Radiation Biology Study Notes
Introduction to Radiation Biology
When x-rays were first discovered, people quickly learned about their potential dangers. The first x-ray burn was reported in 1895, and the first x-ray-induced skin cancer was reported in 1902.
Because it's hard to predict exactly what will happen from radiation exposure, we always assume that any amount of radiation could cause some harm.
Effects of Ionizing Radiation on Body Tissues
Mechanisms of Radiation Injury
Radiation can hurt our body tissues in two main ways:
Ionization: This happens when x-rays hit the cells in your body. When an x-ray hits an atom, it can knock off an electron, creating a charged pair called an "ion pair." This dislodged electron then bumps into other atoms, causing chemical changes and damaging the cells.
Free Radical Formation: This occurs when radiation hits water molecules in your body. This process creates unstable molecules called "free radicals." These free radicals have an extra, unpaired electron, which makes them very reactive and able to cause harm to your cells, leading to damage.
Concepts of Radiation Exposure
The way radiation affects us follows a "linear, non-threshold" rule. This means that even a very small amount of radiation, without any minimum starting point (no threshold), can potentially cause some biological damage to the body. The more radiation exposure, the more damage is likely to occur.
Theories of Radiation Injury
There are two main ideas about how radiation damages cells:
Direct Theory: This theory suggests that cell damage happens when radiation directly strikes important parts within a cell, like its DNA. However, this type of direct hit doesn't happen very often.
Indirect Theory: This theory states that damage occurs when x-ray energy is absorbed by the water inside our cells. This absorption leads to the creation of harmful substances, such as free radicals, which then indirectly damage the cell.
Outcomes of Radiation Exposure
After being exposed to radiation, a cell can have one of five different outcomes:
The cell may not be affected at all and remains unchanged.
The cell may get injured but then fully repair itself and go back to working normally.
The cell might die, but the body replaces it with a new cell through its natural processes.
The cell could be damaged, repair itself, but not work as well as it did before.
The cell might repair itself incorrectly, which can lead to abnormal changes like the development of a tumor or cancer.
Dose-Response Curve
A "dose-response curve" is a graph that shows how tissue damage relates to the amount of radiation received. This curve typically demonstrates a "linear, non-threshold" connection, meaning that the amount of damage is directly linked to the radiation dose, with no minimum safe amount. Even a very small exposure to radiation might cause some biological harm.
Stochastic and Nonstochastic Effects
Radiation effects are categorized into two types:
Stochastic Effects: These effects happen randomly and relate directly to the radiation dose, meaning there's no safe level or "threshold." Examples include developing cancer or genetic mutations that could be passed down. The likelihood of these effects increases with dose, but their severity doesn't.
Nonstochastic (Deterministic) Effects: These effects have a "threshold" dose, meaning they only appear once a certain amount of radiation has been received. The severity of these effects, such as hair loss, cataracts, or reduced fertility, increases with the dose.
Sequence of Radiation Injury
When a body tissue is injured by radiation, it goes through several stages:
Latent Period: This is the time between when you are exposed to radiation and when you actually see any signs or symptoms of injury. How long this period lasts depends on the total dose of radiation and how long the exposure occurred.
Period of Injury: During this time, the cells are actively experiencing damage, which might include problems like cells not being able to divide properly.
Recovery Period: If the radiation dose was not deadly, the damaged cells have a chance to repair themselves during this time.
Cumulative Effects: Any radiation damage that doesn't fully repair can build up over time. This means that repeated small exposures can lead to significant problems eventually.
Determining Factors for Radiation Injury
How badly someone is affected by radiation depends on several key factors:
Total Dose of Radiation: The overall amount of radiation received.
Dose Rate: Whether the radiation was received quickly or slowly over time. A rapid exposure generally causes more damage.
Amount of Tissue Irradiated: The size of the body area that was exposed to radiation.
Cell Sensitivity: Different types of cells in the body react differently to radiation; some are more easily damaged than others.
Age: Younger individuals, especially children, are typically more susceptible to radiation damage.
