Applications and Safety
1. Medical Applications
Learning outcomes
- I can describe medical uses of radioactive isotopes.
- I can explain how radiation is used for medical imaging.
- I can explain how radiation is used to treat cancer.
- I can compare diagnostic and therapeutic uses of radiation.
- I can describe the benefits of radioactive medicine.
Medical Applications of Radioactive Isotopes
Radioactive isotopes have important uses in modern medicine. They can help doctors diagnose disease, investigate how organs are functioning, and treat certain medical conditions, including cancer.
Medical uses of radiation can be divided broadly into two categories:
- diagnostic uses — obtaining information about structures or processes inside the body
- therapeutic uses — using radiation to damage or destroy abnormal cells
Radioactive isotopes used in medicine are often called radioisotopes or, when incorporated into a medically useful compound, radiopharmaceuticals.
Why Radioactive Isotopes Are Useful in Medicine
Radiation can travel through body tissues and can be detected or can deposit energy in cells.
This makes radioisotopes useful because they can be used to:
- trace substances through the body
- examine organ function
- identify abnormal tissue
- detect some cancers
- measure biological processes
- destroy selected diseased cells
The radioisotope chosen depends on its:
- radiation type
- radiation energy
- half-life
- chemical behaviour
- biological destination in the body
Radiopharmaceuticals
A radiopharmaceutical is a radioactive substance prepared for medical use.
It often combines:
radioactive isotope + biologically active molecule
The molecule helps carry the radioactive isotope to a particular organ or tissue.
For example, a radiopharmaceutical might accumulate preferentially in:
- bones
- thyroid tissue
- heart muscle
- kidneys
- certain tumors
The radiation can then be detected for imaging or used to treat the targeted tissue.
Radioactive Tracers
A radioactive tracer is a radioactive substance introduced into the body so that its movement or distribution can be followed.
A tracer may be:
- injected
- swallowed
- inhaled
depending on the investigation.
The tracer moves through the body according to its chemical and biological properties.
A detector outside the body can then detect radiation emitted by the tracer.
Why Tracers Are Useful
Many medical tests need to determine not only what an organ looks like, but also how it is functioning.
For example, a tracer can help investigate:
- blood flow
- kidney function
- thyroid activity
- heart function
- bone metabolism
- glucose use in tissues
This makes nuclear medicine particularly useful for studying physiology and metabolism.
Diagnostic Nuclear Medicine
In diagnostic nuclear medicine, a small amount of a radiopharmaceutical is introduced into the patient.
The process is generally:
radiopharmaceutical administered → tracer accumulates or moves through body → radiation leaves body → detector records radiation → computer constructs image
The goal is to obtain useful information while keeping the radiation dose appropriately low.
Gamma Radiation for Imaging
Gamma radiation is particularly useful for many diagnostic applications because gamma photons:
- can pass through body tissue
- can be detected outside the body
- do not have charge
- can provide information about where the radioactive tracer is located
A detector can determine where gamma photons are coming from and create an image showing tracer distribution.
The Gamma Camera
A gamma camera detects gamma radiation emitted from a radiopharmaceutical inside the patient's body.
A simplified sequence is:
- A radiopharmaceutical is administered.
- It travels to or accumulates in the target tissue.
- The radioisotope emits gamma radiation.
- Gamma photons leave the patient's body.
- The gamma camera detects the photons.
- A computer processes the signals.
- An image showing tracer distribution is produced.
Scintigraphy
Imaging using a gamma camera is often called:
scintigraphy
The resulting image can reveal areas where tracer uptake is:
- normal
- unusually high
- unusually low
These patterns can provide information about organ function or abnormal tissue.
Technetium-99m
One of the most widely used radioisotopes in diagnostic nuclear medicine is:
technetium-99m (⁹⁹ᵐTc)
The "m" means the nucleus is in a:
metastable excited state
Technetium-99m emits useful gamma radiation and has a half-life of approximately:
6 hours
Why Technetium-99m Is Useful
Technetium-99m has several useful characteristics.
