Physics

Key unit Competence

By the end of the unit the learner should be able to analyze the use of radiation in

medicine. 

My goals

• Explain radiation dosimetry.

• Differentiate the terms exposure, absorbed dose, quality factor (relative to

biological effectiveness) and dose equivalent as used in radiation dosimetry.

• Differentiate physical half-life, biological half-life and effective half-life

• Solve radiation dosimetry problems

• Analyse the basics of radiation therapy for cancer.

• Explain safety precautions when handling radiations

• Describe the concept of balanced risk.

Introductory activity

 Radiation has always been present and is all around us. Life has evolved in a world containing significant levels of ionizing radiation. Our bodies are adapted to it. People are constantly exposed to small amounts of ionizing radiation from the environment as they carry out their normal daily activities; this is known as background radiation. We are also exposed through some medical treatments and through activities involving radioactive material.

Fig 13.1 above identifies four major sources of public exposure to natural radiation: cosmic radiation, terrestrial radiation, inhalation and ingestion. Brainstorm and try to answer the following questions:

a. Distinguish artificial source of radiation and natural source of radiation?

b. Explain briefly each major source of public exposure to natural radiation

stated above.

c. Which kind of sources of radiation are mostly preferred to be used in

medicine? Explain why

d. Does exposure to heavy ions at the level that would occur during deepspace missions of long duration pose a risk to the integrity and function

of the central nervous system? Explain to support your idea.

13.1 RADIATION DOSE

13.1.1 Ionization and non-ionization radiations

Activity 13.1: Types of radiation

Radiation is the emission of particles or electromagnetic waves from a source.

Radiation from radioactive materials has the ability to interact with atoms and

molecules of living objects.

a. With the help of the diagram below, distinguish the forms of radiation?

b. Which type do you think is mostly used in medical treatment? Explain

your answer with supporting arguments?

c. Suggest the possible side effects of using radiations in medicine? Which

of the two forms of radiation induces more side effects when exposed to

human body? Explain to support your choice.

In a neutral atom, the positive charge of the nucleus is equal and opposite to the total negative charge of the orbital electrons. If such an atom loses or gains an electron, it becomes an ion. The atom will now have a net positive or negative charge and is called an ion. This process is called ionization, and the radiation responsible for it is called ionising radiation. When discussing the interaction of radiations with matter in particularly in relation to health, two basic types of radiation can be considered:

a. Ionizing radiation.

 Thisis a radiation that carries enough energy to liberate electrons from atoms or molecules, thereby ionizing them. As the more powerful form of radiation, ionizing radiation is more likely to damage tissue than non-ionizing radiation. The main source of exposure to ionizing radiation is the radiation used during medical exams such as X-ray radiography or computed tomography scans. However, the amounts of radiation used are so small that the risk of any damaging effects is minimal. Even when radiotherapy is used to treat cancer, the amount of ionizing radiation used is so carefully controlled that the risk of problems associated with exposure is tiny.

All forms of living things emit a certain amount of radiation, with humans, plants and animals accumulating radioisotopes as they ingest food, air and water. Some forms of radiation such as potassium-40 emit high-energy rays that can be detected using measurement systems. Together with the background radiation, these sources of internal radiation add to a person’s total radiation dose.

Background radiation is emitted from both naturally occurring and man-made sources. Natural sources include cosmic radiation, radon radiation in the body, solar radiation and external terrestrial radiation. Man-made forms of radiation are used in cancer treatment, nuclear facilities and nuclear weapons. Globally, the average exposure to ionizing radiation per year is around 3 milliSieverts (mSv), with the main sources being natural (around 80%). The remaining exposure is due to man-made forms such as those used in medical imaging techniques. Exposure to man-made forms of ionizing radiations is generally much higher in developed countries where the use of nuclear imaging techniques is much more common than in developing countries.

b. Non-ionizing radiations

 Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy to ionize atoms or molecules. Examples of non-ionizing radiations include visible light, microwaves, ultraviolet (UV) radiation, infrared radiation, radio waves, radar waves, mobile phone signals and wireless internet connections. Although UV has been classified as a non-ionizing radiation but it has been proven that high levels of UV-radiation can cause sunburn and increase the risk of skin cancer developing.

Scientific investigations suggest that the use of telecommunications devices such as mobile phones may be damaging, but no risk associated with the use of these devices has yet been identified in any scientific studies. This energy is emitted both inside the body and externally, through both natural and man-made processes.

13.1.2 Radiation penetration in body tissue

Activity13.2:

The figure below shows the penetrating power of radiation represented by A, B

and C. Use the figure to answer the following questions. 

Questions:

a. Interpret the figure and write the names of letters A, B and C labeled on

the figure above?

b. Which of the three types of radiation has high penetrating power?