Radiation Effects
Radiation can cause two main types of effects:
Short-term Effects: These happen when a large amount of radiation is received over a short period. An example is Acute Radiation Syndrome (ARS), with symptoms like nausea, vomiting, diarrhea, and hair loss.
Long-term Effects: These effects appear much later, often from small doses of radiation received repeatedly over a long time. Examples include various types of cancers and birth abnormalities.
Classification of Biological Effects of Radiation
Radiation can affect two different types of cells in your body:
Somatic Cells: These are all the cells in your body except for your reproductive cells (sperm or egg cells). Any radiation damage to somatic cells is seen in the person who was exposed, such as skin burns or cancer.
Genetic Cells: These are your reproductive cells. Damage to these cells won't show up in the exposed person themselves, but it could be passed on to their future children or generations as genetic mutations.
Sensitivity of Cells to Radiation
Cells in the body have different levels of sensitivity to radiation:
"Radiosensitive" cells are easily harmed by radiation.
"Radioresistant" cells are less affected.
The sensitivity of a cell is determined by several factors:
Mitotic Activity: Cells that divide rapidly and frequently are more sensitive to radiation.
Cell Differentiation: Cells that are immature or highly specialized are generally more sensitive.
Cell Metabolism: Cells that have a higher metabolic rate (meaning they are very active) tend to be more sensitive to radiation damage.
Radiosensitive and Radioresistant Organs
Some organs are much more easily damaged by radiation than others:
Organs especially sensitive to radiation (Radiosensitive):
Tissues that produce immune cells (lymphoid tissue).
The inner part of bones that produces blood cells (bone marrow).
Male reproductive organs (testes).
The digestive tract (intestines).
Tissues that are more resistant to radiation (Radioresistant):
Glands that produce saliva (salivary glands).
The organs that filter blood (kidney).
The large organ that processes nutrients and toxins (liver).
Critical Organs
When getting dental x-rays, some specific organs are considered "critical" because damage to them could seriously impact a person's quality of life. These include:
The outer layer of the body (skin).
The gland in the neck that produces hormones (thyroid gland).
The transparent part of the eye that helps focus light (lens of the eye).
The tissue inside bones that produces blood cells (bone marrow).
Radiation Measurement Units
We use specific units to measure radiation:
Exposure: This measures the amount of radiation in the air.
The older unit is "Roentgen."
The modern (SI) unit is "Coulomb per kilogram" (extC/kgextC/kg).
Dose: This measures the amount of radiation absorbed by a substance or tissue.
The older unit is "Rad."
The modern (SI) unit is "Gray" (extGyextGy).
Dose Equivalent: This measures the biological effect of the absorbed radiation, considering different types of radiation.
The older unit is "Rem."
The modern (SI) unit is "Sievert" (extSvextSv).
Sources of Radiation Exposure
We are exposed to radiation from various sources:
Natural Background Radiation: This comes from our environment all the time.
Cosmic radiation comes from space, like from stars and the sun.
Terrestrial radiation comes from naturally radioactive materials found in the earth and the air around us.
In the U.S., the average person receives about 0.00150.0015 to 0.003extGy0.003extGy (which is 150150 to 300extmrads300extmrads) from natural sources each year.
Artificial Sources: These are human-made sources of radiation and include:
Everyday consumer products.
Radiation from atomic weapons fallout.
Medical procedures like nuclear medicine.
Dental x-rays.
Risk and Risk Estimates for Dental Radiography
When it comes to dental x-rays, the estimated risk of developing a fatal cancer from them is very low, about 33 in every 1,000,0001,000,000 people. To put this in perspective, the natural risk of spontaneously developing cancer (not from radiation) is much higher, around 3,3003,300 in every 1,000,0001,000,000 people.
Risk Comparisons
To understand the low risk of dental x-rays better, here are some everyday activities that carry a similar risk of 11 in 1,000,0001,000,000:
Riding a bicycle for 1010 miles.
Driving a car for 300300 miles.
Flying in an airplane for 1,0001,000 miles.
Smoking approximately 1.41.4 cigarettes daily.
