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SI Unit

Older Unit

Conversions

Activity

1 becquerel= 1 nuclear decay per second

1 curie (Ci) = 3.7 x 1010 nuclear decays per second = activity producing the same number of nuclear decays as 1 gram of radium-226

MBq = 37 x mCi mCi = 0.027 x MBq

Absorbed dose

1 gray (Gy) = 1 Joule per kg

1 rad = 100 erg per gram

Gy = 0.01 x rad rad = 100 x Gy

Dose equivalent*

1 sievert (Sv) = 1 gray x Quality Factor 1

1 rem = 1 rad x Quality Factor 1

Sv = 0.01 x rem rem = 100 x Sv

* Quality Factor = 1 for all X-, gamma, and beta radiations; 5 for neutrons; 20 for alpha radiation.

measurement. Effective dose from common radiologic and nuclear medicine procedures are summarized in Appendix 2.

Background Radiation

Living on Earth has always meant continuous exposure to natural background radiation including cosmic radiation, radiation from naturally-occurring minerals in natural surroundings and construction materials as well as internal radiation from radionuclides within our own bodies. Finally there is the inevitable but highly variable inhalation of the ubiquitous radioactive gas radon, a decay product of the uranium-238 which is a component of the Earth's crust.

In Eastern North America typical natural background radiation is in the order of 1.0 mSv per year with exposure to radon contributing a further 1-2 mSv. Natural background radiation varies by a factor of five to eight times from place to place in the world and the variation is even greater when differences in inhaled radon are taken into account. Numerous large surveys have compared cancer incidence and deaths in high and low background regions in an attempt to measure the presumed carcinogenic effect of low level radiation. No association between cancer incidence and level of background radiation has been found. It is ironic that for radiation safety purposes we may struggle to regulate and reduce radiation exposures which are less than those which may be incurred by moving from one city to another because of the geographic variation in background radiation levels!

Table 3. Sample effective doses for bone and thyroid scans

Bone Scan

Thyroid Uptake

Radiopharmaceutical Physical half-life Radionuclide emissions

Activity Critical organs Effective dose

99mTc-MDP 6.02 hours gamma (140 keV)

740 MBq (20 mCi) bone, bladder

131I-Sodium iodide 8.1 days gamma (364 keV) and beta (mean 192 keV) 0.185 MBq (5 ^Ci) thyroid, bladder

Radiation Effects and Carcinogenesis

Biological Effects of Radiation

Cellular DNA is the critical target for the biological effects of ionizing radiation, both as direct target and as a secondary target of the diffusible radiolytic products of water and possibly other cellular constituents. Radiation induced damage may occur within each of the chemical components of DNA. Spontaneous chemical reversal of the damage may occur but if it does not then the site undergoes enzyme-mediated repair. This either restores the DNA to its original state or results in stable DNA damage. Ionizing radiation and other genotoxic agents such as ultraviolet radiation and numerous chemicals can damage DNA nucleotide bases or cause single strand and double strand breaks in the sugar-DNA backbones and DNA-protein cross links. Of these it is the DNA double strand lesions which are most important in the induction of lethal cell events, chromosomal abnormalities and gene mutations.

At high dose rates the natural repair mechanism can be overwhelmed. Lowering the dose rate (e.g., by fractionation of the radiation dose) is likely to diminish its RBE because of the possibility of simultaneous DNA damage and repair at low dose rates. Exposure to natural background radiation is an important example of how low dose rate radiation might be expected to exhibit a lesser biological effect than the equivalent high dose rate exposure.

Stochastic and Deterministic Effects

The clinical manifestations of unrepaired DNA damage caused by ionizing radiation are conventionally divided into those which are "stochastic" and those which are "deterministic" (formerly called "non-stochastic").