Explain to support your idea.

c. Outline four uses of the man-made sources of radiation?

d. How does radiation affect me? Explain clearly with scientific reasoning.

An important characteristic of the various ionising radiations is how deeply they can penetrate the body tissues. X-rays, gamma rays, and neutrons of sufficient energy described below can reach all tissues of the body from an external source.

• Alpha Radiation  

Alpha radiation occurs when an atom undergoes radioactive decay, giving off an α- particle consisting of two protons and two neutrons (essentially the nucleus of a helium-4 atom) following the equation

Due to their charge and mass, alpha particles interact strongly with matter, and can only travel a few centimeters in air. A thin sheet of paper, on the other hand, stops alpha particles. They are also stopped by the superficial dead layer of skin that is only 70 µm thick. Therefore, radionuclides that emit only alpha particles are harmless unless you take them into the body. This you might do by inhalation (breathing in) or ingestion (eating and drinking).

• Beta Radiation

 Beta radiation takes the form of either an electron or a positron (a particle with the size and mass of an electron, but with a positive charge) being emitted from an atom. Due to their smaller mass, they are able to travel further in air, up to a few meters, and can be stopped by a thick piece of plastic, or even a stack of paper. Such radiation can penetrate the skin a few centimeters, posing somewhat of an external health risk. The depth to which beta particles can penetrate the body depends on their energy.

High-energy beta particles (several MeV) may penetrate a cm of a tissue, although most are absorbed in the first few mm. As a result, beta emitters outside the body are hazardous only to surface tissues such as the skin or the lenses of the eye. When you take beta emitters into the body, they will irradiate internal tissues and then become a much more serious hazard.

• Gamma Radiation

 Gamma radiation, unlike alpha or beta, does not consist of any particles, instead consisting of a photon of energy being emitted from an unstable nucleus. Having no mass or charge, gamma radiation can travel much farther through air than alpha or beta, losing (on average) half its energy. Gamma waves can be stopped by a thick or dense enough layer material, with high atomic number. Materials such as lead can be used as the most effective form of shielding.

• X-Rays 

X-rays are similar to gamma radiation, with the primary difference being that they originate from the electron cloud. This is generally caused by energy changes in an electron, such as moving from a higher energy level to a lower one, causing the excess energy to be released. X-Rays are longer-wavelength and (usually) lower energy than gamma radiation, as well.

• Neutron Radiation 

 Neutron radiation consists of a free neutron, usually emitted as a result of spontaneous or induced nuclear fission. They are able to travel hundreds or even thousands of meters in air, they are however able to be effectively stopped if blocked by a hydrogen material, such as concrete or water. Neutron radiation occurs when neutrons are ejected from the nucleus by nuclear fission and other processes. The nuclear chain reaction is an example of nuclear fission, where a neutron being ejected from one fission atom will cause another atom to fission, ejecting more neutrons. Unlike other radiations, neutron radiation is absorbed by materials with lots of hydrogen atoms, like paraffin wax and plastics.

13.1.3 Radiation dosimetry

Activity13.3

a. What does the term Dosimeter in radiation dosimetry mean?

b. Who Should Wear a Dosimeter? Suggest reasons why it is very important to wear a

dosimeter?

Just as for drugs, the effect of radiation depends on the amount a person has received. Therefore, amounts of radiation received are referred to as doses, and the measurement of such doses is known as dosimetry

Dosimeters are used to monitor your occupational dose from radioactive material or radiation-producing equipments. Most individuals working with X-ray producing equipment in the hospital will be issued with a dosimeter. For those individuals working in the research laboratory setting, dosimeters will be issued based on the nuclide and total activity that will be used. 

Dosimeters are integrating detectors; that is, they accumulate the radiation dose and give off an amount of light which is proportional to that dose. The energy absorption properties of dosimeters are designed to be very similar to tissue, so they are very effective as personnel dosimeters. These devices are used to measure exposures from x-ray, gamma ray and high energy beta particles.

Dosimeters are not suitable for measuring exposures to low energy beta particles or alpha particles.

13.1.4 Radiation exposure

Activity13.4:

a. What are the symptoms of radiation exposure?

b. Explain briefly the effects of radiation exposure to the human body?

c. It is possible that side effects can happen when a person undergoes

radiation treatment for cancer. Suggest the common side effects of

radiation exposure to the human body?

d. Does radiation exposure to the human body induce risks? Support your

decision with clear explanations.

Long-term exposure to small amounts of radiation can lead to gene mutations and increase the risk of cancer, while exposure to a large amount over a brief period can lead to radiation sickness.

13.1.5 Absorbed radiation dose

Activity13.5

a. What does the term absorbed dose mean in medical treatment?

b. In the application of radiation in medicine, we use the statement “A

measure of the risk of biological harm”. Brainstorm and explain clearly

what the statement means.

c. Explain why doses of alpha and gamma radiation produce unequal

biological effects?