Dental Radiation and Risks
Here are some specific radiation doses linked to certain effects, along with how dental x-rays compare:
Thyroid gland: It would take about 0.006extGy0.006extGy of radiation to potentially cause cancer in the thyroid gland. The average dose from a dental x-ray is much, much lower, around 0.00006extGy0.00006extGy.
Bone marrow: A dose of about 0.005extGy0.005extGy to the bone marrow is considered. You would need to take between 2,0002,000 to 5,0005,000 dental x-ray films to reach a dose that might lead to leukemia.
Skin erythema (reddening of the skin): A dose of 2.5extGy2.5extGy within 1414 days is needed to cause noticeable skin reddening. This would require more than 500500 typical dental x-ray films.
Cataracts (clouding of the eye lens): About 2extGy2extGy of radiation is required to cause cataracts. There is some discussion about whether the eye lens should be considered a "critical organ" for dental radiography, given how high this required dose is compared to typical dental exposures.
Minimizing Patient Exposure and Dose
To keep dental x-ray exposure as low as possible for patients, dentists use several methods:
Using faster film speeds: Newer film types, like F-speed film, significantly reduce the amount of radiation needed per x-ray. F-speed film can reduce the patient's radiation dose by 60 ext{%} to 70 ext{%} compared to older D-speed film.
Proper collimation and positioning: X-ray beams are precisely aimed and shaped (collimated) to only expose the necessary area, preventing radiation from spreading to other parts of the face or body. Correct positioning also helps to avoid unnecessary re-takes.
Higher kVp settings: Using a higher kilovoltage peak (kVp) setting on the x-ray machine allows for shorter exposure times, which means the patient receives less overall radiation.
Risk vs. Benefit Consideration
Dental x-rays should only be taken when the advantages of finding and diagnosing diseases are greater than the small biological risks involved. When performed correctly, dental x-rays are generally very beneficial for your health, far outweighing their minimal risks.
Introduction to Radiation Biology
Historical Context
The first x-ray burn was reported in 1895.
The first case of x-ray-induced skin cancer was reported in 1902.
Experts cannot always predict specific outcomes from radiation exposure, leading to the conservative assumption that any amount of radiation poses a risk.
Effects of Ionizing Radiation on Body Tissues
Mechanisms of Radiation Injury
Two Primary Mechanisms:
Ionization
Occurs when x-rays strike patient tissue, producing an ion pair (positive atom and a dislodged negative electron).
This electron interacts with other atoms causing chemical changes and biological damage.
Free Radical Formation
A neutral atom or molecule with a single unpaired electron in its outermost shell is formed during the ionization of water, leading to the production of hydrogen and hydroxyl free radicals, resulting in cell damage.
Concepts of Radiation Exposure
Linear, non-threshold dose-response relationship indicates that any amount of radiation causes some biological damage.
Theories of Radiation Injury
Direct Theory
Cell damage occurs when ionizing radiation directly hits critical areas within the cell (occurs infrequently).
Indirect Theory
Damage results when x-ray photons are absorbed by water, leading to the formation of toxins (i.e., free radicals) which harm the cell.
Outcomes of Radiation Exposure
Five Possible Outcomes:
No effect; cell unaffected.
Cell injured but it repairs itself and resumes normal function.
Cell dies but is replaced through normal processes.
Cell damaged, repairs itself, but functions at a reduced level.
Cell repairs itself incorrectly, leading to biophysical changes (e.g., tumor or malignancy).
Dose-Response Curve
Correlates tissue damage with radiation dose.
Describes a linear, non-threshold relationship where damage is directly proportional to radiation dose, implying that even minimal exposure may result in biological harm.
Stochastic and Nonstochastic Effects
Stochastic Effects
Effects that occur as a direct function of dose without a threshold (e.g., cancer, genetic mutations).
Nonstochastic (Deterministic) Effects
Occur depending on severity of dose and have a threshold (e.g., loss of hair, cataracts, decreased fertility).
Sequence of Radiation Injury
Key Periods:
Latent Period: Time between exposure and observable clinical signs, depends on the total dose and exposure duration.