It:
- emits gamma radiation suitable for detection
- has a relatively short half-life
- can be incorporated into different radiopharmaceuticals
- can be used to investigate different organs
- generally disappears from the patient's body relatively quickly through radioactive decay and biological elimination
Its half-life is long enough to perform many examinations but short enough to help limit prolonged radiation exposure.
Half-Life and Medical Tracers
Choosing an appropriate half-life is important.
If the half-life is too short:
- the isotope may decay before the examination is completed
If it is too long:
- the patient may remain radioactive unnecessarily long
An appropriate medical tracer should remain active long enough to complete the procedure but not much longer than necessary.
Imaging the Skeleton
Radioactive tracers can be used in bone scans.
Certain technetium-99m radiopharmaceuticals accumulate in areas of active bone metabolism.
A bone scan can help doctors investigate conditions such as:
- bone metastases
- some fractures
- infections
- areas of abnormal bone activity
Regions of increased tracer accumulation may appear as areas of unusually high uptake.
Hot Spots and Cold Spots
Nuclear medicine images often show differences in tracer uptake.
A hot spot is an area where more tracer accumulates than expected.
A cold spot is an area where less tracer accumulates than expected.
These patterns do not automatically identify a specific disease.
Instead, doctors interpret them together with:
- symptoms
- medical history
- other imaging
- laboratory tests
SPECT Imaging
A gamma camera can also be used to produce three-dimensional information.
One technique is:
SPECT
which stands for:
Single Photon Emission Computed Tomography
During a SPECT scan, detectors collect radiation from different directions around the patient.
A computer reconstructs these measurements into cross-sectional or 3D images.
PET Imaging
Another important nuclear medicine technique is:
PET
which stands for:
Positron Emission Tomography
PET uses radioisotopes that undergo:
beta-plus decay
During beta-plus decay, a:
positron
is emitted.
Positron Annihilation
After travelling a short distance through tissue, the positron interacts with an electron.
The positron and electron undergo:
annihilation
Their mass is converted mainly into electromagnetic radiation.
Two gamma photons are produced, travelling in approximately opposite directions.
Each photon has an energy of:
511 keV
PET scanners detect these pairs of photons.
How PET Produces an Image
A simplified PET process is:
- A positron-emitting radiopharmaceutical is administered.
- The tracer moves through the body.
- Positrons are emitted.
- Positrons encounter electrons.
- Annihilation produces pairs of gamma photons.
- Detectors surrounding the patient detect photon pairs.
- A computer reconstructs the locations of the events.
- A 3D image of tracer distribution is produced.
Fluorine-18
A commonly used PET radioisotope is:
fluorine-18 (¹⁸F)
It has a half-life of approximately:
110 minutes
Fluorine-18 can be incorporated into compounds such as:
fluorodeoxyglucose (FDG)
FDG behaves in some ways similarly to glucose.
PET and Metabolism
Cells require glucose for energy.
Some tissues or tumors may have unusually high glucose metabolism.
An FDG PET scan can therefore reveal areas with increased tracer uptake.
PET imaging can be useful in areas including:
- oncology
- cardiology
- neurology
PET often provides information about function and metabolism rather than simply anatomy.
PET/CT
PET is often combined with:
CT — computed tomography
PET provides information about:
metabolism and function
CT provides detailed information about:
anatomical structure
Combining the two allows doctors to relate areas of unusual tracer activity to precise locations within the body.
Diagnostic Imaging vs Ordinary X-Rays
In ordinary X-ray imaging:
radiation source → outside the patient
X-rays pass through the body and are detected.
In nuclear medicine:
radioactive source → inside the patient
The radiopharmaceutical emits radiation that is detected outside the body.
This is an important distinction.
Diagnostic Uses of Radiation
The main purpose of diagnostic nuclear medicine is:
to obtain information
Diagnostic procedures usually aim to use the smallest radioactive dose that provides adequate information.
Examples include:
- bone scans
- thyroid scans
- kidney scans
- cardiac imaging
- PET imaging
- SPECT imaging
Radiation Therapy
Radiation can also be used to treat disease.
Radiation therapy uses ionizing radiation to damage cells.
Cancer cells divide uncontrollably.