What is important when we analyze the effect of radiation on human being is not so much the total dose to the whole system but the dose per kg. That’s why a doctor will prescribe smaller doses of medicine for children than for adults. A similar approach is used in radiation protection measurements, where the unit of absorbed dose is specified in terms of the amount of energy deposited by radiation in 1 kg of material. This unit is the Gray, abbreviated Gy.

It was named in honor of Louis Gray, who was a very big name in the early days of radiation dosimetry. An absorbed radiation dose of 1 Gray corresponds to the deposition of 1 joule of energy in 1 kg of material. The gray is a measure of energy absorbed by 1 kg of any material, be it air, water, tissue or whatever. A person who has absorbed a whole body dose of 1 Gy has absorbed one joule of energy in each kg of its body tissue.

As we shall see later, the gray is a fairly hefty dose, so for normal practical purposes we use the milligray (abbreviated mGy) and the microgray (abbreviated µGy). The gray is a physical unit. It describes the physical effect of the incident radiation (i.e., the amount of energy deposited per kg), but it tells us nothing about the biological consequences of such energy deposition in tissue. Studies have shown that alpha and neutron radiation cause greater biological damage for a given energy deposition per kg of tissue than gamma radiation does.

In other words, equal doses of, say, alpha and gamma radiation produce unequal biological effects. This is because the body can more easily repair damage from radiation that is spread over a large area than that which is concentrated in a small area. Because more biological damage is caused for the same physical dose.

13.1.6 Quality factors 

Quality factors are used to compare the biological effects from different types of radiation. For example, fast neutron radiation is considered to be 20 times as damaging as X-rays or gamma radiation. You can also think of fast neutron radiation as being of “higher quality”, since you need less absorbed dose to produce equivalent biological effects. This quality is expressed in terms of the Quality Factor (Q). The quality factor of a radiation type is defined as the ratio of the biological damage produced by the absorption of 1 Gy of that radiation to the biological damage produced by 1 Gy of X or gamma radiation. The Q of a certain type of radiation is related to the density of the ion tracks it leaves behind it in tissue; the closer together the ion pairs, the higher the Q.

13.1.7 Equivalent dose 

The absorbed radiation dose, when multiplied by the Q of the radiation delivering the dose, will give us a measure of the biological effect of the dose. This is known as the equivalent dose. The unit of equivalent dose H is the Sievert (Sv). An equivalent dose of one Sievert represents that quantity of radiation dose that is equivalent, in terms of specified biological damage, to one gray of X or gamma rays. In practice, we use the millisievert (mSv) and microsievert (µSv). The sievert is the unit that we use all the time, because it is the only one that is meaningful in terms of biological harm. In calculating the equivalent dose from several types of radiation (we call this “mixed radiation”), all measurements are converted to Sv, mSv or µSv and added.

Most of the radiation instruments we use to measure doses or dose rates read in mSv or µSv. Few other instruments can read in mGy or µGy, but they measure only gamma radiation.

13.1.8 Radiation protection

 The effects of radiation at high doses and dose rates are reasonably well documented. A very large dose delivered to the whole body over a short time will result in the death of the exposed person within days. We know from these that some of the health effects of exposure to radiation do not appear unless a certain quite large dose is absorbed. However, many other effects, especially cancers are readily detectable and occur more often in those with moderate doses. At lower doses and dose rates, there is a degree of recovery in cells and in tissues. Radiation protection sets examples for other safety disciplines in two unique respects:

• First, there is the assumption that any increased level of radiation above natural

background will carry some risk of harm to health.

• Second, it aims to protect future generations from activities conducted today.

The use of radiation and nuclear techniques in medicine, industry, agriculture, energy and other scientific and technological fields has brought tremendous benefits to society. The benefits in medicine for diagnosis and treatment in terms of human lives saved are large in size. No human activity or practice is totally devoid of associated risks. Radiation should be viewed from the perspective that the benefit from it to mankind is less harmful than from many other agents.

Quick check 13.2:

At what level is radiation harmful? Explain your idea

Note: The optimization of patients’ protection is based on a principle that the dose to the irradiated target (tumor) must be as high as it is necessary for effective treatment while protecting the healthy tissues to the maximum extent possible.

13.1.9 Checking my progress

1. Does receiving external-beam radiation make a person radioactive or able

to expose others to radiation? Explain to support idea

2. How can I be sure that the external-beam radiating machine isn’t damaging

normal, healthy tissue in my body? Explain clearly with scientific reasoning.

3. I am having an imaging test using radioactive materials. Will I be radioactive

after the test? Comment to support your decision.

4. All my radioactive material is secured properly and I have empty waste

containers in the lab. Do I have to lock the room? Explain clearly to justify

your decision.