With deterministic effects the probability of causing harm will be zero at doses up to some known threshold (usually hundreds or even thousands of mSv) and then will increase steeply and proportionately to dose above the threshold (for clinical effect). Radiation burns, decreased salivary gland secretion, lens opacities (cataracts) and loss of fertility from gonadal irradiation are deterministic effects. Because the threshold levels are relatively high, such effects will not be seen in the practice of diagnostic nuclear medicine. In radionuclide therapy however deterministic effects are deliberately sought but may also be encountered as side effects. For example when iodine-131 is administered to treat hyperthyroidism we deliberately seek to reduce thyroid gland function while with the larger doses used in the treatment of thyroid carcinoma radiation sialadenitis may be encountered as a side effect. Similarly, when treating painful bony metastases with an agent such as radioactive strontium-89 bone marrow depression may occur as a side effect.

With stochastic effects the probability of the event increases with radiation dose with the assumption that there is no threshold (i.e., no dose too small not to have an effect). In practice there may well be a point below which the effect is imperceptible or so small as to be negligible. Examples of stochastic effects are the increased probability of developing leukemia or a solid cancer following radiation exposure.

Linear No Threshold Hypothesis

This hypothesis states that the stochastic dose/response effects (e.g., cancer induction) observed at high doses and dose rates can be extrapolated in a straight line passing through the origin (zero dose and zero effect). This implies that there is no dose so low that it does not have an adverse effect even though no effect is directly observable. This is a perfectly valid scientific and safety device providing it is appreciated that it is not proven or provable, and that other hypotheses may fit the observable phenomena such as a linear/quadratic relationship between dose and effect for low doses and even that there is indeed a threshold (Fig. 2).

Sources of Radiation

Along with natural background radiation there comes exposure to man-made radiation. Some of this is unavoidable such as the fall-out from nuclear weapons testing (now at very low levels), the generation of electricity by nuclear power, and the radiation from luminous dials and signs. By far the greatest contribution to the man-made radiation in the developed world comes from medical uses in diagnostic radiology and nuclear medicine. When distributed over the adult North American population this is approximately equivalent to two-thirds of natural background.

Radiation-Induced Cancer

The clinical presentation of leukemia induced by ionizing radiation is likely to be delayed by several years from the time of exposure. In the case of solid tumours the latent period may be several decades. It is believed that excess cancers are still arising in Japanese atomic bomb survivors. The risk of inducing additional neoplasms is generally quoted as the average for an adult population. The additional lifetime risk of developing a fatal cancer is generally taken as 1 in 4,000 for an effective dose of 10 mSv. Lifetime risk from radiation exposure is relatively less in elderly individuals (due to limited life expectancy from other illness) and relatively greater in healthy children.

Atomic bomb survivor data show an excess of leukemias and solid cancers over the expected spontaneous occurrence but the excess of solid cancers only exceeds 95% confidence limits for victims with estimated doses in the range 50-250 mSv and higher with no excess demonstrable for exposures below 50 mSv.

Effects of Ante-Natal (in utero) Radiation Exposure

The effects of radiation exposure of the conceptus depends upon the time of exposure relative to conception (Table 4). Prior to the beginning of organogenesis (three weeks after conception) damage to the small number of relatively undifferentiated cells is most likely to result in failure of implantation or undetectable death of the conceptus rather than a damaged liveborn child. When major

Figure 2. Possible mathematical relationships between radiation dose and cancer risk: (a) threshold, (b) linear no threshold and (c) quadratic models. The threshold model (a) suggests that there is a threshold below which stochastic effects such as cancer induction do not occur. Above this threshold, cancer induction is predicted to be proportional to dose. The linear no threshold model (b) assumes no such threshold. The quadratic (c) model assumes a low rate of cancer induction at lower radiation doses and a higher rate at higher doses.

