Doses and Risks in DiagnosticRadiology, Interventional

Radiology and Cardiology, andNuclear Medicine

By

Dr. Ayush Garg

The purpose of this chapter is to review the doses involved and to estimate the associated risks in radiology, cardiology, and nuclear medicine.

DOSES FROM NATURAL BACKGROUND RADIATION• Natural sources of radiation include

• Cosmic rays from outer space and from the sun,

• Terrestrial radiation from natural radioactive materials in the ground, and radiation from radionuclides

• naturally present in the body, ingested from food, or inhaled.

• Enhanced natural sources are sources that are natural in origin but to which exposure is increased because of human activity.

• Examples include air travel at high altitude, which increases cosmic ray levels, and movement of radionuclides on the ground in phosphate mining.

• Indoor radon exposure might be consideredin some instances an enhanced natural source, in as much as it is not natural to live in an insulated house.

• Cosmic Radiation

• Cosmic rays are made up of radiations originating from outside the solar system and from charged particles (largely protons) emanating from the surface of the sun.

• Cosmic ray intensity is least in equatorial regions and rises toward the poles.

• There is an even larger variation in cosmic ray intensity with altitude, because at high elevations above sea level, there is less atmosphere to absorb the cosmic rays, so their intensity is greater

Natural Radioactivity in the Earth’s Crust

• Naturally occurring radioactive materials are widely distributed throughout the earth’s crust, and humans are exposed to the y-rays from them like Thorium & Uranium in U.S.

Internal Exposure

• Small traces of radioactive materials are normally present in the human body, ingested from the tiny quantities present in food or inhaled as airborne particles.

• Radioactive thorium, radium, and lead can be detected in most persons, but the amounts are small and variable, and the figure usually quoted for the average dose rate resulting from these deposits is less than 10 Sv per year.

• Only radioactive potassium-40 makes an appreciable contribution to human exposure from ingestion. The dose rate is about 0.2 mSv per year, which cannot be ignored as a source of mutations in humans.

• The biggest source of natural background radiation is radon gas, which seeps into the basements of houses from rocks underground.

• Radon, is a decay product in the uranium series, a noble gas.

• Radon progeny emit α-particles that, it is believed, are responsible for lung cancer.

Areas of High Natural Background

• The highest natural background radiation is in Kerala, where more than 100,000 people receive an average annual dose of about 13 mSv, reaching a high in certain locations on the coast of 70 mSv.

• It is because of radioactivity in rocks, soil, or in building materials from which houses are made.

COMPARISON OF RADIATION DOSES FROM NATURAL SOURCES AND HUMAN ACTIVITIES• In addition to natural background

radiation, the human population is exposed to various sources of radiation resulting from human activities.

DIAGNOSTIC RADIOLOGY

• The radiation doses involved in radiology other than interventional procedures are seldom sufficiently large to result in deterministic effects.

• By definition, a deterministic effect has a practical threshold in dose, the severity of the effect increases with dose, and it results from damage to many cells. One exception is inadvertent exposure of the developing embryo or fetus, with a possible consequence of reduced head diameter (microcephaly) and mental retardation.

• The threshold for radiation-induced mental retardation is about 0.3 Gy (International Commission on Radiological Protection [ICRP], 2006), so few procedures are likely to cause this effect.

• The potential deleterious consequences of diagnostic radiology involve stochastic effects, that is, carcinogenesis and heritable effects.

• The characteristic of stochastic effects is that there is no threshold in dose; that is, there is no dose below which the effect does not occur, and the probability of carcinogenesis or heritable effects increases with dose.

• A stochastic effect may result from irradiation of one or a few cells, and the severity of the response is not dose related.

• As a consequence, absorbed dose to a limited portion of a person’s body does not provide by itself the overall perspective on risk associated with a given procedure.

• Effective dose is a more relevant quantity; it takes into account the tissues and organs irradiated, as well as the dose involved.

• It is defined as the sum of the equivalent doses to each tissue and organ exposed multiplied by the appropriate tissue weighting factors (WT).

• The unit of absorbed dose is the gray, whereas the unit of effective dose is the sievert.

Adult Effective Doses for Various Diagnostic Radiology Procedures

Adult Effective Doses for Various CT Procedures

• The overall population impact of diagnostic radiology can be assessed in terms of the collective effective dose, the product of effective dose and the number of persons exposed.

• In this case, the unit is the person-sievert.

• This quantity is a surrogate for “harm” resulting from a given event involving radiation exposure.

• For example, the collective effective dose from the Chernobyl accident multiplied by the risk coefficient (5% per sievert for fatal cancer) gives an estimate of the number of cancer cases resulting from the accident, and is therefore a measure of the harm done.

• Some of the largest doses in diagnostic radiology are associated with fluoroscopy. In this case, the dose rate is greatest at the skin, where the x-ray beam first enters the patient.