Period of Injury: Resulting cellular injuries, e.g., lack of mitosis.
Recovery Period: Damage repair occurs if the dose is non-lethal.
Cumulative Effects: Unrepaired radiation damage accumulates over time.
Determining Factors for Radiation Injury
Factors influencing injury:
Total dose of radiation.
Dose rate (rapid vs. slow exposure).
Amount of tissue irradiated.
Cell sensitivity (some cells are more sensitive to radiation).
Age (children are more susceptible).
Radiation Effects
Types of Effects:
Short-term Effects: Associated with large doses over a short duration (e.g., acute radiation syndrome - ARS).
Symptoms include nausea, vomiting, diarrhea, hair loss.
Long-term Effects: Result from small doses absorbed repeatedly over a long period (e.g., cancers, birth abnormalities).
Classification of Biological Effects of Radiation
Somatic Cells: All body cells except reproductive cells; effects seen in irradiated individuals.
Genetic Cells: Reproductive cells; genetic effects are not visible in the individual but may be passed to future generations.
Sensitivity of Cells to Radiation
Cells termed radiosensitive are more susceptible to radiation, while radioresistant cells are less affected.
Determining Sensitivity Factors:
Mitotic activity: Cells that divide frequently are more sensitive.
Cell differentiation: Immature or specialized cells are more sensitive.
Cell metabolism: Cells with higher metabolic rates are more sensitive.
Radiosensitive and Radioresistant Organs
Radiosensitive Organs:
Lymphoid tissue.
Bone marrow.
Testes.
Intestines.
Radioresistant Tissues:
Salivary glands.
Kidney.
Liver.
Critical Organs
Organs whose damage can significantly reduce quality of life during dental radiographic procedures include:
Skin.
Thyroid gland.
Lens of the eye.
Bone marrow.
Radiation Measurement Units
Units of Measurement:
Exposure (Traditional: Roentgen, SI Unit: Coulomb/kg).
Dose (Traditional: Rad, SI Unit: Gray (Gy)).
Dose Equivalent (Traditional: Rem, SI Unit: Sievert (Sv)).
Sources of Radiation Exposure
Natural Background Radiation:
Cosmic radiation (from stars and the sun) and terrestrial radiation (radioactive materials in the earth and air).
Average dose in the US: ranges from $0.0015 - 0.003 ext{ Gy}$ (150 to 300 mrads) per year.
Artificial Sources:
Includes consumer products, fallout from atomic weapons, nuclear medicine, dental radiography.
Risk and Risk Estimates for Dental Radiography
Risk of inducing fatal cancer from dental radiography is estimated at $3$ in $1,000,000$.
Natural cancer risk (spontaneous) is $3,300$ in $1,000,000$.
Risk Comparisons
Example Risks (1 in 1 million):
Riding $10$ miles on a bicycle.
Driving $300$ miles in a motor vehicle.
Flying $1,000$ miles.
Smoking $1.4$ cigarettes daily.
Dental Radiation and Risks
Specific Dose estimates for radiation effects:
Thyroid gland: $0.006 ext{ Gy}$ needed to induce cancer (average dental dose: $0.00006 ext{ Gy}$).
Bone marrow: $0.005 ext{ Gy}$ (2000 to 5000 films needed for leukemia).
Skin erythema: $2.5 ext{ Gy}$ in $14$ days (over $500$ films needed).
Cataracts: $2 ext{ Gy}$ required (some debate over classification as critical).
Minimizing Patient Exposure and Dose
Factors Influencing Patient Exposure:
Use of faster film speeds (F-speed reduces dose by $60 ext{ ext{-}}70 ext{ ext{%}}$ compared to D-speed).
Proper collimation and positioning to limit exposure area.
Higher kVp settings lead to lesser exposure time, minimizing radiation.
Risk vs. Benefit Consideration
Dental radiographs should be prescribed only when the benefits of disease detection outweigh the biological risks involved. Properly performed dental radiographs generally prove beneficial compared to their risks.