13.2 BIOLOGICAL EFFECTS OF RADIATION EXPOSURE

13.2.1 Deterministic and stochastic effects

Activity 13.6:

Is the use of ionizing radiation in medicine beneficial to human health? Explain to

support your point.

1. Are there risks to the use of ionizing radiation in medicine? Explain your

answer.

2. How do we quantify the amount of radiation?

3. What do we know about the nature (mechanism) of radiation-induced

biological effects?

4. How are effects of radiation classified? 

Effects of radiations due to cell killing have a practical threshold dose below which the effect is not evident but in general when the effect is present its severity increases with the radiation dose. The threshold doses are not an absolute number and vary somewhat by individual. Effects due to mutations (such as cancer) have a probability of occurrence that increases with dose.

a. Deterministic effects: 

These effects are observed after large absorbed doses of radiation and are mainly a consequence of radiation induced cellular death. They occur only if a large proportion of cells in an irradiated tissue have been killed by radiation, and the loss can be compensated by increasing cellular proliferation.

b. Stochastic effects:

 They are associated with long term, low level (chronic) exposure to radiation. They have no apparent threshold. The risk from exposure increases with increasing dose, but the severity of the effect is independent of the dose. Irradiated and surviving cells may become modified by induced mutations (somatic, hereditary). These modifications may lead to two clinically significant effects: malignant neoplasm (cancer) and hereditary mutations.

The frequency or intensity of biological effects is dependent upon the total energy of radiation absorbed (in joules) per unit mass (in kg) of a sensitive tissues or organs. This quantity is called absorbed dose and is expressed in gray (Gy). In evaluating biological effects of radiation after partial exposure of the body further factors such as the varying sensitivity of different tissues and absorbed doses to different organs have to be taken into consideration.

To compare risks of partial and whole body irradiation at doses experienced in diagnostic radiology and nuclear medicine a quantity called equivalent or effective dose is used. A cancer caused by a small amount of radiation can be just as malignant as one caused by a high dose. 

Activity 13.7: 

1. What is magnitude of the risk for cancer and hereditary effects?

2. Is ionizing radiation from medical sources the only one to which people is

exposed?

3. What are typical doses from medical diagnostic procedures?

4. Can radiation doses in diagnosis be managed without affecting the

diagnostic benefit? Explain to support your decision.

5. Are there situations when diagnostic radiological investigations should be

avoided? Explain to support your decision. 

The lifetime value for the average person is roughly a 5% increase in fatal cancer after a whole body dose of 1 Sv. It appears that the risk in fetal life, in children and adolescents exceeds somewhat this average level (by a factor of 2 or 3) and in persons above the age of 60 it should be lower roughly by a factor of ~ 5.

Animal models and knowledge of human genetics, the risk of hereditary deleterious effects have been estimated to not be greater than 10% of the radiation induced carcinogenic risk. All living organisms on this planet, including humans, are exposed to radiation from natural sources. An average yearly effective dose from natural background amounts to about 2.5 mSv. This exposure varies substantially geographically (from 1.5 to several tens of mSv in limited geographical areas).

Various diagnostic radiology and nuclear medicine procedures cover a wide dose range based upon the procedure. Doses can be expressed either as absorbed dose to a single tissue or as effective dose to the entire body which facilitates comparison of doses to other radiation sources (such as natural background radiation. There are several ways to reduce the risks to very, very low levels while obtaining very beneficial health effects of radiological procedures. Quality assurance and quality control in diagnostic radiology and nuclear medicine play also a fundamental role in the provision of appropriate, sound radiological protection of the patient.

There are several ways that will minimize the risk without sacrificing the valuable information that can be obtained for patients’ benefit. Among the possible measures it is necessary to justify the examination before referring a patient to the radiologist or nuclear medicine physician. Failure to provide adequate clinical information at referral may result in a wrong procedure or technique being chosen by radiologist or nuclear medicine specialist. An investigation may be seen as a useful one if its outcome - positive or negative influences management of the patient. Another factor, which potentially adds to usefulness of the investigation, is strengthening confidence in the diagnosis. Irradiation for legal reasons and for purposes of insurance should be carefully limited or excluded.

Activity 13.8:

1. Are there special diagnostic procedures that should have special

justification? Explain to support your decision.

2. Do children and pregnant women require special consideration in

diagnostic procedures?

3. What can be done to reduce radiation risk during the performance of a

diagnostic procedure?

While all medical uses of radiation should be justified, it stands to reason that the higher the dose and risk of a procedure, the more the medical practitioner should consider whether there is a greater benefit to be obtained. Among these special position is occupied by computed tomography (CT), and particularly its most advanced variants like spiral or multi slice CT.

