Hall Book Ch 15 (Doses and Risks in Diagnostic Radiology)

Question 1

Everyone is exposed to radiation from unperturbed natural sources, enhanced natural sources, and sources resulting from human activity, including medical x-rays.

Natural background radiation comes from ( ) from the earth’s crust, and inhaled or ingested radioactivity.

Cosmic ray levels vary with ( ).

Answer 1

cosmic rays, terrestrial radiation

altitude and latitude

Question 2

Terrestrial radiation levels vary widely with ( ).

Radon and its progeny result in irradiation of lung tissue with ( ); this is the largest source of natural radiation.

( ) is by far the largest source of radiation resulting from human
activities.

Answer 2

locality, γ-particles?

Medical radiation

Question 3

By the year 2006, the collective effective dose to the US population from medical radiation had increased by about ( )% from the 1980s and now approximately equals that from ( ).

Answer 3

700, natural background radiation

Question 4

The radiation doses involved in diagnostic radiology, except for ( ) procedures, do not result in ( ); the risks are ( ) effects (i.e., carcinogenesis and heritable effects).

Answer 4

interventional, tissue reactions (deterministic effects), stochastic

Question 5

Computed Tomography

The use of CT has increased dramatically in the past 35 years.

By 2010, the number of CT scans in the United States reached 85 million per
year, with about 10% of these in children. There was a small reduction in the
number of scans performed for a few years, but by 2015, the number started
to rise again. There is a wide variation in the number of scanners per million
people between countries with different health care systems, with Japan
having by far the most.

CT scans involve relatively large effective doses because larger volumes of
tissue are exposed to higher doses than with common x-rays. The effective
dose varies from about 2 mSv for a head scan to about 15 mSv for chest or
abdomen. Values vary significantly for different scanners and for the same
make scanner in different departments.
Effective doses from CT scans are larger in small children than in adults,
unless care is taken to adjust for kilovolt and milliamperes.
Effective Dose and Cancer
The cancer risk to a person is expressed in terms of the effective dose, the
equivalent dose to the various organs and tissues exposed, multiplied by the
appropriate WT.
The collective effective dose is the product of the effective dose and the
number of persons exposed. It gives an indication of the harm or “detriment”
to an exposed population.
The collective effective dose to the US population in 2006 from medical
radiation was estimated by NCRP to be 899,000 person-Sv, a 700% increase
from the previous comprehensive survey conducted in the 1980s.
The “detriment” from medical radiation may be calculated from the collective
effective dose and the risk estimate of 5.5% per sievert for cancer (fatal and
nonfatal) and 0.2% per sievert for serious heritable effects.
Detriment includes an allowance for loss of quality of life as well as for death.
The use of medical radiation for 1 year in the United States may benefit more
than half of the population but may result in about 50,000 cases of cancer
(fatal plus nonfatal) and about 1,798 cases of serious heritable effects.
Interventional Procedures
The past two decades have witnessed a major increase in high-dose
fluoroscopically guided interventional procedures in medicine. Interventional
radiology and cardiology now represent one of the three fastest growing areas
of radiation medicine (together with CT and nuclear medicine).
Radiation doses to patients from interventional radiology and cardiology are, in
general, much higher than from general diagnostic radiology.
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NCRP estimated that, in 2006, there were 16.7 million interventional
procedures performed in the United States, with a corresponding collective
effective dose of 128,394 person-Sv.
Long interventional procedures can result in effective doses to the individual of
5 to 70 mSv, with TIPS being one of the largest.
Because patients undergoing interventional procedures are, in general, older
and/or with life-threatening illnesses, the possibility of radiation-induced
malignancy 20 years down the road is largely academic. By contrast, doses
are occasionally so high that tissue reactions can be an immediate problem.
There are reports in the literature of serious skin damage resulting from
fluoroscopically guided interventional procedures, including erythema, deep
ulceration, and occasionally necrosis requiring a skin graft. The number of
such cases is very small compared with the number of procedures performed,
but they do occur occasionally.
Doses to personnel involved in interventional procedures are among the
highest recorded routinely in medical centers, and there is evidence that
radiation-induced cataracts are not uncommon.
Nuclear Medicine
Nuclear medicine is the medical specialty in which unsealed radionuclides,
chemically manipulated to form radiopharmaceuticals, are used for diagnosis
and therapy.
Most nuclear medicine procedures are diagnostic examinations. Therapeutic
procedures account for only about 1% to 2%.
NCRP estimated that, in 2006, there were 19.7 million nuclear medicine
procedures performed in the United States, three-quarters of them in persons
older than 45 years. This resulted in a collective effective dose of 22,000
person-Sv, an increase of about 600% from the 1980s, with cardiology
accounting for much of this trend.
UNSCEAR estimated that, with 5% of the world population, the United States
was responsible for 50% of the nuclear medicine procedures performed in the
world.
Cardiac patients frequently undergo as many as 36 examinations, with a
median effective dose of greater than 60 mSv.
In a 2009 survey, Fazel et al. concluded that every year, about 4 million
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“nonelderly” adults are subject to nuclear cardiology procedures resulting in
an effective dose in excess of 20 mSv.
In general, there are three doses of interest following a nuclear medicine
procedure.
1. Dose to the target organ
2. Dose to the critical organ, which may be much higher
3. Effective dose, which determines the risk of stochastic effects (cancer and
heritable effects)
PET displays physiologic and metabolic data that can be fused with the
anatomic data from a CT scan.
The most commonly administered PET agent is [18F]-FDG that highlights
areas of metabolism and can detect cancer metastases.
A PET image with [18F]-FDG results in an effective dose of about 11 mSv.
However, this is usually accompanied by a CT scan, which increases the
effective dose by an amount that depends on the site being imaged.
Other PET agents used in oncology can detect areas of rapid cellular
proliferation in a tumor or areas of hypoxia. Still, other agents can be used to
measure blood flow in cardiology.
The most common form of nuclear medicine therapy is the use of iodine-131
for the treatment of hyperthyroidism. The dose to the thyroid may be many
tens of grays. In addition, there is a total body dose of 70 to 150 mGy, which
is caused by the circulation of the radioactive iodine in the bloodstream. The
second most common nuclear medicine therapy procedure is the treatment of
thyroid cancer. Following surgical removal of the cancer and the thyroid
gland, radioiodine may be used to destroy any residual iodine-accumulating
cancer cells that had spread to lymph nodes, lung, or bone. The third most
common therapy with radionuclides is the treatment of bony metastases.
Radioimmunotherapy uses antibodies labeled with iodine-131 or yttrium-90
and injected against specific antigens to treat various malignancies.
Medical Radiation of Children and Pregnant Women
Children are more sensitive than adults to radiation-induced malignancies by a
factor of 10 to 15. Physicians and patients alike are more cautious about
nuclear medicine procedures in children than about diagnostic x-rays, even
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when the radiation dose is comparable.
It is rare for radionuclides to be administered to a pregnant woman; great care
should be exercised if in an emergency situation, this is deemed to be
essential. Particular care is needed if the radionuclide involved is iodine
because after the 12th week of gestation, the fetal thyroid avidly takes up
iodine and can be seriously damaged.
Summary
The three principal contributors to the collective effective dose from medical
radiation are (1) CT scans, (2) nuclear medicine (including nuclear
cardiology), and (3) interventional radiology and cardiology. The millions of
conventional radiographs, including chest x-rays and mammography, account
for no more than 10% of the total collective effective dose.