Radiation can damage important cellular molecules, particularly:
DNA
If enough damage occurs, a cell may:
- stop dividing
- lose reproductive ability
- undergo cell death
This can reduce or destroy a tumor.
Why Radiation Can Treat Cancer
Ionizing radiation transfers energy to matter.
Inside biological tissue, this can:
- ionize molecules
- create reactive chemical species
- damage DNA
- damage chromosomes
- interfere with cell division
Cancer cells often divide rapidly and may be particularly vulnerable to radiation damage.
However, healthy cells can also be damaged.
Therefore, treatment must be carefully planned.
External Beam Radiotherapy
One major type of cancer treatment is:
external beam radiotherapy
A machine directs high-energy radiation toward the tumor from outside the body.
Modern systems commonly use high-energy X-rays, electrons, or other radiation beams rather than relying on radioactive material inside the machine for every treatment type.
The aim is:
maximum useful dose to tumor + minimum unnecessary dose to healthy tissue
Linear Accelerators
Many hospitals use a:
linear accelerator (LINAC)
A LINAC accelerates electrons.
These electrons can:
- be used directly for some treatments
- produce high-energy X-rays for deeper tumors
The radiation beam can be shaped and directed very precisely.
Treatment from Several Directions
Radiotherapy beams may be directed at a tumor from several angles.
Each individual beam passes through some healthy tissue.
However, the beams are arranged so that they overlap strongly at the:
tumor
This allows the tumor to receive a larger total dose while reducing the dose delivered to any one region of surrounding healthy tissue.
Fractionation
A total radiation treatment is often divided into multiple smaller treatments called:
fractions
For example, instead of delivering the entire dose at once, treatment may be spread over several sessions.
Fractionation can allow:
- healthy cells time to repair some damage
- repeated damage to tumor cells
- improved control of side effects
The exact treatment schedule depends on the cancer and treatment plan.
Brachytherapy
Another form of radiation treatment is:
brachytherapy
In brachytherapy, a sealed radioactive source is placed:
inside or very close to the tumor
This allows a high radiation dose to be delivered locally.
Because radiation intensity decreases strongly with distance, tissues farther from the source receive less radiation.
Brachytherapy Applications
Brachytherapy can be used for certain cancers, including some:
- prostate cancers
- cervical cancers
- uterine cancers
- breast cancers
- other localized tumors
Depending on the procedure, radioactive sources may be:
- temporarily inserted
- permanently implanted
Radioactive Isotopes in Therapy
Some treatments use radioactive isotopes that are taken into the body and preferentially accumulate in particular tissues.
This is called:
radionuclide therapy or systemic radionuclide therapy.
The radioactive material delivers radiation from within the body.
Iodine-131 and the Thyroid
An important example is:
iodine-131 (¹³¹I)
The thyroid naturally absorbs iodine because iodine is required to make thyroid hormones.
Doctors can therefore administer radioactive iodine.
The thyroid takes up the iodine-131.
Radiation is then delivered directly to thyroid tissue.
This can be used in treatment of conditions including:
- hyperthyroidism
- certain thyroid cancers
Why Iodine-131 Is Effective
Iodine-131 undergoes beta-minus decay and also emits gamma radiation.
The beta radiation deposits energy over a relatively short distance and can damage thyroid cells.
The radioactive iodine becomes concentrated in thyroid tissue because of the thyroid's normal biological function.
This is an example of:
targeted radionuclide therapy
Targeted Radionuclide Therapy
Modern nuclear medicine can attach a radioactive isotope to a molecule that preferentially travels to particular cells or receptors.
The molecule acts like a:
delivery system
The radioisotope acts as the:
radiation source
This can allow radiation to be concentrated more strongly in diseased tissue.
Diagnostic vs Therapeutic Radiation
The fundamental goals are different.
| Diagnostic Use | Therapeutic Use |
|---|---|
| Obtain information | Treat disease |
| Usually relatively small administered activity/dose | Sufficient dose to damage target cells |
| Radiation must be detectable | Radiation must deposit useful energy |
| Imaging is produced | Tissue is intentionally damaged |
| Examples: PET, SPECT, gamma imaging | Examples: radiotherapy, brachytherapy, radionuclide therapy |
Choosing Radiation for Diagnosis
For diagnostic purposes, doctors generally want radiation that:
- can escape from the body
- can be detected externally
- provides useful image information
- gives an appropriately low patient dose
- comes from an isotope with a suitable half-life
Gamma photons are therefore useful in many diagnostic procedures.