Both the fetus and children are thought to be more radiosensitive than adults. Diagnostic radiology and diagnostic nuclear medicine procedures (even in combination) are extremely unlikely to result in doses that cause malformations or a decrease in intellectual function. The main issue following in childhood exposure at typical diagnostic levels (<50 mGy) is cancer induction.

Medically indicated diagnostic studies remote from the fetus (e.g. radiographs of the chest or extremities, ventilation/perfusion lung scan) can be safely done at any time of pregnancy if the equipment is in proper working order. Commonly the risk of not making the diagnosis is greater than the radiation risk. If an examination is typically at the high end of the diagnostic dose range and the fetus is in or near the radiation beam or source, care should be taken to minimize the dose to the fetus while still making the diagnosis. This can be done by tailoring the examination and examining each radiograph as it is taken until the diagnosis is achieved and then terminating the procedure

For children, dose reduction in achieved by using technical factors specific for children and not using routine adult factors. In diagnostic radiology care should be taken to minimize the radiation beam to only the area of interest. Because children are small, in nuclear medicine the use of administered activity lower than that used for an adult will still result in acceptable images and reduced dose to the child. The most powerful tool for minimizing the risk is appropriate performance of the test and optimization of radiological protection of the patient. These are the responsibility of the radiologist or nuclear medicine physician and medical physicist.

The basic principle of patients’ protection in radiological X-ray investigations and nuclear medicine diagnostics is that necessary diagnostic information of clinically satisfactory quality should be obtained at the expense of a dose as low as reasonably achievable, taking into account social and financial factors.

13.2.2 Effects of radiation exposure

Quick check13.1:

Will small radiation doses hurt me? Some effects may occur immediately (days or months) while others might take tens of years or even get passed to the next generation. Effects of interest for the person being exposed to radiation are called somatic effects and effects of

I. Radiation Health 

Effects Ionizing radiation has sufficient energy to cause chemical changes in cells and damage them. Some cells may die or become abnormal, either temporarily or permanently. By damaging the genetic material (DNA) contained in the body’s cells, radiation can cause cancer.  Fortunately, our bodies are extremely efficient at repairing cell damage. The extent of the damage to the cells depends upon the amount and duration of the exposure, as well as the organs exposed.

Exposure to an amount of radiation all at once or from multiple exposures in a short period of time. In most cases, a large acute exposure to radiation causes both immediate ( radiation sickness) and delayed effects (cancer or death), can cause  sickness or even death  within hours or days.  Such acute exposures are extremely rare.

II. Chronic Exposure 

With chronic exposure, there is a delay between the exposure and the observed health effect. These effects can include cancer and other health outcomes such as benign tumors, cataracts, and potentially harmful genetic changes. Some radiation effects may occur immediately (days or months) while others might take years or even get passed to the next generation. Effects of interest for the person being exposed to radiation are called somatic effects and effects of interest that affect our children are called genetic effects.

Activity13.5: Low levels of radiation exposure

a. What is the safe level of radiation exposure? Explain your answer.

b. What is the annual radiation exposure limit? Explain your answer

Radiation risks refer to all excess cancers caused by radiation exposure (incidence risk) or only excess fatal cancers (mortality risk). Risk may be expressed as a percent, a fraction, or a decimal value.

For example, a 1% excess risk of cancer incidence is the same as a 1 in a hundred (1/100) risk or a risk of 0.01. However, it is very hard to tell whether a particular cancer was caused by very low doses of radiation or by something else.

While experts disagree over the exact definition and effects of “low dose”. Radiation protection standards are based on the premise that any radiation dose carries some risk, and that risk increases directly with dose.

Note:

• The risk of cancer from radiation also depends on age, sex, and factors such as

tobacco use.

• Doubling the dose doubles the risk. 

Acute health effects occur when large parts of the body are exposed to a large amount of radiation. The large exposure can occur all at once or from multiple exposures in a short period of time. Instances of acute effects from environmental sources are very rare.

13.2.3 Safety precautions for handling radiations

Activity 13.10: Safety precautions to be recognized when handling radiations

a. Who is involved in planning my radiation treatment?

b. How is the treatment plan checked to make sure it is best for me?

c. What procedures do I have in place so that the treatment team is able

to treat me safely?

d. How can I be assured that my treatment is being done correctly every

day?

e. What is the difference between a medical error and a side effect?

f. Outline the measures taken to reduce doses from external exposure

Shortening the time of exposure, increasing distance from a radiation source and shielding are the basic countermeasures (or protective measures) to reduce doses from external exposure.

Note: Time: The less time that people are exposed to a radiation source, the less the absorbed dose Distance: The farther away that people are from a radiation source, the less the absorbed dose.