Figure 2. Possible mathematical relationships between radiation dose and cancer risk: (a) threshold, (b) linear no threshold and (c) quadratic models. The threshold model (a) suggests that there is a threshold below which stochastic effects such as cancer induction do not occur. Above this threshold, cancer induction is predicted to be proportional to dose. The linear no threshold model (b) assumes no such threshold. The quadratic (c) model assumes a low rate of cancer induction at lower radiation doses and a higher rate at higher doses.

organogenesis is underway irradiation may cause congenital malformations. These effects are deterministic with an estimated threshold in humans of 10 to 25 mSv. Mental retardation, sometimes associated with microcephaly, has been seen in the children of atomic bomb survivors. The threshold for this effect is estimated at 60 to 300 mSv.

The fetal thyroid gland does not concentrate iodide (or radioiodide) until after 12 weeks of gestation. Radioiodine administered to the pregnant woman after this time has the potential to cause radiation damage to the fetal thyroid including the induction of congenital hypothyroidism. Cases have been reported in the literature in which therapeutic doses of iodine-131 administered to thyrotoxic pregnant women have caused hypofunction of the fetal thyroid. This has happened sufficiently often for it to be known that during intra-uterine existence the fetus survives and develops normally on maternal thyroxine of either endogenous or exogenous (supplemental)

Table 4. Human fetal developmental milestones in weeks

Implantation 1-2 weeks

Neural plate formation 1-3

Organogenesis 3-7

Upper limb bud formation 4

Heart septation 7

Palate closure 8

Concentration of iodine in fetal thyroid 12

origin. Providing that the possibility of congenital hypothyroidism is recognized and treated within a few weeks of birth, normal post natal development can occur.

The evidence for the stochastic effect of intrauterine irradiation conferring an increased probability of developing cancer or leukemia in childhood or in adult life is not consistent but it is generally thought reasonable to assume that in utero sensitivity may be several times that of the adult population.

The possibility of pregnancy or breast feeding should always be excluded when a nuclear medicine procedure is contemplated in a female of reproductive age. Most departments post a notice in waiting rooms and washrooms asking any woman who knows or suspects herself to be pregnant, who has missed an expected menstrual period or who is breast feeding to report this to the receptionist or a technologist before starting the procedure. It is also the duty of the technologist to ask the patient concerning pregnancy and breastfeeding prior to administering a radiopharmaceutical. Many departments will record that the question has been asked and record the response and some may require the patient to sign a declaration. Once the woman's status is known the nuclear medicine specialist and the referring physician can decide to go ahead, cancel, modify or postpone the procedure according to a risk/benefit analysis. Clearly a proposed ventilation/perfusion lung scan for suspected pulmonary embolism at 32 weeks of pregnancy requires different consideration from a speculative bone scan at 8 weeks in a woman with a tentative diagnosis of fibromyalgia. The risk/benefit analysis must consider radiation dose to the fetus, including that due to any transplacental passage of the radiopharmaceutical, and its residence time in maternal organs adjacent to the uterus (particularly the bladder).

Practice Point

Exclusion of pregnancy is particularly important when therapeutic radiopharmaceuticals are to be administered. Serum beta-HCG becomes detectable about two weeks after ovulation (around the time of implantation) and should be tested whenever pregnancy cannot be reliably excluded otherwise. Particular tact may be required when dealing with a thyrotoxic teenage candidate for radioiodine therapy accompanied by a parent.

Genetic Disorders

Theoretically genetic disorders may occur in first and subsequent generation children born to parents with radiation exposure prior to conception due to radiation-induced mutation in gamete precursors. This effect has never been demonstrated to occur in human populations (including the atomic bomb survivors) but its occurrence is predicted from plant and animal work. By extrapolation, it has been estimated that a doubling of the spontaneous mutation rate (non-radiation induced plus background radiation induced) requires on the order of 50 to 250 mSv.

Principles of Radiation protection

Increased Distance from the Source

Increasing the distance from a point source of radiation causes the exposure rate to drop off rapidly according to the Inverse Square Law. For example moving from 10 cm to 30 cm from a point source will decrease exposure rate by a factor of nine. You can illustrate this effect by warming your hands at an open fire and then taking a step backwards.

