Dental Radiography: Radiation Biology and Safety

Midterm Exam & Study Priorities

• Midterm Exam Timing: The midterm exam will be held in week seven. Students are encouraged to start reviewing for it in week six.
• Study Resources: The primary study resource recommended is Kahoot. Students should use Kahoot at any time to prepare.
  • When reviewing Kahoot questions, students should not just memorize the correct answer but also understand why other options are wrong and consider alternative wordings or related factors.
  • For example, if a question is about kVp (kilovoltage peak), students should also know about mA (milliamperage).
• Supporting Resources: If a concept from Kahoot is not fully grasped, students should refer to PowerPoints and the textbook for deeper understanding.
• Week 7: Midterm exam only, no regular class.
• Week 8: No classes.

Quiz Feedback

• One quiz question from last week, concerning waves and crests, was removed due to poor writing; everyone received a point for it.
• Another question dealt with ionization, specifically that ionization involves either gaining or losing an electron. This was the only correct answer; no extra points were awarded as it was a straightforward correct/incorrect question.
• An extra point question was included on the exam, which many students capitalized on, leading to high overall scores.

Current Course Content (Chapters 3 & 4)

• This week's focus is on Chapters 3 and 4, covering:
  • Ionization and free radical formation.
  • Stochastic and nonstochastic radiation effects.
  • Short-term, long-term, somatic, and genetic effects of radiation exposure.
  • Principles of patient protection: before, during, and after exposure.
  • Guidance documents like ALARA (As Low As Reasonably Achievable) regarding frequency and type of X-rays.
  • Operator protection, as operators receive more exposure than patients.
  • CODA (Commission on Dental Accreditation) standards for student competency in exposing, processing, and interpreting dental X-rays.
  • Ethical and legal standards for radiation safety.
  • Risk assessment and management of radiation exposure.

Radiology Radiation Biology

• Definition: The study of the effects of ionizing radiation on living tissues.
• Key Concepts:
  • Ionization.
  • Free radical formation.
  • Direct and indirect theories of injury.
  • Dose response curve (with no safe threshold for radiation exposure).
  • Stochastic effects (e.g., cancer, genetic mutations).
  • Nonstochastic effects (e.g., erythema, cataracts).
• These topics are a continuation from the previous week's discussion.
• When an X-ray is produced and strikes patient tissue, it results in either a photoelectric effect or Compton scatter.
  • Compton scatter: $62\%$
  • Photoelectric effect: <30\%

Ionization

• Mechanism: The removal of orbital electrons from atoms. When X-ray photons ionize water in the tissue, it can lead to damage.
• Effect: Leads to cellular changes, resulting in tissue and DNA changes.
• Cumulative Nature: Ionization effects are cumulative over time, meaning they build up with repeated exposures.
• Natural Occurrence: Ionization also occurs naturally in the environment from sources such as:
  • Eating a banana.
  • Flying on an airplane.
  • Sun exposure.
• Dental X-rays are considered much safer than other forms of X-ray exposure.

Free Radical Formation

• Process: When radiation hits tissue, it ionizes the water within the cells (the body is primarily made of water).
• Result: This creates highly reactive and unstable molecules with an unpaired electron, known as free radicals.
• Damage Mechanism: These free radicals attack and damage cell structures, leading to cellular injury.

Theories of Radiation Injury (Direct vs. Indirect)

• Definition: Damage to living tissue caused by exposure to ionizing radiation.
• Direct Theory: The X-ray photon directly strikes and damages the DNA of a cell.
  • Occurs infrequently.
• Indirect Theory: X-radiation ionizes water in the cell, producing toxins (free radicals) that then cause damage.
  • Occurs more frequently.
• Quiz Question Example: Damage may result from direct or indirect theory. Which occurs more frequently? (Answer: Indirect). Which is caused by radiation directly hitting DNA? (Answer: Direct). Which produces free radicals? (Answer: Indirect).

Dose Response Curve

• Concept: Describes the relationship between the amount of radiation received (dose) and the amount of damage to the tissue (response).
• Linear Relationship: In dental hygiene and dental X-rays, there is a linear relationship—as radiation dose increases, tissue response (damage) increases.
• Non-Threshold Curve: Indicates that any radiation dose, no matter how small, can cause biological damage.
  • There is no safe threshold for X-ray radiation exposure; no amount is considered entirely safe.
  • This is why patients often express concern about radiation exposure.
• Analogy: Similar to a sunburn; the longer the exposure, the more significant the sunburn.