Note: Shielding: Barriers of lead, concrete or water can stop radiation or reduce radiation intensity. There are four main factors that contribute to how much radiation a person absorbs from a source. The following factors can be controlled to minimize exposure to radiation:

I. The distance from the source of radiation

 The intensity of radiation falls sharply with greater distance, as per the inverse square law. Increasing the distance of an individual from the source of radiation can therefore reduce the dose of radiation they are exposed to. For example, such distance increases can be achieved simply by using forceps to make contact with a radioactive source, rather than the fingers.

II. Duration of exposure 

The time spent exposed to radiation should be limited as much as possible. The longer an individual is subjected to radiation, the larger the dose from the source will be. One example of how the time exposed to radiation and therefore radiation dose may be reduced is through improving training so that any operators who need to handle a radioactive source only do so for the minimum possible time.

III. Reducing incorporation into the human body

 Potassium iodide can be given orally immediately after exposure to radiation. This helps protect the thyroid from the effects of ingesting radioactive iodine if an accident occurs at a nuclear power plant. Taking Potassium iodide in such an event can reduce the risk of thyroid cancer developing.

IV. Shielding 

Shielding refers to the use of absorbent material to cover the source of radiation, so that less radiation is emitted in the environment where humans may be exposed to it. These biological shields vary in effectiveness, depending on the material’s crosssection for scattering and absorption. The thickness (shielding strength) of the material is measured in g/cm2 . Any amount of radiation that does penetrate the material falls exponentially with increasing thickness of the shield.

Some examples of the steps taken to minimize the effects of radiation exposure are

described below;

• The exposed individual is removed from the source of radiation.

• If radiation exposure has led to destruction of the bone marrow, the number of healthy white blood cells produced in the bone marrow will be depleted.

• If only part of the body has been exposed to radiation rather than the whole body, treatment may be easier because humans can withstand radiation exposure in large amounts to non-vital body parts.

In every medicine there is a little poison. If we use radiation safely, there are benefits and if we use radiation carelessly and high doses result, there are consequences. Ionizing radiation can change the structure of the cells, sometimes creating potentially harmful effects that are more likely to cause changes in tissue. These changes can interfere with cellular processes so cells might not be able to divide or they might divide too much.

Radioactive rays are penetrating and emit ionizing radiation in the form of electromagnetic waves or energetic particles and can therefore destroy living cells. Small doses of radiation over an extended period may cause cancer and eventually death. Strong doses can kill instantly. Marie Curie and Enrico Fermi died due to exposure to radiation.

Several precautions should be observed while handling radioisotopes. Some of

these are listed in the following:

• No radioactive substance should be handled with bare hands. Alpha and beta

emitters can be handled using thick gloves. Gamma ray emitters must be

handled only by remote control that is by mechanical means. Gamma rays are

the most dangerous and over exposure can lead to serious biological damage.

• Radioactive materials must be stored in thick lead containers.

• Reactor and laboratories dealing with and conducting experiments with

radioactive metals must be surrounded with thick concrete lined with lead.

• People working with radioactive isotopes must wear protective clothing

which is left in the laboratory. The workers must be checked regularly with

dosimeters, and appropriate measures should be taken in cases of overdose.

• Radioactive waste must be sealed and buried deep in the ground.

13.2.3 Checking my progress

1.

a. What does the term background radiation mean?

b. Hat is radiation – am I exposed to background radiation each day even

if I do not have an X-ray examination?

2. What are the risks associated with radiation from diagnostic X-ray imaging

and nuclear medicine procedures?

3. How do I decide whether the risks are outweighed by the benefits of

exposure to X-radiation when I have a radiology test or procedure?

4. Are there alternatives to procedures that involve ionizing radiation that

would answer my doctor’s question? Justify your answer with clear facts.

5. What kinds of safety checks do you perform each day?

6. How often does the medical physicist check the various machines involved

during my treatment are working properly?

7. If I have side effects after my treatment, who can I call?

a. My best friend

b. My primary care doctor

8. I have a question about a radiation treatment I had many years ago. Who

should I call?

13.3 CONCEPT OF BALANCED RISK.

13.3.1 Risks of ionizing radiation in medical treatment

Activity13.12: balanced risk

Brainstorm and write briefly how balance risks in medical treatment occur?

Risk in the area of radiation medicine has several dimensions that are less common in other areas of medicine. First, there may be risks from overexposure that do not cause immediate injury. For example, the causal connection, if any, may be difficult or impossible to verify for a malignancy that surfaces several years after an inappropriate exposure. Second, the risks associated with the medical use of ionizing radiation extend beyond the patient and can affect health care workers and the public.

In amplifying these and other aspects of the risks that attend medical uses of ionizing radiation, the discussion addresses the following issues: human error and unintended events; rates of misadministration in radiation medicine; inappropriate and unnecessary care; and efforts that reduce misadministration and inappropriate care.

13.3.2 Human Error and Unintended Events 

Errors occur throughout health care: A pharmacist fills a prescription with the wrong medicine; an x-ray technician takes a film of the wrong leg; a surgeon replaces the wrong hip. The advent of complex medical technology has increased the opportunity for error even as it has increased the opportunity for effecting cures.