PET indirectly uses gamma photons produced following positron annihilation.
Choosing Radiation for Therapy
For treatment, the aim is different.
Radiation should:
- transfer sufficient energy to the target
- damage abnormal cells
- limit unnecessary exposure to healthy tissue
Beta particles, alpha particles, X-rays and gamma rays can all have therapeutic roles in different situations.
The best choice depends on:
- tumor location
- tumor size
- radiation penetration
- radiation energy
- biological targeting
- surrounding healthy tissue
Penetrating Power Matters
Different radiation types travel different distances through tissue.
Alpha
Very short range but highly ionizing.
This can be useful when alpha-emitting isotopes can be delivered extremely close to target cells.
Beta
Moderate penetration.
Useful in some radionuclide treatments because energy can be deposited within nearby tissue.
Gamma
Highly penetrating.
Useful for detection outside the body and in some treatment applications.
Half-Life Matters
Medical radioisotopes should have an appropriate half-life.
For diagnostic tracers, the isotope should ideally:
- remain active long enough for imaging
- decay reasonably quickly afterward
For therapy, it should:
- remain active long enough to deliver the intended dose
- avoid unnecessary prolonged exposure
Therefore, choosing an isotope involves balancing:
half-life + radiation type + radiation energy + biological behaviour
Physical and Biological Half-Life
In medicine, the amount of radioactive material in the body can decrease for two reasons.
Physical decay
The radioactive nuclei decay according to their physical half-life.
Biological elimination
The body may remove the substance through processes such as:
- urination
- metabolism
- exhalation
- excretion
Therefore, radioactive material may disappear from the body faster than radioactive decay alone would predict.
Effective Half-Life
The combined effect of physical decay and biological removal is described by the:
effective half-life
If both processes remove radioactive material, the effective half-life is shorter than either the physical or biological half-life alone.
This is important when estimating how long a radiopharmaceutical remains in the body.
Benefits of Radioactive Medicine
Radioactive medicine provides several important benefits.
It can allow doctors to:
- investigate organ function
- detect disease at an early stage
- locate abnormal tissue
- monitor treatment response
- obtain metabolic information
- target certain tumors
- treat some cancers without conventional surgery
- deliver radiation selectively to specific tissues
Functional Imaging
One of the greatest strengths of nuclear medicine is that it can show:
what tissue is doing
rather than only:
what tissue looks like
For example, nuclear imaging may reveal:
- glucose metabolism
- blood flow
- bone activity
- kidney filtration
- thyroid activity
before major structural changes are visible with some other imaging techniques.
Benefits of Targeted Treatment
Targeted radionuclide therapy can exploit biological differences between healthy and diseased tissue.
If a radioactive compound preferentially accumulates in a tumor, radiation can be delivered more directly to that tumor.
This can reduce exposure to some surrounding tissues compared with less targeted approaches, although healthy tissues can still receive radiation.
Risks and Limitations
Ionizing radiation can damage healthy cells as well as diseased cells.
Potential risks depend on:
- radiation dose
- radiation type
- exposed tissue
- exposure duration
- patient characteristics
Medical professionals therefore weigh:
expected benefit against radiation risk
before using radioactive materials.
Radiation Dose
Activity and radiation dose are different quantities.
Activity
Measures:
decays per second
Unit:
Bq
Absorbed dose
Measures energy deposited per kilogram of material.
Unit:
gray (Gy)
1 Gy = 1 J/kg
Equivalent/effective dose
Accounts for biological effects of radiation.
Common unit:
sievert (Sv)
Therefore:
Bq, Gy and Sv measure different things.
Activity Does Not Equal Dose
A source with an activity of:
1 MBq
does not automatically give a patient:
1 Gy
or:
1 Sv.
Dose depends on factors such as:
- radiation energy
- radiation type
- time
- location in the body
- biological distribution
- how much radiation is absorbed
This distinction is important in medical radiation physics.
