Page 362 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Ionizing Radiation in Veterinary Medicine Chapter | 19 329

VetBooks.ir quantities of radionuclides over Belarus, Ukraine, and the chemotherapy (Kent, 2017). The forms most commonly
used for malignancies are teletherapy, usually delivered
Russian Federation, and the 2011 Fukushima Daiichi NPP
using a linear accelerator, and plesiotherapy, which places
accident. At Chernobyl, radioactive material dispersed
90
into the atmosphere eventually settled back to the ground a radioactive source such as strontium 90 ( Sr) directly
as influenced by local wind and rainfall conditions, on the tumor. Total delivered dose and the number of
spreading the contamination throughout a wide area and treatment sessions (fractionation) will vary on a case-by-
2
prompting human evacuations from a roughly 4300-km case basis.
area. Similar to nuclear warhead detonations, NPP acci- Radiation burns to the skin can occur with X-rays or
90
dents release 131 I, 137 Cs, strontium 90 ( Sr), uranium, gamma rays used for teletherapy and interventional radiology
and plutonium. The estimated releases for Fukushima are (Waghmare, 2013). Chemotherapy, other medical treatments,
150 160 petabecquerel (PBq) 131 I and 10 15 PBq 137 Cs and individual susceptibility may increase this risk, though
into the atmosphere, and 4 27 PBq 137 Cs and 0.1 1 PBq full epidermal recovery can be complete within 6 weeks
90
Sr into water (Povinec et al., 2013). Large volumes of unless permanent damage has occurred (ICRP, 1992).
fresh water were used during emergency cooling of the
reactor and some of this was unintentionally discharged
into coastal waters. Other released radionuclides included ABSORPTION, DISTRIBUTION, AND
134 132 132 136
Cs, tellurium 132 ( Te), I, Cs, and the radioac- ACCUMULATION OF RADIOACTIVE
tive noble gases xenon 133 ( 133 Xe) and 135 Xe. Radiation CONTAMINANTS IN ANIMALS
dispersion devices or “dirty bombs” will probably involve
a single radionuclide, most likely 137 Cs, but potentially Shielding and distance greatly decrease exposures and
60
also cobalt 60 ( Co) or 90 Sr (Yamamoto, 2013). health hazards associated with external sources of radia-
As of December 2015, there were 441 operational tion. For people in the United States, the U.S. Nuclear
nuclear reactors worldwide (IAEA, 2016). Increased Regulatory Commission (NRC) regulates radiation expo-
leukemia clusters have been reported near civilian NPPs sures by setting exposure limits for individuals through a
in the United States, United Kingdom, and Germany system of practices that keeps all exposures as low as rea-
(Nussbaum and Ko ¨hnlein, 2003). sonably achievable.
External contamination occurs when radioactive parti-
Sterilization of Food cles are deposited on an object or a living organism, such
as what would be expected following the detonation of a
An irradiated object or living organism does not become radiological dispersal device (“dirty bomb”) or accidental
radioactive, meaning they pose no risk to others or to the spill in a medical, laboratory, or industrial setting. Risk
environment. Irradiation of food is accepted by the AMA, can be relatively low with evacuation, appropriate decon-
CDC, and WHO as a method of safely and effectively con- tamination, and controlling further access to the source of
trolling Escherichia coli, Salmonella,and Campylobacter in the contamination. Unless decontamination of the ground
food without decreasing its nutritional quality. and surrounding environment is possible, affected areas
may need to be abandoned for an extended period of time.
Medical Uses of Radiation in Human Internal contamination is when a radionuclide enters
and Veterinary Medicine the body, usually by ingestion or inhalation. Incorporation
occurs when the radionuclide is then taken up by tissue,
In the United States, medical X-rays and nuclear medicine such as what happens with diagnostic bone scans ( 99m Tc)
18
make up about 79% of man-made radiation exposures or PET scans ( F) and the use of radioactive iodine ( 131 I)
(National Research Council, 2006). Multiple computed to treat thyroid disease in cats. Incorporation can also lead
tomography (CT) scans are associated with a small but to tissue damage. Internal exposures only diminish as a
measureable increase in certain childhood cancers. result of radioactive decay or physiologic elimination.
Radionuclides such as 60 Co and 137 Cs have medical appli- Determining the degree of internal versus external expo-
cations, and unintended exposures have occurred follow- sure can be difficult. However, following a release of
ing accidental releases. Other examples include diagnostic radioisotopes, it is reasonable to expect that increased
bone scans (technetium 99 m [ 99m Tc]) or positron emis- levels of internal contamination may occur in animals
18
sion tomography (PET) scans (Fludeoxyglucose 18 [ F]) compared to people in the same affected area. In addition
and the use of radioactive iodine ( 131 I), a gamma and beta to being more likely to ingest contaminated food and
emitter, to treat thyroid disease in people and cats. water than people, animals may also internally contami-
Radiation is currently used in veterinary medicine to nate themselves through normal grooming behaviors.
cure oncology patients or provide palliative care, most The physical half-life of a radioisotope is the time
commonly as a localized treatment either with or without required for its initial rate of emissions to decrease by