By educating health care workers, and by circumscribing their actions, human error may be minimized. However, some number of mistakes will always, unavoidably, be made, and no amount of training or double-checking can erase that fact.

13.3.3 Comparison of risks in the use of ionizing radiation 

The comparison of relative risks of misadministration in by-product radiation medicine to error rates and events in other medical practice settings, as well as the comparison of disease and death rates with the risks of the therapeutic administration itself, help to some extent to place ionizing radiation use in a broader context.

To achieve this success requires the highest standards of performance (accuracy of delivered dose), both when planning irradiation for an individual patient and in actual delivery of the dose. In a large number of cases, decreasing the dose to the target volume is not possible since it would unacceptably decrease the cure rate. In these cases present technological developments aim at optimizing the patients’ protection, keeping the absorbed tumor dose as high as is necessary for effective treatment while protecting nearby healthy tissues.

It should be remembered that successful eradication of a malignant tumor by radiation therapy requires high-absorbed doses and there is a delayed (and usually low) risk of late complication. The above mention techniques are used to provide the best benefit/risk ratio.

A malignant tumor in a pregnant woman may require radiotherapy in attempt to save life of the patient. If a tumor is located in a distant part of the body, the therapy with individually tailored protection of the abdomen (screening) - may proceed.

When thyroid cancer with metastases is diagnosed in a pregnant woman, treatment with ¹³¹I is not compatible with continuation of the pregnancy. The treatment should then be delayed until delivery if doing so wouldn’t put the mother’s life in danger. Medical radiation can be delivered to the patient from a radiation source outside the patient. Regardless of how much dose the patient received, they do not become radioactive or emit radiation.

• Balancing risks are often summarized in the following:

• The demand for imaging, especially computed tomography, that has increased

vastly over the past 20 years

• An estimated 30% of computed tomography tests that may be unnecessary

• Ionizing radiation that may be associated with cancer.

• The risks of radiation exposure that is often overlooked and patients are

seldom made aware of these risks

• The requesting doctor who must balance the risks and benefits of any high

radiation dose imaging test, adhering to guideline recommendations if

possible

• Difficult cases that should be discussed with a radiologist, ideally at a clinic

radiological or multidisciplinary team meeting.

13.3.4 Checking my progress

1. When patients are intentionally exposed to ionizing radiation for medical

purposes, do they suffer unintentional exposures as a result of error or

accident? Comment to support your idea.

2. What can be done to reduce radiation risk during conduct of radiation

therapy?

3. Can pregnant women receive radiotherapy? Explain to support your decision.

4. Can patients’ treatment with radiation affect other people? Explain to

support your decision.

13.4 THE HALF-LIVES: PHYSICAL, BIOLOGICAL, AND EFFECTIVE

Activity 13.14:

Distinguish between physical half-life, biological half-life and effective half-life.

Brainstorm and write the distinction between physical half-life, biological half-life and effective half-life in your note books.

The half-life is a characteristic property of each radioactive species and is independent of its amount or condition. The effective half-life of a given isotope is the time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological elimination.

There are three half-lives that are important when considering the use of radioactive drugs for both diagnostic and therapeutic purposes. While both the physical and biological half-lives are important since they relate directly to the disappearance of radioactivity from the body by two separate pathways (radioactive decay, biological clearance), there is no half-life as important in humans as the effective half-life

The half-life takes into account not only elimination from the body but also radioactive decay. If there is ever a question about residual activity in the body, the calculation uses the effective half-life; in radiation dosimetry calculations, the only half-life that is included in the equation is the effective half-life.

13.4.1 Physical half Lives

The physical half-life is unaffected by anything that humans can do to the isotope. High or low pressure or high or low temperature has no effect on the decay rate of a radioisotope.

14.4.2 Biological half lives

 Biological Half-life is defined as the period of time required to reduce the amount of a drug in an organ or the body to exactly one half its original value due solely to biological elimination. It is typically designated T_b . There are a few things to note about the T_b :

• For radioactive compounds, we have to calculate the T_b because the mass of the isotope is usually on the nanogram scale and, when distributed throughout the body, and especially in the target organ, concentrations are in the pictogram/ml range, much too small to measure directly.

• For non-radioactive compounds, we can measure the Tb directly. For example,

assuming that a person is not allergic to penicillin, we could give 1 000 mg of

the drug and then measure the amount present in the blood pool and in the

urine since we administered such a large amount of the drug.

• T_b is affected by many external factors. Perhaps the two most important are

hepatic and renal function. If kidneys are not working well, we would expect

to see a high background activity on our scans.