Sequence of Radiation Injury

• This sequence describes the progression of damage from radiation exposure:
  1. Latent Period: The time between exposure to radiation and the appearance of observable clinical signs.
     • The duration depends on the total dose and the dose rate (e.g., a cloudy day sunburn might take longer to appear than a sunny day one).
  2. Period of Injury: The time during which cellular injuries become evident.
     • The type and extent of injuries vary.
  3. Recovery Period: Cells can repair damage to a certain extent. However, severe damage can make cells more susceptible to future injury in that area (e.g., repeatedly sunburned skin).
  4. Cumulative Effects: Unrepaired damage accumulates over time, and these effects are additive, potentially leading to mutations or long-term issues.
• Example: Getting a sunburn on a cloudy day, not noticing it immediately (latent period), then feeling the burn (injury), eventual healing (recovery), and increased susceptibility to future burns (cumulative effect).

Determining Factors for Radiation Injury

• The extent of injury depends on several factors:
  • Amount of the dose.
  • Dose rate (how quickly the dose is received).
  • Overall health of the exposed individual.
  • Amount of tissues irradiated (the specific area exposed).
  • Cell sensitivity.
  • Age of the person (younger individuals are generally more sensitive).

Types of Radiation Injury

• Short-term Injury (Acute Radiation Syndrome):
  • Occurs with large doses over a short period.
  • Symptoms include hair loss, nausea, vomiting, diarrhea, hemorrhage.
• Long-term Injury:
  • Associated with smaller doses over a long period.
  • Includes cancer and genetic defects.
• Somatic Effects:
  • Affects all cells in the body except reproductive cells.
  • Causes damage to the irradiated individual (e.g., cancers, cataracts, poor health).
  • Damage is not passed on to future generations.
• Genetic Effects:
  • Affects reproductive cells.
  • Damage is hereditary and passed down to future generations.
  • Does not directly affect the exposed individual but impacts offspring.
  • Genetic damage generally cannot be repaired.

Cell Sensitivity to Radiation Exposure

• A cell's response to radiation is determined by its activity:
  • Mitotic Activity: Cells that divide more frequently are more sensitive.
  • Cell Differentiation: Immature, undifferentiated cells are more sensitive.
  • Metabolism: Cells with a high metabolism are more sensitive.
• Radiosensitive Cells/Organs/Tissues (highly affected by radiation):
  • Small lymphocyte (most radiosensitive cell in the body).
  • Immature reproductive cells (testes, ovaries).
  • Young bone cells.
  • Bone marrow.
  • Lymphoid tissues.
  • Intestines.
• Radioresistant Cells/Organs/Tissues (less affected by radiation):
  • Bone cells (mature).
  • Muscle cells.
  • Nerve cells.
  • Salivary glands.
  • Kidney.
  • Liver.
• Critical Organs: Damage to these organs by X-radiation can significantly impact a person's life (i.e., permanent damage) due to their vital function or high sensitivity.
  • Thyroid gland.
  • Bone marrow.
  • Eyes.
  • Skin.

Pregnancy and Dental X-rays

• General Safety: Current dental X-ray material reports that dental X-rays are generally safe at any stage of pregnancy.
• Recommendation: It is usually recommended to perform X-rays after the first trimester if possible.
• Protocol: If a patient is pregnant, obtain medical clearance from their OB/GYN, especially for non-emergency X-rays (e.g., for a toothache).
• Lead Aprons: Always provide a lead apron if the patient requests one. Some offices may use two lead aprons, though one is theoretically sufficient given the low radiation dose.
• Ideal Approach: Avoid taking radiation X-rays if at all possible, or defercing until the second trimester.

Patient Protection and Education (Chapter 4)

Recommendations for Prescribing Dental X-rays

• ADA Guidelines: The American Dental Association's (ADA) 2012 guidelines for prescribing dental X-rays are still relevant and should be monitored for changes.
  • These guidelines inform dental professionals on how often and what types of X-rays should be taken (e.g., a full set every two years).
• Patient Education: Educate patients on the safety of dental X-rays.
  • If an office does not use a lead apron (following EU guidelines), have documentation from the ADA available to explain why it might do more harm than good in certain scenarios.

Proper Equipment

• Equipment must comply with state and federal radiation guidelines to reduce unnecessary exposure to both patients and operators.
• Key components include:
  • Filtration.
  • Collimation.
  • Position Indicating Device (PID).