Practice Point

Increasing distance from the source brings major benefits in reducing radiation exposure in nuclear medicine but the major source of radiation exposure, the patient, is not a point source. Simple inverse square law calculations do not apply when the distance from the patient is less than 3 m. This principle can be easily demonstrated by taking a survey meter and measuring exposure rates at increasing distances from a patient recently injected with a 99mTc agent for a bone or myocardial perfusion scan.

Minimize Time of Exposure

Carefully plan procedures to require the least time handling or in the vicinity of a radiation source.

Shielding

Gamma and X-ray radiation is attenuated by the material through which it passes to an extent determined by the energy of the radiation, the density of the material and its thickness. The thickness of a shielding material required to reduce radiation exposure from a given radionuclide by half is called its half value layer (HVL). The HVL of lead for 99mTc is 0.3 mm whereas for the more energetic emission of 131I a thickness of 3.0 mm is needed (Table 5). Lead is frequently used for shielding because its high density minimizes the thickness and hence the volume occupied by the shielding needed to provide a required attenuation. Where space is available, concrete, compacted earth or water may provide effective and economical shielding.

Practice Point

The wearing of lead or lead equivalent (usually 0.25 mm lead equivalent) aprons to shield the trunk is popular among technologists and mandated in some jurisdictions. These are effective shields at typical diagnostic X-ray energies but the theoretical attenuation achieved for 99mTc is < 50%, while for 131I it is negligible. The risk of wearing the heavy device may not be negligible in terms of back strain.

Elements of distance, time and shielding are often used together to achieve the desired reduction in radiation exposure. Occasionally over emphasis on one principle can have a net negative effect. For example particularly heavy and clumsy shielding of a radioactive source may increase the time needed to handle it and increase the possibility of a spill.

Table 5. Common half-value layers (HVL)

99mTc 131I

Photon energy keV 140 364

As low as reasonably achievable (ALARA)

The adoption of the linear no threshold hypothesis, with its implication that no level of exposure no matter how small can be considered safe, logically led to the adoption of the imperative that exposure should always be as low as reasonably achievable—ALARA. This had its beginning more than 40 years ago with ICRP and NCRP. The phrase "reasonably achievable" is open to misinterpretation, and to avoid preoccupation with inconsequentially small amounts of radiation has been qualified by the phrase—too often forgotten or ignored—"social and economic factors taken into account." ALARA remains a valuable principle for radiation protection, providing it is interpreted with common sense and does not become a tyranny. In the institutional setting that common sense may be provided by a broadly representative Radiation Safety Committee.

Practical Aspects of Radiation protection

Expression of Radiation Risk

For most nuclear medicine diagnostic procedures the risk comes down to the stochastic risk of a small increment in the individual's pre-existing risk of developing a fatal leukemia or cancer at some time in their remaining lifespan. The radiation dose is determined by the radiopharmaceutical injected, body size and anything influencing biodistribution and is independent of the actual imaging procedure ie. once a patient is injected with the radiopharmaceutical the radiation dose will be identical regardless of whether one image is taken or twenty.

A radiation dose of 10 mSv is estimated to confer a lifetime risk of fatal malignancy in the order of 1 in 4,000, a small increment to the lifetime risk of cancer death of 1 in 5 for every newborn child (rising to 1 in 3 for an adult of 65 years). Thus a bone scan following the injection of 99mTc- MDP 750 MBq results in an effective dose of 6 mSv and an estimated lifetime risk of fatal cancer of 1 in 6,700. A rest/stress myocardial perfusion scan with each component receiving 99mTc-sestamibi 1100 MBq results in a total radiation dose of 25 mSv and a lifetime cancer risk of 1 in 1,600. In the first case the procedure will have increased the probability of the dreaded event from 0.25000 to 0.25015 for a young adult and in the second from 0.33333 to 0.33396 for an elderly man.