• Each individual organ in the body has its own T_b and the whole body also has

a T_b representing the weighted average of the T_b of all internal organs and

the blood pool. It is therefore very important to have a frame of reference. For

example, do you need to know the T_b of the drug in the liver or in the whole

body?

• All drugs have aT_b , not just radioactive ones. Drug package inserts often refer

to the half-time of clearance of a drug from the blood pool or through the

kidneys.

• Since the whole body has a T_b representing the weighted average of the T_b of

all internal organs, it will almost never equal that of an internal organ. 

13.4.3 Effective half lives

 Effective half-life is defined as the period of time required to reduce the radioactivity level of an internal organ or of the whole body to exactly one half its original value due to both elimination and decay.

It is designated T_e can be measured directly. For example, one can hold a detection device 1 m from the patient’s chest and count the patient multiple times until the reading decreases to half of the initial reading.

The patient is permitted to use the rest room between readings as needed, so both elimination and decay are taking place. The half-life being measured in this case is the T_e andT_e is affected by the same external factors that affect T_b since T_e is dependent uponT_b .

13.4.4 Checking my progress

1. I-131 sodium iodide has a T_b of 24 days. What is Te if T_p = 8 d ?

2. A Tc-99 m compound has a T_e = 1 days. What isTb if T_p = 6 d

3. A radiopharmaceutical has a biological half-life of 4.00 hr and an effective

half-life of 3.075 hr. What isotope was used? 

13.5 END UNIT ASSESSMENT

13.5.1 Multiple choice

1. Which of the following would reduce the cell damage due to radiation for a

lab technician who works with radioactive isotopes in a hospital or lab?

a. Increase the worker’s distance from the radiation source.

b. Decrease the time the worker is exposed to the radiation.

c. Use shielding to reduce the amount of radiation that strikes the worker.

d. Have the worker wear a radiation badge when working with the

radioactive isotopes.

e. All of the above.

2. If the same dose of each type of radiation was provided over the same

amount of time, which type would be most harmful?

a. X-rays. c. γ rays.

b. α Rays. d. β particles.

3. Which of the following is true?

a. Any amount of radiation is harmful to living tissue.

b. Radiation is a natural part of the environment.

c. All forms of radiation will penetrate deep into living tissue.

d. None of the above is true.

4. Which radiation induces the most biological damage for a given amount of

energy deposited in tissue?

a. Alpha particules.

b. Gamma radiation.

c. C. Beta radiation.

d. D. All do the same damage for the same deposited energy.

5. Which would produce the most energy in a single reaction?

a. The fission reaction associated with uranium-235.

b. The fusion reaction of the Sun (two hydrogen nuclei fused to one

helium nucleus).

c. Both (A) and (B) are about the same.

d. Need more information.

6. The fuel necessary for fusion-produced energy could be derived from

a. Water.        d. Superconductors.

b. Uranium.    e. Helium.

c. Sunlight.

13.5.2 Structured questions

7. If the equipment isn’t working and my treatment is delayed or postponed,

who checks that it is safe to use again? And will this delay affect my cancer?

8. Do you have weekly chart rounds where you review patient-related

information in peer review?

9. Will you take imaging scans regularly during my treatment to verify position

of my treatment? Who reviews those scans?

10. People who work around metals that emit alpha particles are trained that

there is little danger from proximity or touching the material, but they must

take extreme precautions against ingesting it. Why? (Eating and drinking

while working are forbidden.)

11. What is the difference between absorbed dose and effective dose? What are

the SI units for each?

12. Radiation is sometimes used to sterilize medical supplies and even food.

Explain how it works.

13. How might radioactive tracers be used to find a leak in a pipe?

14. Explain that there are situations in which we may or may not have control

over our exposure to ionizing radiation.

a. When do we not have control over our exposure to radiation?

b. When do we have control over our exposure to radiation?

c. Why might we want to limit our exposure to radiation when possible?

15. Does exposure to heavy ions at the level that would occur during deepspace missions of long duration pose a risk to the integrity and function of

the central nervous system?

16. Radiation protection of ionizing radiation from radiation sources is

particularly difficult. Give a reason for this difficulty. 

13.5.3 Essay questions

17. I always lock my radioactive material-use rooms. However, renovators came

in during the weekend, worked, and left the door open while they were

on their lunch break. Am I responsible and how can I prevent this from

happening? Debate on the situation above to support your answer.

18. How can I ensure that personnel who work in my lab, but do not use

radioactive material, do not violate the security requirements? Debate to

support your idea.

19. 19. A Housekeeping staff member opens my radioactive material-use room

after working hours and does not lock it when they leave. What should I do?

Explain clearly to support your idea.

20. Make a research and predict what steps that can or might be taken to reduce

the exposure to radiation (consider if living near a radioactive area like an

abandoned uranium mine, if finding a radioactive source, or in the event of

a nuclear explosion or accident).