Radiation Protection
Medical staff follow radiation protection principles to minimize unnecessary exposure.
Three important ideas are:
time
Reduce unnecessary exposure time.
distance
Increase distance from radiation sources when appropriate.
shielding
Use suitable materials to absorb radiation.
These principles are especially important when handling radioactive sources or radiopharmaceuticals.
ALARA
A common radiation protection principle is:
ALARA
meaning:
As Low As Reasonably Achievable
The goal is not necessarily to eliminate all radiation exposure, because the radiation may provide an important medical benefit.
Instead, unnecessary exposure should be minimized while still achieving the medical purpose.
Diagnostic Example
A patient receives a technetium-99m radiopharmaceutical for a bone scan.
The tracer accumulates in bone.
Gamma photons leave the body.
A gamma camera detects the radiation.
The computer produces an image showing tracer distribution.
The purpose is:
diagnosis
The radiation is primarily being used to obtain information.
Therapeutic Example
A patient receives iodine-131 to treat thyroid disease.
The thyroid absorbs the radioactive iodine.
Beta radiation deposits energy in thyroid tissue.
The radiation damages selected thyroid cells.
The purpose is:
treatment
The radiation is being used intentionally to damage tissue.
Comparing the Two Examples
Technetium-99m bone scan
Goal:
detect and investigate
Radiation role:
provide information
Iodine-131 therapy
Goal:
destroy or reduce abnormal tissue
Radiation role:
deposit damaging energy
This illustrates the central difference between:
diagnostic nuclear medicine
and:
therapeutic nuclear medicine
Worked Half-Life Example
Technetium-99m has a half-life of approximately:
6 hours.
Suppose a patient receives a radiopharmaceutical with an initial activity of:
800 MBq.
After 6 hours:
800 → 400 MBq
After 12 hours:
400 → 200 MBq
After 18 hours:
200 → 100 MBq
After 24 hours:
100 → 50 MBq
This calculation considers radioactive decay only; biological elimination can reduce the activity in the body further.
Worked Activity Example
A radioisotope has:
initial activity = 640 MBq
half-life = 4 hours
How much activity remains after:
12 hours?
Number of half-lives:
n = 12/4
n = 3
Activity:
640 → 320 → 160 → 80
Therefore:
80 MBq remains
before considering biological removal.
Why Short Half-Lives Can Be Beneficial
A relatively short half-life can reduce the length of time that a patient remains radioactive.
However, an isotope cannot have such a short half-life that it decays before:
- transportation
- preparation
- administration
- imaging or treatment
The ideal half-life depends on the medical procedure.
Medical Radioisotopes and Safety
Radioactive medicine involves controlled exposure to ionizing radiation.
Before a procedure, medical professionals consider whether the expected diagnostic or therapeutic benefit justifies the radiation exposure.
For diagnostic examinations, the aim is generally to obtain the required information using an appropriately optimized dose.
For therapy, a much larger dose may intentionally be delivered to diseased tissue.
Common Mistakes
Mistake: Saying all medical radiation comes from radioactive isotopes.
X-ray machines and medical linear accelerators can produce radiation electrically without using radioactive isotopes as the beam source.
Mistake: Saying radioactive tracers are only used to show anatomy.
Nuclear medicine is especially useful for investigating function and metabolism.
Mistake: Saying PET directly detects positrons outside the body.
PET scanners primarily detect the gamma photons produced when positrons annihilate with electrons.
Mistake: Saying diagnostic and therapeutic radiation have the same purpose.
Diagnosis obtains information; therapy intentionally damages diseased tissue.
Mistake: Saying gamma radiation is always used for treatment because it penetrates deeply.
Different treatments may use gamma rays, X-rays, beta particles, alpha particles or other radiation.
Mistake: Confusing activity and dose.
Activity is measured in Bq; absorbed dose is measured in Gy.
Mistake: Assuming a short half-life is always better.
The isotope must remain radioactive long enough to complete the medical procedure.
Key Terms
Nuclear medicine — medical field that uses radioactive materials for diagnosis and treatment.
Radioisotope — radioactive isotope of an element.