These risks will appear small to the informed physician and very small when set against the possible benefit to be obtained from the bone scan and the myocardial perfusion scan to which they relate. Yet how are they to be expressed to the patient and his or her relatives who may find it difficult to accept any risk and be unfamiliar with thinking in epidemiological terms and dealing with the concept of stochastic risk? When told the odds of death are 1 in 10,000 the expert concentrates on the odds and dismisses the risk. Conversely the layperson concentrates on the dread event, ignores the odds and is concerned by the risk. (If you doubt the above consider the person who dreams the afternoon away planning how he will spend a lottery prize while ignoring the odds against winning.)

Equivalent risks may be helpful in putting radiation risks in context but be warned that your attempt to be helpful may be counterproductive. Expressing the risk of a nuclear medicine procedure in terms of an equivalent number of chest radiographs may be thought helpful in that the latter is more familiar to most patients. Anyone told that a bone scan is equivalent to 60 chest X-rays or that a rest/stress myocardial perfusion scan equates to 250 chest X-rays is unlikely to know that annual natural background radiation is equivalent to 30 chest X-rays or focus on the safety of chest X-rays rather than the apparent danger of the nuclear medicine procedure. Describing the risk of nuclear medicine procedures in terms of cigarettes smoked, slices of cream pie eaten or minutes of mountain climbing is more likely to make the physician look foolish than to educate the patient. It may be more acceptable to relate risk to that of a fatal accident when driving a car since that is such a familiar, recurrent and difficult to avoid activity.

The risk of a fatal accident during a working lifetime in a "safe"occupation, such as a bank manager or sales assistant, is approximately 1 in 10,000. This is equivalent to a 4 mSv radiation dose. For many people this will help to put a bone scan in context. The Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 50 mSv in one year or a lifetime dose of 100 mSv in addition to background radiation.

Dose limits

Legislation changes over time and differs between countries and the reader should consult local authorities for the current dose limits. Current radiation effective dose limits in Canada, Britain and Europe are 1 mSv (0.1 rem) annually for the general public and 20 mSv (2 rem) annually for someone who is occupationally exposed. The corresponding limits for the United States are 1 mSv (0.1 rem) and 50 mSv (5 rem) respectively. At these limits it can be seen that nuclear medicine technologists with a typical exposure of 1.5-2.0 mSv annually exceed the limit for the general public but fall far short of that for the occupationally exposed.

Dose limits for pregnant women are less uniform. In Canada the dose limit for the occupationally-exposed pregnant worker is 4 mSv (0.4 rem) effective dose to the worker from the summed internal and external sources of radiation exposure for the remainder of the pregnancy after the pregnancy has been declared. In the United States the dose to an embryo/fetus during the entire pregnancy due to occupational exposure of a declared pregnant woman must not exceed 5 mSv (0.5 rem). There is no dose rate restriction on this maternal exposure. In Europe and the United Kingdom the dose limits for the pregnant worker are 2.0 mSv to the surface of the abdomen and 1.0 mSv to the fetus for the remainder of the pregnancy after the pregnancy has been declared. The rate of fetal exposure must not exceed 0.06 mSv in any two week period in the remainder of the pregnancy. In practical terms this means that pregnant women who wish to carry out the normal work of a nuclear medicine technologist may generally do so.

The "Ten Day Rule"

It has been advocated that elective nuclear medicine procedures should be carried out only during the first ten days following the first day of menstruation in women of reproductive capacity because of the relative certainty that the patient is not pregnant during that time. This "Ten Day Rule" is theoretically sound but is not enforced in many nuclear medicine departments for diagnostic procedures on grounds of the administrative difficulty of its implementation and with the knowledge of the relative radiation insensitivity of the pre-implantation and pre-organogenesis embryo and the unlikely event of congenital defects in liveborn survivors. All female patients of childbearing potential referred for radionuclide therapy should have a pregnancy test performed to prevent inadvertent exposure to the embryo/fetus.