• The entrance exposure limit for standard operation of a fluoroscope is 10 R per minute.

• A typical fluoroscopic entrance exposure rate for a medium-built person is approximately 3 R per minute (corresponding to an absorbed dose rate of about 30 mGy per minute).

• Much higher dose rates may be encountered during recorded interventional and cardiac catheterization studies

INTERVENTIONAL RADIOLOGY AND CARDIOLOGY• Interventional fluoroscopy refers to any procedure in which the use or

application of a medical device is fluoroscopally guided in the body and includes procedures that may be for diagnostic or therapeutic purposes.

• It is one of the fastest growing areas of medical radiation.

• These procedures include • Cardiac radiofrequency ablation, • Coronary artery angioplasty and stent placement, • Neuroembolization, • Transjugular intrahepatic portosystemic shunt (TIPS) placement.

• Such procedures tend to be lengthy and involve fluoroscopy of a single area of the anatomy for a prolonged period—frequently for longer than 30 minutes and occasionally for over an hour.

Adult Effective Doses for Various Interventional Radiology Procedures

Patient Doses and Effective Doses

• Radiation doses received by patients from interventional radiology and cardiology are much higher than from general diagnostic radiology; so much so that there is a risk of deterministic effects, such as early or late skin damage.

• During these procedures, typical fluoroscopic absorbed dose rates to the skin can range from 20 to more than 50 mGy per minute.

Potential Effects of Fluoroscopic Exposures on the Reaction of the Skin

Dose to Personnel

• Physicians involved in cardiology, angiography, and fluoroscopically guided interventional work routinely receive radiation doses higher than any other staff in a medical facility and comparable to doses received in the nuclear industry.

• Frequently, doses received by interventional radiologists are close to the annual dose limits, and there is also evidence that ocular cataracts are not uncommon. This is principally because of prolonged fluoroscopy.

NUCLEAR MEDICINE

• Nuclear medicine is the medical specialty in which unsealed radionuclides, chemically manipulated to form radiopharmaceuticals, are used for diagnosis and therapy.

• Radiopharmaceuticals localize in various target tissues and organs, and although nuclear medicine images have less spatial and anatomic resolution than do radiographic or magnetic resonance images, they are better able to display physiology and metabolism.

Historical Perspective

• The first person to suggest using radioactive isotopes to label compounds in biology and medicine was the Hungarian chemist Georg von Hevesy, whose work, beginning before World War II, earned him a Nobel Prize in 1943.

• The cyclotron was invented and developed by Ernest Lawrence in the 1930s, also leading to a Nobel Prize, and devices of this type have been used to produce short-lived isotopes and positron emitters.

Effective Doses for Adults from Various Nuclear Medicine Examinations

Principles in Nuclear Medicine

• They are produced artificially, using four principal routes of manufacture:

• (1) cyclotron bombardment (producing, for example, gallium-67, indium-111, thalium-201, cobalt-57, iodine-123, carbon-11, oxygen-15, nitrogen-13, and fluorine-18),

• (2) reactor irradiation (e.g., chromium-51, selenium-75, iron-59, cobalt-58, iodine-125, and iodine-131),

• (3) fission products (e.g., iodine-131, xenon-133, and strontium-90),

• (4) generators that provide secondary decay products from longer lived parent radionuclides. The most common example is the column generator incorporating molybdenum-99 for the provision of technetium-99m.

• Most technetium-99m generators use fission-produced molybdenum-99, although techniques of neutron irradiation could provide a viable alternative source of this important parent radionuclide.

• Other generators include those incorporating tin-113 (for the provision of indium-113m), rubidium-81 (for krypton-81m), and germanium-68 (for gallium-68).

• The use of radiopharmaceuticals for diagnosis or therapy is based on the accumulation or concentration of the isotope in the organ of interest, referred to as the target organ.

• A radiopharmaceutical may have an affinity for a certain organ that is not necessarily the organ of interest, in which case this organ is termed a critical organ.

• Often the dose to a critical organ limits the amount of radioisotope that may be administered. The risk to which the patient is subjected is clearly a function of the doses received in all organs and is expressed in terms of the effective dose.

• In addition to conventional planar imaging, two basic modalities have evolved.

• First, there is single photon emission computed tomography (SPECT).

• This uses conventional y-emitting radiopharmaceuticals and is often performed in combination with planar imaging.

• The second modality is the more specialized technique of PET. This is based on the simultaneous detection of the pairs of photons (511 keV) arising from positron annihilation and mostly uses the short-lived biologically active radionuclides oxygen-15, carbon-11, fluorine-18, and nitrogen-13.

Positron Emission Tomography

• The important and unique feature of PET studies is that they document physiologic abnormalities, or changes in metabolism, rather than simply alterations in anatomy.