Filtration

• Purpose: Filters out long wavelength, lower energy X-rays from the beam, resulting in a higher energy, more penetrating, and useful beam.
• Inherent Filtration: Built into the X-ray tube itself, consisting of the glass window, insulating oil, and tube head seal.
  • Typically $0.5$ to $1.0$ mm of aluminum equivalent.
  • This is not sufficient to meet federal regulations.
• Added Filtration: Aluminum discs added to the machine (between the collimator and tube head seal) to increase total filtration.
• Total Filtration: The sum of inherent and added filtration.
  • For machines operating at $>70$ kilovolts (kVp), the total filtration must be at least $2.5$ mm of aluminum.
  • If inherent filtration is $1.0$ mm, an additional $1.5$ mm must be added.
  • Machines operating at $<70$ kVp require less total filtration, but the focus for exams is often on the $>70$ kVp requirement.

Collimation

• Purpose: Restricts the size and shape of the X-ray beam, thereby reducing patient exposure.
• Types:
  • Round cone-shaped beam.
  • Rectangular shaped beam (preferred).
• Benefits of Rectangular Collimation: Reduces radiation exposure by $60\%$ compared to round collimation, as it matches the size of a standard #2 film/sensor.
• Beam Diameter Limit: The X-ray beam at the patient's skin surface must be less than or equal to $2.75$ inches in diameter (for round) or across its widest dimension (for rectangular).

Position Indicating Device (PID)

• Function: A cone-shaped or rectangular extension used to aim the X-ray beam.
• Types: Rectangular or round (clinic typically uses round).
• Length: Commonly $8$ inches (short) or $16$ inches (long).
• Advantage of Longer PID: Produces the least divergence of the X-ray beam, leading to less tissue exposure to radiation because the beam is narrower.
  • A shorter PID results in a wider beam divergence, meaning more tissue exposure.

During Exposure - Patient Protection

• Thyroid Collar & Lead Apron: Historically, debated, but generally accepted protective measures.
  • Thyroid collars are important for protecting the sensitive thyroid gland.
  • Lead aprons are used to shield radiosensitive areas.
• Beam Alignment Devices: These devices help align the receptor and beam, which reduces errors and the need for retakes, thus minimizing radiation exposure.
• Exposure Factor Selection: Adjusting exposure factors (e.g., from adult to child settings) is critical for appropriate dosing.
• Proper Technique: Ensuring receptors are covered, and the PID is correctly positioned (horizontally and vertically) to get a diagnostic image on the first attempt.
• Faster Film/Digital Receptors: Using faster film or digital receptors (sensors and phosphor plates) significantly reduces exposure time.
  • Digital receptors can reduce exposure by $50\%$ to $90\%$ compared to traditional film.

Operator Protection - Distance and Position Recommendations

• Distance: Operators must stand at least $6$ feet (1.83 meters) away from the tube head during exposure.
  • For portable X-ray machines, operators must stand behind a protective shield.
• Angle: Stand perpendicular ($90^ ext{o}$) or at a $135^ ext{o}$ angle to the beam, never in the direct path of the primary beam.
• Never: Operators should never:
  • Hold the receptor for the patient.
  • Hold or stabilize the tube head.
  • While patients might occasionally hold a receptor (e.g., in endodontic procedures), this should be avoided frequently, especially by operators who receive repeated exposure.

Minimizing Errors

• Focus on proper image acquisition and alignment of the X-ray tube and receptor to reduce retakes.
• Phosphor Plates: These are bendable and susceptible to damage (e.g., finger marks, bends, bite marks). If a phosphor plate is damaged (e.g., by a bite), it must be discarded because the pixels are destroyed, and it cannot be reused.

Radiation Monitoring

• Personal Monitoring (Dosimeters): Some offices use personal monitoring devices (dosimeters) to track individual radiation exposure, though they are not mandated in Michigan.
  • In some clinics, a select number of staff might wear them.
• Equipment Monitoring: X-ray equipment must be monitored for leakage by the state of Michigan.

Guidelines and Legislation

• Legislation: Two public acts govern radiation safety legislation.
• Radiation Safety Principles:
  • ALARA: As Low As Reasonably Achievable.
  • ALADA: As Low As Diagnostically Acceptable.
• Dose Limits: Due to the cumulative nature of radiation, there are different maximum permissible doses (MPD) for various groups:
  • Occupational Exposed Persons (us): MPD for occupational exposure is $50$ millisieverts (mSv) or $5.0$ rem per year.
  • Pregnant Occupational Workers: Limited to $0.5$ mSv per month during pregnancy.
  • Public/Non-occupationally Exposed (patients): Limited to $5$ mSv 0.5 rem per year.
  • Cumulative Lifetime Dose: Age in years multiplied by $10$ mSv.
    • Formula: $Cumulative ext{ }MPD = Age ext{ } (in ext{ }years) imes 10 ext{ }mSv$
    • For example, a $70$-year-old's cumulative MPD would be $70 imes 10 = 700$ mSv.