Breast feeding

Breast feeding by a woman who has recently received a radiopharmaceutical for therapeutic or diagnostic purposes may pose a risk to the child because of the proximity of the woman to the child during the time of breast feeding and/or because of the excretion of certain radiopharmaceuticals or their labelled breakdown products in the breast milk. In these circumstances an elective procedure may be cancelled or postponed. Alternatively complete cessation or temporary interruption of breast feeding may be advised (Table 6). A major consideration is the woman's desire to continue breast feeding and the age of the infant in question. With temporary interruption of breast feeding it should be borne in mind that the woman may be able to express and "bank" her milk prior to the procedure so that it may be used to bottle feed the infant during the interruption. It is also likely that the woman will need to express and discard her milk during the interruption. In practice the interruption of breast feeding for more than a few days is likely to become its complete cessation. The objective is to reduce the maximum radiation dose to the infant to under 1.0 mSv.

Safe Handling of Radiopharmaceuticals

Radioactive materials must be stored in a secure, shielded location that has appropriate warning signs and that is appropriate to the physical and chemical nature of the material. For example, solutions of potentially volatile radioiodine should be stored in a fume hood directly vented to the exterior. Materials awaiting disposal should be clearly marked and securely stored while awaiting decay to a safe activity level.

Always wear surgical gloves when handling radioactive materials. Never eat, drink or smoke in a place where radioactive materials are stored or handled. Never mouth pipette. Whenever possible manipulate radioactive material behind a barrier which shields the trunk. Use metal syringe shields with heavy lead glass for all injections of radiopharmaceuticals Ensure that face masks and mouth pieces are tightly fitting when administering radioaerosols and gases. Remember that the injected patient is in effect an unshielded container and as such the greatest source of technologist radiation exposure. Therefore minimize close contact with the patient to what is clinically and technically required. Know how to clean up radioactive spills including those of bodily fluids of patients who have received radiopharmaceuticals.

Disposal of radioactive materials may be by venting to the outside air, to the sewer or as solid garbage. In each case measurements must be made to ensure that

Table 6. Breast feeding restrictions following administration of common radioactive nuclear medicine materials

Material

Breast feeding Restriction

99mTc-labelled agents for renal scans no interruption

(MAG3, DMSA, glucoheptonate, DTPA), bone scans (MDP, PYP), cardiac perfusion scans (sestamibi), red blood cells, liver scans (colloids, biliary materials such as disofenin, mebrofenin);

123I-sodium iodide; 111In-leukocytes

99mTc-labelled agents for lung scans (MAA)

interruption for 12 hours

99mTc-pertechnetate, leukocytes

67Ga-gallium citrate, 201Tl-thaNium chloride interruption for 24 hours interruption for at least one week

131I-sodium iodide administered diagnostically or therapeutically complete cessation quantities and concentrations of the particular radionuclides permitted by regulatory authorities are not exceeded. Such responsibility is not avoided when a commercial waste disposal organization is employed.

Frequently Asked Questions (FAQs)

Is radiation exposure dangerous to nuclear medicine technologists and other professionals?

Figures from the Canadian National Dose Registry based on readings from thermoluminescent dosimeter badges show that a nuclear medicine technologist may expect to receive approximately 1.5 mSv annually as a result of occupational exposure. Patients injected with 99mTc-based imaging radiopharmaceuticals are the major source of this exposure. By comparison the radiation dose received by diagnostic radiology technologists is approximately one-tenth of that received by their nuclear medicine counterparts. However neither group is exposed to significant risk as compared with the numerous stochastic and non-stochastic risks cheerfully accepted as part of everyday living.

Radiation doses for nuclear medicine physicians are no greater than those of other radiologists and are significantly smaller than those received by technologists because of the limited exposure to patients.

What do you do if a woman mistakenly undergoes a nuclear medicine procedure at a time when she is at an early stage of pregnancy?

