has instituted the ALARA (as low as reasonably achievable) concept
to reduce radiation exposure to individuals to a minimum. The ALARA concept
calls for a reasonable effort to maintain individual and collective radiation exposure
as low as possible. Under this concept, techniques, equipment, and procedures are
all critically evaluated. According to the NRC Regulatory Guide, under the ALARA
concept, when the exposure to a radiation worker exceeds 10% of the occupational
exposure limit in a quarter (Action Level I), an investigation is made by the RSO,
and the report is reviewed by the RSC.When the exposure exceeds 30% of the
occupational exposure limit (Action Level II), corrective actions are taken or the
licensee must justify a higher dose level for ALARA in that particular situation, but
not to exceed the annual occupational dose limit.
18.2.5 Principles ofRadiation Protection
Of the various types of radiation, the α-particle is most damaging because of its
charge and large mass, followed in order by the β-particle and the γ-ray. Heavier
particles have shorter ranges and therefore deposit more energy per unit path length
in the absorber, causing more damage. On the other hand, γ-rays and x-rays have no
charge or mass and therefore have a longer range in matter and cause relatively less
damage in tissue. Knowledge of the type and energy of radiation is essential in
understanding the principles of radiation protection.
The cardinal principles of radiation protection from external sources are based
on four factors: time, distance, shielding, and activity.
18.2.5.1 Time
The total radiation exposure to an individual is directly proportional to the time of
exposure to the radiation source. The longer the exposure, the higher the radiation
dose. Therefore, it is wise to spend no more time than necessary near radiation
sources.
18.2.5.2 Distance
The intensity of a radiation source, and hence the radiation exposure, varies
inversely as the square of the distance from the source to the point of exposure. It is
recommended that an individual should keep as far away as practically possible
from the radiation source. Procedures and radiation areas should be designed so that
individuals conducting the procedures or staying in or near the radiation areas
receive only minimal exposure.