Radiopharmaceutical — radioactive substance prepared for medical diagnosis or treatment.
Radioactive tracer — radioactive substance used to follow a biological process or determine the distribution of material in the body.
Gamma camera — detector system used to produce images from gamma radiation emitted by radiopharmaceuticals inside the body.
Scintigraphy — imaging technique using gamma-emitting radiopharmaceuticals and a gamma camera.
SPECT — Single Photon Emission Computed Tomography; nuclear imaging technique that produces tomographic images.
PET — Positron Emission Tomography; imaging technique using positron-emitting radioisotopes.
Positron — antimatter counterpart of the electron with positive charge.
Annihilation — interaction in which a particle and its antiparticle are converted into other forms of energy, producing two 511 keV gamma photons in PET.
Technetium-99m — widely used gamma-emitting diagnostic radioisotope with a half-life of about 6 hours.
Fluorine-18 — positron-emitting isotope commonly used in PET imaging.
Radiotherapy — treatment using ionizing radiation to damage or destroy diseased cells.
Brachytherapy — treatment involving radioactive sources placed inside or close to a tumor.
Radionuclide therapy — treatment using radioactive substances administered to the patient.
Iodine-131 — radioactive isotope used particularly in treatment of certain thyroid conditions.
Absorbed dose — radiation energy absorbed per kilogram of matter, measured in gray (Gy).
Sievert (Sv) — unit used for quantities related to biological effects of ionizing radiation.
Effective half-life — time for radioactive material in the body to decrease by half due to both radioactive decay and biological removal.
Key Takeaways
- Radioactive isotopes can be used for both diagnosis and treatment.
- Medical radioactive compounds are often called radiopharmaceuticals.
- Radioactive tracers allow doctors to investigate processes inside the body.
- Diagnostic nuclear medicine often provides information about organ function and metabolism.
- Gamma radiation is useful for many imaging applications because it can escape the body and be detected externally.
- Gamma cameras are used to detect gamma-emitting radiopharmaceuticals.
- Technetium-99m is widely used for diagnostic imaging.
- Technetium-99m has a half-life of approximately 6 hours.
- PET uses positron-emitting isotopes.
- Positron-electron annihilation produces two 511 keV gamma photons.
- Fluorine-18 is commonly used in PET.
- PET can provide information about tissue metabolism.
- Radiation therapy uses ionizing radiation to damage diseased cells.
- Radiation can damage DNA and prevent cancer cells from reproducing.
- External beam radiotherapy directs radiation toward tumors from outside the body.
- Brachytherapy places radioactive sources inside or near a tumor.
- Iodine-131 can be used to treat certain thyroid diseases.
- Diagnostic procedures primarily use radiation to obtain information.
- Therapeutic procedures use radiation to damage or destroy target tissue.
- The choice of medical radioisotope depends on radiation type, energy, half-life and biological behaviour.
- Activity is measured in Bq, while absorbed radiation dose is measured in Gy.
- Medical use of radiation balances potential benefits against radiation risks.
Check Your Understanding
- What is a radiopharmaceutical?
- Explain how a radioactive tracer can be used to investigate the body.
- Why is gamma radiation useful for diagnostic nuclear imaging?
- Describe how a gamma camera is used.
- What is technetium-99m?
- Give two reasons why technetium-99m is useful for diagnostic imaging.
- Explain the difference between activity and detector count rate.
- What does PET stand for?
- What type of radioactive decay is used by PET radioisotopes?
- Explain what happens when a positron meets an electron.
- What energy does each annihilation photon used in PET have?
- Explain why PET can provide information about metabolism.
- What is the main purpose of diagnostic radiation?
- What is the main purpose of therapeutic radiation?
- Explain how ionizing radiation can damage cancer cells.
- What is brachytherapy?
- Explain why iodine-131 can be useful for treating thyroid disease.
- Compare diagnostic and therapeutic uses of radiation.
- Technetium-99m has a half-life of approximately 6 hours. A radiopharmaceutical initially has an activity of 640 MBq. Calculate the activity remaining after 18 hours, considering radioactive decay only.
- Explain two major benefits and two potential risks of using radioactive isotopes in medicine.