• The principle of PET imaging is that the scanner locates the tracer by detecting the collinear pairs of 0.511-MeV photons emitted if a positron annihilates after uniting with an electron.

• Examples of radionuclides used for PET imaging include oxygen-15, carbon-11, and fluorine-18; these radionuclides have short half-lives of 2, 20, and 110 minutes, respectively

• The most commonly administered positron emitting radionuclide is fluorine-18, which is used for the production of [18F]-2-deoxy-2-fluoro-D-glucose, usually referred to as FDG.

• [18F]-FDG has also found a use in the early diagnosis of Alzheimer disease through the direct visualization of amyloid plaques in the brain.

• Other metabolic radiopharmaceuticals are rapidly finding a place in clinical practice—for example, [18F]- fluorothymidine (FLT) to map areas of rapid cell proliferation within tumors, or compounds such as [18F]-fluoromisonidazole(FMISO), [18F]- fluoroazomycin-arabinoside(FAZA), or 64-Cudiacetyl- bis(N4-methylthiosemicarbazone) (64- Cu-ATSM) to highlight areas in tumors that are hypoxic.

The Therapeutic Use of Radionuclides

• The most common form of nuclear medicine therapy is the use of radioactive iodine-131 for the treatment of hyperthyroidism.

• It involves an absorbed dose to the thyroid gland that varies with the person and is very nonuniform within the tissue itself.

• In addition, there is a total body dose of typically 70 to 150 mGy, which results from the isotope circulating in the blood.

• Because radiation is known to be a potent carcinogen, the risk of leukemia or of thyroid cancer following iodine-131 therapy has been appreciated from the outset.

• Pregnancy is a contraindication to the treatment of hyperthyroidism with iodine-131.

• Treatment of fertile women should be preceded by the taking of a careful history and a pregnancy test. Treatment should be delayed, if possible, to eliminate the potential effects during pregnancy.

• The second most common form of therapy with unsealed radionuclides is the treatment of thyroid cancer.

• Following complete surgical removal of the cancer and the thyroid gland, radioiodine may be given to destroy any residual iodine-accumulating cancer cells that had spread to lymph nodes, lungs, or bone.

• Such treatments involve large doses of iodine-131 that may result in a total body dose of 0.5 to 1.0 Gy, which is sufficient to cause severe depression of the bone marrow.

The therapy of bone metastases

• Several cancers, including prostate and breast, have a predilection for diffuse spread throughout the skeleton. There are several radiopharmaceuticals (such as strontium-89 chloride) that will localize in the metastatic lesions to provide palliation (but not cure).

Polycythemia vera

• It is a relatively rare disease that is characterized by overproduction of red and white blood cells by the bone marrow.

• Phosphate-32 is given intravenously, which localizes in the bone so that the y-rays emitted result in a mild bone marrow suppression and reduction in the production of many blood elements.

Radioimmunotherapy

• It uses radiolabeled antibodies directed against specific antigens. These agents can be used for the treatment of chemotherapy-resistant lymphomas.

• The antibodies are most commonly labeled with iodine-131 or yttrium-90 and injected intravenously in relatively large activities.

MEDICAL IRRADIATION OF CHILDREN AND PREGNANT WOMENIrradiation of Children

• The hazards associated with medical radiation in children are basically the same as in adults— namely, cancer and heritable effects.

• Concern for possible heritable effects induced by radiation is likewise greater in children, because they have their entire reproductive lives ahead of them.

• The general principle is that radiation exposures should be kept at the lowest practical level and in each case, the expected benefit should exceed the risk clearly.

Irradiation of Pregnant Women

• The risks involved in exposure to radiation of the embryo or fetus are summarized as follows:

• 1. For the first 10 days following conception (i.e., during preimplantation), the most significant effect of radiation may be to kill the embryo, leading to resorption.

• 2. Between 10 days and 8 weeks postconception (i.e., during organogenesis), the risks include congenital malformations and small head size, as well as carcinogenesis.

• 3. Between 8 and 15 weeks, and to a lesser extent 15 to 25 weeks, the risks include mental retardation, as well as small head size and carcinogenesis.

• 4. Beyond 25 weeks, the only risk of externally delivered diagnostic radiation is carcinogenesis, which is much reduced, compared with the risk during the first trimester.

• Radiation-induced carcinogenesis is considered a stochastic effect; that is, there is no threshold and the risk increases with dose.

• The other serious effects, such as mental retardation and congenital malformations, are considered deterministic; that is, there is a dose threshold of about 0.3 Gy.

• If a woman requires an emergency radiologic examination, however, there should be no hesitation to do the study.

• The health of the woman is of primary importance, and if serious injury or illness is suspected, this takes priority in determining the need for a study.

SUMMARY

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