Occasionally, despite all precautions, it will be discovered that a procedure has been performed on a woman who is subsequently found to have been pregnant at the time. The question of performing a therapeutic abortion may then be raised. In such cases as much factual information as possible must be supplied as quickly as possible to what will inevitably be an anxious situation. This will include calculation of the radiation dose likely to have been received by the products of conception (taking a worst case scenario), consideration of the stage of the pregnancy and the threshold considerations for deterministic effects as discussed above. These must be seen in the context of the rates of spontaneous occurrence of the effects it is feared that radiation may induce and all the other risks of pregnancy. Consideration should also be given to the opportunities for detection of fetal abnormalities by ultrasound and amniocentesis. Rarely if ever will the additional risk to an unborn child resulting from in utero irradiation alone justify interruption of the pregnancy. The nuclear medicine physician can be a helpful resource to the woman, her family and her obstetrician/gynecologist by dispassionately presenting them with the facts and the risks.

Why is explaining radiation risk so difficult?

Nuclear medicine professionals may sometimes think that radiation risks are somehow unfairly magnified in comparison with all the other threats to continuing health and happiness. They may wonder why they seem to be so bad at explaining this. The following factors may have something to do with it.

1. The Hiroshima and Nagasaki legacy. The atomic bombing of these Japanese cities and their largely civilian populations was such a dramatic and horrifying event that the general perception of the scale of their long term radiation consequences is greatly magnified. Consequently the risks of radiation generally are exaggerated.. Visitors to the Radiation Effects Research Foundation official web page may be surprised by the figures for excess cancers and leukemias among atomic bomb survivors with average exposure of 200 mSv The 428 excess cancer deaths attributable to radiation in the survivor population studied represented 8.8% of the total cancer deaths observed in 1950-1990.

2. The difficulty in explaining the small incremental risk of death from cancer seen against a several thousand fold greater pre-existing risk.

3. Obsessional concentration on the feared (anticipated) event rather than the remote odds of it happening (lottery syndrome).

4. The ALARA principle and linear no threshold hypothesis preclude a 'safe" radiation dose making it difficult to reassure an anxious patient

5. The difficulty in explaining stochastic risk.

6. Dependence on thermoluminescent dosimetry which provides only delayed reassurance rather than using a direct reading device whenever concern over radiation exposure is expressed.

7. Irresponsible colleagues who either treat radiation in a cavalier fashion describing the radiation dose received from a rest/stress myocardial perfusion scan as

"about equivalent to a chest X-ray" (whereas it is equivalent to approximately 250 chest X-rays) or who by their actions exaggerate radiation dangers.

8. The extreme sensitivity with which radioactive emissions can be detected.

9. The long delay (latency) in the presentation of radiation-induced cancer which indicates a lurking danger still present 40 years after a bone scan.

Additional Reading

http://www.radscice.com/dowd.html/

An Extensive Listing of Radiation protection Resources on the Internet

Hall EJ. Radiobiology for the Radiologist. 5th ed. JP Lippincott Company, 2000.

A perennial classic that takes the reader from the basic principles of radiobiology to practical aspects of radiation protection and carcinogenesis. ICRP Publication 52, "Protection of the patient in nuclear medicine". 1987. ICRP Publication 53, "Radiation dose to patients from radiopharmaceuticals". 1987. ICRP Publication 80, "Radiation doses to patients from radiopharmaceuticals: Addendum

2 to ICRP Publication 53". 1998. Selected monographs from the International Commission on Radiological Protection (ICRP) listings at http://www.icrp.org/.

Chapter 3

New Mothers Guide to Breast Feeding

New Mothers Guide to Breast Feeding

For many years, scientists have been playing out the ingredients that make breast milk the perfect food for babies. They've discovered to day over 200 close compounds to fight infection, help the immune system mature, aid in digestion, and support brain growth - nature made properties that science simply cannot copy. The important long term benefits of breast feeding include reduced risk of asthma, allergies, obesity, and some forms of childhood cancer. The more that scientists continue to learn, the better breast milk looks.

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